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Studies on organic foulants in the seawater feed of reverse osmosis plants of SWCC
DESALINATION ELSEVIER
Desalination 132 (2000) 217-232 www.elsevier.com/locate/desal
Studies on organic foulants in the seawater feed of reverse osmosis plants of SWCC Abdul Ghani I. Dalvi*, Radwan A1-Rasheed, Mohammad A. Javeed Research and Development Center, Saline Water Conversion Corporation, P. O. Box 8328, Al-Jubai131951, Saudi Arabia Tel. +966 (3) 361-0333; Fax +966 (3) 362-1615; E-mail: [email protected]
Received 3 July 2000; accepted 17 July 2000
Abstract
In the seawater desalination inorganic constituents (40,000-50,000 ppm) pose serious problems to multistage flash (MSF) process and more acutely to seawater reverse osmosis (SWRO) process which is also greatly influenced by organic content of (2-4 ppm). It is well-known that fouling on the RO membrane causes serious problems including (i) gradual decline of membrane flux thereby decreasing permeate production, (ii) an increase in AP thereby increasing the requirement of high pressure pump rating and (iii) degradation of membrane itself. The dissolved organic matter is not a single substance but a mixture of ill-defined aliphatic and aromatic compounds. Humic substance constitute major portion of total organics in seawater, thus to identify the nature, and compositional characteristic of such substance along with its estimation is of utmost importance to overcome the problems associated with fouling of RO membranes. Humic substances from Al-Juball, A1-Khobar (Gulf Coast) and Jeddah (Red Sea Coast) were isolated and analyzed by different techniques, viz elemental analysis, UV-visible speclrometry, IR Spectrometry and fluorescence spectrometry. Method of isolation and purification of humic substance has been standardized to recover gram quantifies. Two methods of quantitative estimation have also been developed. The first method involves estimation of humic substance from seawater by UV-visible spectrometer atler isolation. In the second method, humic substance was determined using fluorescence technique by spectro-photofluorometer either after isolation and pre-concenlration or directly in the seawater without isolation. The precision of methods in terms of percentage relative standard deviation for UV-visible method is 3.8% while for fluorescence method is 1.75%. Elemental composition, nature, IR speclra and concentration of humic substances of A1-Jubail and A1-Khobar are similar. However, humic substances from Jeddah differed in many respects including the composition. The remarkable deviation in elemental composition indicates that perhaps nature and origin of humic substances fi'om Gulf and Red Sea coasts are different. High nitrogen, sulphur and hydrogen to carbon ratio indicate high bacterial activity in the Red Sea region. Keywords: Organic foulants; Humic substances; Fulvic acid; Humic acid; Reverse osmosis; Quantitative estimation; Fluorescence; UV-visible spectrometry; Fluorometry; Infra-red spectroscopy; XAD resins
*Corresponding author. Presented at the Conference on Membranes in Drinking and Industrial Water Production, Paris, France, 3-6 October 2000 International Water Association, European Desalination Society,American Water Works Association, Japan Water Works Association 0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: SO0 l 1 - 9 1 6 4 ( 0 0 ) 0 0 1 5 3 - 3
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I. Introduction
Desalination in the broad sense is a process through which water of low salinity is produced to an extent that it becomes potable. Among the known desalination processes, multistage flash (MSF) distillation and reverse osmosis (RO) membrane filtration are most popular and widely used techniques. Major chemical constituents (40,000-50,000 ppm) of seawater are of inorganic origin plus a minute quantity (2-4 ppm) of organic origin. Though organics are negligible in concentration as compared to inorganic constituents, they pose more acute problems in reverse osmosis desalination process. It is well known that fouling in RO membranes causes serious problems including (i) a gradual decline of membrane flux thereby decrease in permeate production, (ii) an increase in AP thereby increasing requirement of high pressure pump rating and (iii) degradation of membrane itself. All these factors reflect on the cost of water production. Hence, now-a-days attempts are being made to deplete the concentration of organic and some of the inorganic constituents from the feed to RO to overcome these problems by various pretreatment methods. Other than conventional methods such as coagulation [1], filtration and separately passing through activated carbon or clays for decreasing the organic load from the feed of RO, some of the more recent techniques [2,3] are also emerging such as nanofiltation and ultrafiltration which are felt to be quite promising pretreatment method to overcome the said problems and to reduce the overall cost of production. Moreover, these organic contaminants have been found to be the precursors for the formation of organic derivatives, some of which are carcinogenic. The dissolved organic matter is not a single substance but a mixture of many ill-defined aliphatic and aromatic compounds. However, among the total dissolved organic substance (DOS) in seawater
---90% is represented by the humic materials or substances. Much attention has, therefore, been paid to isolate them and study their chemistry. Humic substances account for significant and variable proportions of organic matter in soils, sediments, and they are also found as soluble organic matter in fresh and seawaters [6--8]. Humic substances comprise a general class of ubiquitous substances in terrestrial and aquatic environments. Studies on humic substance in the earlier days [9,10] were confined only to soil scientists. The term 'humus' (Latin equivalent to soil) was initially introduced to describe the dark colored organic matter in soil, and later, the term was modified as 'humus acid' or 'humic acid'. These substances are known to play a role in the food chain, affect the aesthetic quality of water by imparting color and act as a complexing agent for inorganic ions. Thus there has been a growing interest to determine the contribution by humic substances to biological, physical and geochemical processes in natural water systems. Chloroform was found to be ubiquitous in chlorinated drinking water, hence the US Environmental Protection Agency (USEPA) issued a regulation, limiting the concentration of chloroform and its sister trihalomethanes (THMs) to 100 ppb [11,12]. Various authors have supported the hypothesis that it is the na~ral organic matter (humic substance) which is the most common reaction precursor to trihalomethane formation [13-15]. Aquatic humic substances are organic acids that are derived from soil humus and aquatic plants. Generally more than half of the dissolved organic carbon in water is due to humic substances. As there is growing concern over the role of humic substance in the various aspects of water chemistry, more and more efforts are now concentrated to understand the role, structure and chemistry of humic acid in aqueous systems mainly because of their complexation with metal ions, mobilization of toxic trace metal [16], formation of chlorinated
A.G.L Dalvi et al. /Desalination 132 (2000) 217-232
methanes in water treatment, role in bacterial growth after chlorination of feed and interaction with organics like solublization of pesticides and hydrocarbon [17]. To asses the potential for bacterial growth and THMs formation potential due to humic substance in sea water, it is essential to isolate and characterize the humic substances. Further-more, there is a need to develop methods to estimate the humic substances in the seawater and other aqueous systems. The isolation and estimation of humic substances are two difficult tasks to be achieved but they are very important, as various aspects are related to humic sub-stances. Low concentration of humic substances in seawater or aqueous system and non-availability of efficient sorbent to obtain gram quantities, makes the isolation and characterization, a tedious, lengthy and difficult task. The methods once developed will enable to map out the areas of high humic substances content where preventive measures could be taken to avoid biofouling on RO membrane. Humic molecules as such are refractory in nature, not easily assimilable by microorganisms. However, during the chlorination of feed water, humic substances are broken down into smaller fragments thereby loosing their inert nature and become primary source of bacterial nutrient promoting in membrane biofouling. Smaller fragments formed during the chlorination process are also major source for the formation of carcinogenic organic compound such as trihalomethanes, haloacetic acids and other disinfection by-products. Both these problems are of utmost importance in RO process. Thus much attention is being paid recently to better understanding the situation. In this direction research work leading to abatement of organic fouling is actively being pursued. Present study was carried out with a view to standardize the procedure for extraction of dissolved organic substances from seawater, their isolation and characterization, and to study the concentration and distribution of humic
219
substances, in the aquatic systems near the RO desalination plants located in the Eastern Province (Arabian Gulf) and Western Province (Red Sea). To achieve the above said objectives, the project was carried out with the following: • To standardize extraction and purification procedure of humic substances in seawater to get at least gram quantities. • To isolate humic substances from sea near the locations of SWCC plant in (a) Eastern Province (Arabian Gulf) and (b) Western Province (Red Sea). • Characterization and estimation of pure isolated humic substances by physicochemical techniques, viz., UV, IR, TOC and CHNS (carbon, hydrogen, nitrogen and sulphur) analyzer. • Development of a method for the determination of concentration of humic substances in aqueous system by direct or indirect methods using techniques like absorption spectrometry or flourometry.
2. Experimental There are several methods and techniques such as precipitation [18], solvent extraction [19], ultrafiltration [20], for isolation and extraction of humic substances. However, all these methods are tedious, time consuming and involve complicated procedures to isolate and concentrate humic substances from large volumes of water. After the development of macroporous resins for chromatography [21-23], these resins were fully exploited for removing trace organic solutes from water [24-27]. Humic acid concentration is less than 2 ppm whereas the total dissolved organic substance (DOS) is itself only about 2-3 ppm. To get gram quantities of humic substances, large amount of seawater under specific conditions is passed through exchange column of specific resins Amerlite XAD-2
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A.G.I. Dalvi et al./ Desalination 132 (2000) 217-232
and/or XAD-7 having high efficiency absorption or adsorption for humic substance and then eluted under different conditions to obtain concentrated matter.
ii........................
ii
2.1. Procedure o f isolation
Glass column of 55 cm length and 5 cm diameter fitted with high porosity disc and socket at bottom and with B24/40 female socket at the top end was used. Schematic diagram of experimental set up used for isolation of humic substance from seawater is given in Fig. 1. Unchlorinated seawater from intake was pumped into a cleaned 200 L drum through bolting silk cloth (75 la mesh size) to remove suspended particles. After filtration, the pH of seawater was adjusted to 1.9-2.00 using sulphuric acid and pumped through column with help of master flex pump at the rate of 110-120 ml/min. Thus about 160-170 L of seawater was passed through the column per day. After passage of 2500-3000 L of seawater in about 20 d through the column, it was washed with distilled water, the pH of which was adjusted to 1.9-2.0. Washing of column with distilled water o f p H 2.0 was carried out till the effluent was free of chloride. This indicated that all salts have been removed from column. After thorough washing of column with distilled water of pH = 2.0, humic acid which was adsorbed on the column was eluted with a mixture of 5M NHnOH and methanol in equal proportion. Elution was kept very slow at the rate of 5-10 ml/h. In about 2.0-3.0 L of eluting solution all adsorbed humic substances were eluted which was further concentrated using a Rotavapour assembly consisting of buchi water bath and water condenser coupled with refrigerated constant temperature circulator. Evaporation was carried out under vacuum at a temperature of 40-45°C. When the solution containing concentrated humic substances reduced from 2-3 L down to about 30-40 ml, it was transferred to a
Fig. 1. Schematic diagram for the extraction of humic substances from the seawater.
dish which was kept in desicator in vacuum. Here, the solution was further concentrated by slow evaporation under vacuum and finally obtaining a solid, dark brown colored residue after about 8-10 days. This solid residue was ground into fine powder and stored in a glass bottle in desicator for further studies.
2.2. Instruments
To obtain UV-visible spectrum of humic substance, Shimadzu UV-2100S dual beam spectrometer having wave-length range of 190-800 nm was used. To record the infra red spectra of humic substances, KBr pallets (disc) were used. 5 mg of the humic substance was thoroughly mixed with 0.3 gm of spectroscopic grade KBr and pressed using pelletizer resulting in transparent disc. Disc was mounted on window holder and IR spectra was recorded using Hitachi 270-50 spectrometer in the range of 2504000 cm-l. To obtain the excitation and fluorescence spectrum of humic acid Shimadzu RF1501 was used. This spectrometer is capable of varying the wavelength of both excitation and emission (fluorescence) from 220 to 900 nm. Carlo Erba EA 1108 was used for elemental analysis of carbon (C), hydrogen (H), nitrogen
A.G.1. Dalvi et al. / Desalination 132 (2000) 217-232
(N) and sulphur (S). To estimate total organic carbon (TOC) Shimadzu TOC 500 model was used.
2. 3. Quantification of humic substances For quantification of humic substance a small glass column about 30 cm in length and 1.2 cm diameter filled with XAD-2 resins was used. Two liters of unchlorinated seawater from sites of estimation were taken. The pH value of seawater was adjusted to 2 and passed through the column which was already conditioned at pH = 2. Seawater was passed at the rate of 6-8 drops/ min. After loading 2 L of seawater, column was washed with distilled water whose pH is adjusted to 2 in order to remove all salts. Then humic acid adsorbed on the column was eluted with eluting mixture of equal proportion of 5 M NHnOH and methanol of equal proportion. Elution rate was adjusted to 3--4 drops/min. About 10 times column volume (=300 ml) eluting solution was used for complete removal of humic substances absorbed on the column. This effluent was concentrated at 40°C to 125-130 ml to remove ammonia. Later the eluted concentrate solution was transferred to a 250 ml volumetric flask and made up to mark. Portion of these solution were used for quantification of humic substance by UV-visible and fluorescence techniques.
221
3. Results and discussion
3.1. Estimation and characterization of isolated humic substances It is practically difficult to characterize and estimate any material by means of single technique. In order to get more reliable and dependable result, it is always recommended to use as many techniques as possible to characterize or estimate any material or substance. Hence, various techniques were adopted to characterize and estimate humic substances isolated from seawater at three different locations of Red Sea and Arabian Gulf coasts. The techniques used to characterize and estimate were: (a) Elemental analysis (b) Total organic carbon (TOC) (c) Ultraviolet and visible absorption spectroscopy (d) Infrared spectroscopy (e) Fluorescence spectroscopy Table 1 indicates that a huge amount of seawater was used to obtain humic substance in gram quantities. Last column of the Table 1 gives approximate concentrations of humic substances in seawater based on amount recovered from the total volume of seawater used,
Table 1 Volume of seawater used for isolation ofhumic substance and their concentration Station
Volume of seawater, Amount isolated by resin Concentration ofhumic substances in seawater, ppm L (dry form), g Based on 100% retention on Based on 50% retention
resin AI-Jubail AI-Khobar Jeddah
2400 2600 3100
1.8 2.1 1.77
on resin 0.75 0.80 0.59
1.5 1.6 1.18
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Table 2 Percentageof fulvic and humicacid ofhumic substances Components Fulvic acid, % Humic acid, % Total acid, meq/g
Stations AI-Jubail AI-Khobar Jeddah 80.4 79.5 84 19.6 20.5 16 3.36 3.229 5.01
The humic substances usually consist of humic and fulvic acids in addition to other components. Humic acid components are soluble at higher pH but insoluble at low pH while fulvic acid component is soluble in both bases and acids [28]. Humic substances isolated from three locations viz. A1-Jubail, A1-Khobar and Jeddah were subjected to analysis, it was found that 80.4, 79.5 and 84% of substances contained fulvic acid component respectively in above said sites. It can be seen from Table 2 that humic acid component in isolated humic substance conrained only 19.5% in AI-Jubail, 20.5% in A1Khobar and 16% in Jeddah. High percentage of fulvic acid in Jeddah sample (=84%) means more aliphatic components in humic substance and this was also confirmed by high hydrogen to carbon ratio compared to other samples. In general, aliphatic components are higher than aromatic component in all the samples from three sites. Furthermore, total acidity was high in Jeddah sample which was 5.01 meq/g compared to 3.36 and 3.23 meq/g for AI-Jubail and AIKhobar samples respectively.
3.2. Elemental analysis
Elemental analysis is probably the most common technique used in characterization of humic substances [29]. In fact, chemical analysis is the cornerstone of all chemical inquiry. Elemental analysis could also be useful in establishing the purity of humic substances
preparations methods. Elemental composition, along with elemental ratios such as C/H, O/C, N/C, are fundamental approaches used in describing and understanding the geochemistry of the isolated substances and often are single most useful indicator of unique nature of a given humic substance. Having stated the importance of elemental data for the interpretation, it is necessary to obtain reliable and accurate results. However, there are limitations in obtaining accurate data for various reasons. Foremost limitation is the availability of very small samples with high purity. The result of elemental analysis for C, H, N, S for all three samples collected from AI-Jubail, A1-Khobar and Jeddah are tabulated in Table 3. However, oxygen values are computed after taking into account the ash content of the sample. A standard of a known composition was also run along with the above samples to ascertain the analysis. It can be seen that C % in AI-Khobar and A1-Jubail were around 42% while in Jeddah it was only 27%. However, hydrogen in Jeddah was 5.46% compared to 6.23% in A1-Jubail and A1-Khobar samples. In other words, percentage ratio of H/C in Jeddah was very high, thereby indicating that aliphatic components were more in Jeddah sample compared to samples from AI-Khobar and AI-Jubail. Nevertheless, H/C ratio in A1-Khobar and A1Jubail plant samples were also high (-1.77) which indicated that aliphatic components in humic substances from these sites were also high but not as high as was in Jeddah sample (which was highest among the samples from all three locations). In addition, ratio of O: C was also above 0.7 in all three samples indicated high proportion of fulvic acid in humic substances. This fact was supported by solubility test in acid as shown in Table 2 where it was indicated that all the three samples from AI-Jubail, AI-Khobar and Jeddah contained 80.4%, 79.5% and 84% of fulvic acid, respectively compared to 19.6, 20.5
A.G.I. Dalvi et al. /Desalination 132 (2000) 217-232
223
Table 3 Elemental analysis Sample Element,% identity C H Khobar HA Jubail HA Jeddah HA Standard Actual value
S
O
Ash
Atom, % C H
N
42.25 6.23 7.19
0
40.43
3.9
3.52
42.19 6.23 6.38
0
37.5
7.7
N
O
Atomratio H/C C/H
0.514 0
2.53
1.77
3.516 6.23 0.456 0
2.34
1.772 0.564 0.666 0.130
6.23
S
O/C
N/C
0.565 0.719 0.146
26.95 5.46 10.28 10.23 38.98 8.1
2.246 5.46 0.734 0.319 2.44 2.131 0.41
30.14 3.8 8.23 9.9 27.85 - (31.35) (4.0) (8.13) (9.3) (27.85) - -
2.512 3.8 2.612 4.0
1.08
0.327
0.588 0.309 1.74 1.513 0.661 0.693 0.237 0.581 0.290 1.74 1.531 0.653 0.666 0.222
Values in brackets are actual value of standard
and 16% of humic acid component respectively in these plants. Hence, it can be stated that aromatic components in all three humic substances were less compared to aliphatic components. However Jeddah sample contains higher aliphatic component compared to other samples. The decrease in aromacity of Jeddah humic substance indicated less refractory or inert nature and thus more viable or assimilable nature o f carbon as a nutrients for microorganisms. This could be one of the important factors for high SDI and biofouling in Jeddah. The difference in nature of humic substance isolated from Jeddah compared to AI-Jubail and A1-Khobar can also be noted from N/C ratios. This indicates a lot of bacterial activity in Jeddah in comparison with other two sites. Secondly relatively high N/C in Jeddah samples indicated that large proportion of humic substances produced were from aquagenic refractory material. Sulphur content in Jeddah sample was around 10% whereas it was absent in A1-Jubail and A1-Khobar samples. High nitrogen and sulphur content may provide an ideal breeding ground for microorganism. However the source and origin of sulphur need further investigation.
Based on elemental analysis the most likely empirical formula for humic substances in A1Jubail and AI-Khobar area is CsH14NO whereas for Jeddah sample it is C4I-I8NOS05. Visual observation of samples also revealed that A1Jubail and A1-Khobar were similar of having dark brown color fine powder which formed lumps while Jeddah sample was very fine crystalline powder with light yellowish brown color. It did not form lump.
3.3. Molecular weight Humic substances molecular weight determination using gel permeation chromatography for AI-Jubail, AI-Khobar and Jeddah lied in the range of 3000-3700. Average molecular weight of three site samples are 3440, 3620, and 3718 for AI-Jubail, A1-Khobar and Jeddah, respectively.
3.4. Temperature dependence o f weight loss Weight loss determination of humic substances has been carried out on three samples
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A.G.I. Dalvi et al. /Desalination 132 (2000) 217-232
Table 4 Weight loss analysis and ash content Station
Table 5 Total organic carbon of isolated humic substances
% weight loss @ temp.
% ash
55°C Al-Jubail 6.6 AI-Khobar 6.1 Jeddah 2.6
1O0°C
750 °C
content
9.1 8.2 4.0
92.2 96.1 91.9
7.7 3.9 8.1
from different sites of AI-Jubail, AI-Khobar and Jeddah and results are shown in Table 4. All three samples indicated gradual increase of weight loss as temperature increased up to 100°C. This is due to loss of moisture. It has been reported [30] that loss of weight at 5560°C corresponds to water determined by Karl Fisher water content in humic substances. Here in this case also loss of weight at 55°C corresponds to water content in these three samples. Loss in weight beyond the temperature of 80°C is considered as loss due to decomposition o f humic substances. Samples dried at different temperatures up to 80°C regained the weight when again exposed to air whereas the samples heated at temperature 80°C or above, did not regain or regained negligible weight [30] was the basis for the conclusion that the humic substances heated beyond the temperature of 80°C start decomposing. Present samples was heated up to 750°C to determine the ash content and results of ash content are also shown in Table 4. Ash content were 7.5-8.0% in AI-Jubail and Jeddah sample whereas A1-Khobar sample contained only 4%.
3.5. Total organic carbon (TOC)
Total organic carbon of the isolated humic substances from all three sites were determined by dissolving known amount of dry humic substance in water (free of carbon). The results are shown in Table 5. TOC of humic substances
S. No.
Humic substances,ppm
TOC, ppm A1-Jubail AIJeddah Khobar
1 2 3 4
1 5 10 20
0.32 1.8 3.8 7.4
0.36 2.44 3.7 7.9
8,
0.12 1.1 2.2 4.5
0
0
y - 0.386x- 0.08
s
Al-,lubai] &
[ 3
.~36
J~ld~
2
5
~0 IS Hlmb Ikll~Immmllml
20
Fig. 2. Variation of total organic carbon (TOC) as a function ofhumic substances.
obtained from AI-Jubail and AI-Khobar were higher compared to Jeddah. In fact, AI-Jubail and AI-Khobar humic substances as function of TOC can be represented by Eq. (1) while Eq. (2) represents the case for Jeddah, Al-Khobar and AI-Jubail: Y = 0.386 X - 0.0894
(1)
Jeddah: Y = 0.228 X - 0.0636
(2)
where Y --- TOC and X = humic substance both in ppm.
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A.G.I. Dalvi et al. / Desalination 132 (2000) 217-232
Fig. 2 represents the graphic relation of humic substance with TOC. Both the graph and the equations show that values of TOC obtained were in reasonable agreement with the values of carbon obtained by elemental analysis for these samples as shown in Table 3. For Al-Jubail and AI-Khobar (eastern province) samples carbon values by TOC were about 38% whereas for Jeddah it was 22%. However values of carbon obtained by elemental analysis were 42% and 27% for eastern province location and Jeddah, respectively. As it can be seen in Fig. 2 that two different equations and graphs followed by humic substances obtained from eastern and western provinces indicate that the composition and perhaps nature and origin are different for humic substances isolated from these sites.
too
so % T
7o
~o ao 9o 20 10 i
I
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3ooo
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2OOO1 laoo
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Wave Nmnb~
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I
~
I
;
|coo
a¢o
:
6oo
;
4oo
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2.so
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Fig. 3. Infra red spectrum of humic substances isolated from A1-Jubail site.
9c 80 TO ~o
3.6. 1R spectroscopic studies
50
Spectroscopic method like other investigation techniques are severely limited when applied to humic substances. This is because humic substances comprised of very complicated illdefined mixtures of various long chained organic compounds and macromolecules. Thus the spectra of humic substances represent the summation of the responses of many different species and functional groups present in these substances. In some cases only small fraction of total number of molecules or groups contribute to total spectra. Occasionally, spectral contribution due to some groups may be masked partially or totally, thereby causing hindrance, hence no clear understanding which in certain cases could lead to totally different inferences or interpretations. Infrared spectrum in the region of 4000 to 250 cm -1 obtained by KBr pallet are shown in Figs. 3-5. It can be seen that all IR spectra obtained for samples from three sites viz. A1Jubail (Fig. 3), A1-Khobar (Fig. 4) and Jeddah (Fig. 5) had in general similar features and bands.
40 Jo 2 10 , 4000
3~10
)riO0
~
, ~
,
,
a
,
ll0O
1600
1~
12100
, I~
, ~
, a0O
,
,
~
W n v e N g m b ~ m -n
Fig. 4. Infra red spectrum of humic substances isolated from AI-Khobar site.
10o
io
% T
60 SO a
2o
~o0
i 3ga~
loa0
~oo
ao00
i~o
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Fig. 5. Infra red spectrum of humic substances isolated from Jeddah site.
226
A. G.I. Dalvi et al. / Desalination 132 (2000) 217-232
The samples of AI-Khobar and AI-Jubail showed the closer resemblance whereas Jeddah sample IR showed distinct difference due to the presence of some additional peaks. Prominent bands which were observed in all samples are 31503500 cm -~, 2800-3050 cm -~, 1550-1725 cm -~, 1375-1450 cm-1, 1220 cm-1, and 1050-1150 cm-1 region. However, in the case of Jeddah additional sharp intense band at 600 cm-1 was observed, in addition to the absence of a band around 1220 which could be clearly observed in samples from other two sites. Bands around 1230 cm-1 due to C-O and OH deformation were more clearly seen in AI-Khobar and A1-Jubail samples which was absent or perhaps merged into the band around 1100 cm-~ in Jeddah sample. The band around 1100 cm-l is very intense in the case of Jeddah sample. In addition, very sharp absorption was noticed at 600 cm-~ with shoulder peak at 460cm -1 in Jeddah sample. However peaks of 600 and 480 cm -1 also observed in AI-Jubail and A1-Khobar samples but spectra were diffused and peaks were broad and less intense. The broad peaks in the region 31503500 cm -1 which was observed in all the three samples corresponds to absorption where OH stretching occurs, [31] and its broadness is generally attributed to hydrogen bonding [32,33]. When Hydrogen is bonded to the more electronegative atoms such as oxygen or nitrogen the bond is polarized, leaving the hydrogen atom with partial positive charge. This partially positive charge hydrogen atom can then interact electrostatically with other atoms which are oppositely charged. This interaction can be intramolecular (occur within the same molecule) or intermolecular (between functional group of the different molecules). Thus hydrogen bonding results in increased separation between the hydrogen atom and the atom to which it is covalently attached or bonded. This brings the change in frequency of absorption and broadening result due to the statistical distri-
bution in the extent of hydrogen bonding in the molecule. All the three samples of humic substances indicated the band around 3400 cm-l due to OH stretching broadened because of hydrogen bonding. A shoulder peak around 2920-2950 cm-1 was also evident in the spectra of all the three humic substances. These bands were attributed to the asymmetric and symmetric stretching vibration of aliphatic C-H bonds in methyl and/or methylene units in the compound [32]. The assignment of this band was confirmed by some worker in this field by methylation of humic substance and corresponding observation of increased absorbance in the spectra [31-34]. Absorbance band observed around 17001710 cm-1 is generally attributed to the C=O stretching vibration mainly due to carbonyl group [31,32]. This band was very sharp and distinct in the spectra of the Jeddah sample compared to AI-Jubail and AI-Khobar samples. This probably points out to the fact that fulvic acid component is high in concentration in case of Jeddah sample compared to AI-Jubail and AIKhobar samples. The band at 1650 cm-1 is assigned to aromatic C=C stretching vibration, i.e., carboncarbon double bonds conjugated C=O or COO [35]. Generally C=C bond in benzene is infrared inactive, however benzene derivatives which decrease the symmetry of the molecule, bond becomes infrared active. This band was more pronounced in AI-Jubail and A1-Khobar samples compared to Jeddah again support the aromatic component in Jeddah is relatively less compared to aliphatic component. Band at 1450 cm-1 (as shoulder) has been attributed to the bending vibration of aliphatic C-H group [33] and band at 1400 cm-x is due to O-H bending vibrations of alcohol or carboxylic acids as occurs with simple model compounds. These two bands were superimposed on each other and were not resolved completely. These bands were present in all three samples. The band at 1200-1230cm -~ has been
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A.G.1. Dalvi et al. / Desalination 132 (2000) 217-232
assigned to C-O stretching vibration and OH bonding deformation mainly due to carbonyl groups. This band disappears on producing salts [33]. This band 1050--1100 cm-1 was present in all three sites samples but more intense in Jeddah sample. This band is attributed to the C-H bond (single bond) stretching in a branched CH3 unit [36]. More intensity of this band in Jeddah compared to other samples again support the idea that the Jeddah sample is characterized to be more aliphatic in nature. Presence of intense and sharp band at 600 cm -I in Jeddah sample makes the Jeddah sample IR spectra look very different from those of AI-Jubail and A1-Khobar IR spectrum. This band was attributed to C-S bond [36]. As it can be seen from Table 3 that elemental analysis indicates the presence of 10.23% S in the Jeddah samples whereas A1-Khobar and A1-Jubail did not contain any sulphur. Thus the presence of intense band at 600 cm-l only in Jeddah sample was as expected• Further it was reported that treated and untreated sewage is discharged into the Red Sea and this could be one of the source of high sulphur content in seawater and its humic substances.
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.
.
.
.
.
!
.
.
.
.
.
.
.
" • !
(a) AI-Jubail
L~----JJ
~00
. . . . . . . . .
6OO
i . . . . . . . . .
I
* . . . . . .
8O
~'°"
(b) AI-Kh0bar
400
60O
!
(rm)
II*
(c) Jeddah
3. 7. UV-visible spectroscopic 0
UV-visible spectra of samples generally appear as broad band, thus f'me structural information is lost due to this broadening. Olsen [37] demonstrated that how one can get misleading information due to impurities present in the samples. Considering the ill-defined multicomponent nature of humic substances, UVvisible spectra results from overlap of absorbances of various chromophores or functional groups [38] are very broad and diffused. The UV-visible absorptivities ofhumic substances do vary as a function of pH [39] and these are due to the ionization of carboxylic and phenolic functional groups. Though, UV-visible spectro-
0
0
0
0
0
,
!
•
400 I.
G00
(rim)
Fig. 6. UV-visible spectra of humic substances from a, Al-Jubail; b, A1-Khobar;c, Jeddah.
228
A.G.I. Dalvi et al./ Desalination 132 (2000) 217-232
scopy is a valuable tool in identification of chromophoric functional groups in discrete organic molecules but there are limitations. In spite of the limitation of UV-visible spectroscopy, it has been exploited and used for estimation of degree of humification by Chen et al. 1977 [40]. Fig. 6 shows the UV-visible absorption spectrum obtained with isolated humic substance from three sites. Irrespective of site of sample, spectrum with a broad band having absorption peaks around 300 nm was observed. This absorption band could be utilized for estimation of humic substances in seawater. After isolation of humic substance from seawater by procedure, which is described in experimental part, was subjected to estimation by UV-visible spectral studies. Absorbance of isolated solution containing the humic substance was used to compute the concentration of humic substance. It can be seen from Table 6 that concentration of humic substances estimated using UV-visible spectroscopy technique after separation from 2 L
Absorbance vs Humlc substances
0,3-
0.25,
0.2,
0.15.
0.1.
0.05.
0
5
10 15 Humlc $ubsblncu Conc ppm
20
25
Fig. 7. Calibrationcurveof humic substance (UV-visible region).
of seawater of all three sites, viz. A1-Jubail, AIKhobar and Jeddah were 1.99, 1.93 and 1.2 ppm respectively. The percentage relative standard deviation of the method is 3.8%. Calibration curve used for estimation of humic substances was obtained by dissolving known quantities of humic substances and taking spectrum as shown in Fig. 7. This figure shows a linearity up to 20 ppm and expressed by the following equation: Y = 0.0125 X + 0.0184
(3)
Hence from the above equation and the absorbance (Y) of the unknown humic substance solution, it would be possible to compute concentration (X) of humic substances in the unknown solution.
3. 8. Fluorescence spectroscopy studies
Absorption of radiation by an atom or a molecule raises the molecule from electronic ground state to higher level. Most of the absorbed energy is dissipated in the form of heat and come back to ground state by radiationless transitions depending upon the excitation. However, in some molecules major portion of absorbed energy is emitted as electromagnetic radiation at higher wavelength (low energy as part of it is dissipated in radiationless transition) than the excitation radiation wavelength which depends on allowed electronic transitions according to spectroscopic rules. Relatively few organic compounds exhibit fluorescence mainly in UVvisible region of electromagnetic radiation spectrum. Absorption is a pre-requisite of fluorescence. Some functional groups if present in certain molecules (or compound) may enhance the fluorescence intensity while other functional groups may quench it. Humic substances are known to fluoresce [41-44]. Fluorescence Spectroscopic studies suffer from all the limitations of UV-visible spectroscopy in regard
229
A.G.I. Dalvi et al. / Desalination 132 (2000) 217-232
to obtaining information about functionality in humic substances. It is known that only small fraction of the absorbed radiation is emitted as fluorescence and since humic substances are comprised of multicomponent mixtures, it is likely that fluorescence spectra may represent even small portion or part of the total molecule rather than those represented by UV-visible spectra. These limitations have been highlighted in number of the review [43-45]. In short, it can be concluded that fluorescence spectroscopy can not be used for direct determination of functional groups in humic substances. However, fluorescence technique could be utilized for estimation of humic substance like UV-visible spectroscopy. Initially, it is necessary to determine the absorption maximum wavelength for excitation of molecules to get fluorescence at the optimum intensity. This is obtained by taking excitation spectrum i.e. observing the fluorescence at a fixed wavelength and varying excitation wavelength. The excitation spectra for humic substances in water are shown in Fig. 8. It can be seen from the figure that there is broad bands absorption which peaks at 256 nm and 661 nm in addition to direct emission from excitation source at 421 nm and 842 nm which are sharp in nature. Broad bands at 256 nm and 661 nm are due to molecular absorption levels which are diffused and overlap with molecular orbital levels of longer life time compared to atomic levels which are sharp and have short life time [46]. Excitation spectra of any molecule, in fact reveals their absorption bands or level. Fig. 8 shows the fluorescence spectrum of isolated humic substance. It was evident from the fluorescence spectrum that there is a broad band emission at 462 nm. This fluorescence emission at 462 nm is a characteristic of humic substance and could be used for quantification. Pure humic substance isolated from seawater was used for obtaining fluorescence calibration curve. Calibration was found to be linear up to 20 ppm and can be seen in Fig. 9. Using this calibration
500
I;
o,o
220
i'i
A.:"i Wavelength (nm)
900
ExcitationSpectrum
00
0.0
, 220
, Wave length (nm) Fluorescence Spectrum
Excitation spectrum (1) 256 (broad) (2) 421 (sharp) (3) 661 (broad) (4) 843 (sharp)
900
Fluorescence Spectrum 256(sharp) 462 (broad 514 (sharp) 706 (broad)
Fig. 8. Excitation and fluorescence spectrum.
curve, the concentration of humic substance could be obtained directly either in seawater by obtaining fluorescence intensity at excitation wavelength of 256 nm or from eluted humic substance solution obtained after separating it from 2 L of seawater by XAD-2/XAD-7 resin as discussed in detail in experimental section. The results obtained by fluorescence spectroscopic technique using two type of samples i.e. seawater as such or isolated humic solution from seawater are shown in Table 6. It can be seen that humic substance concentration estimated after isolation were 2.09, 2.06 and 1.39 ppm whereas amount estimated directly in seawater prior to separation were 2.7, 2.34 and 1.4 ppm in Al-Jubail, A1-Khobar and Jeddah, respectively. The percentage relative standard deviation of the method is 1.75% for isolated samples. Amount estimated prior to separation is higher compared to isolated samples. This is due to interference of impurities in seawater. Perhaps these impurities or other species present in seawater also fluoresce at the same wavelength as humic substance thereby enhancing the estimation.
230
A.G.I. Dalvi et al. / Desalination 132 (2000) 217-232
other techniques could also be used for their quantitative estimation. 2. Elemental analysis of isolated humic substances from all three sites indicated that humic matter isolated from eastern and western provinces is of two different nature and compositions. Their empirical formula (CsHlnNO and C4I-IsNOS0.5) and degree of aromacity were different.
Fluorescence Intensity v$ Humlc substances 400
y = 16.701X + 10.412 300
i
200,
J
100.
0
5
10 15 Huml¢ eull~tanmm Cone. ppm
20
25
Fig. 9. Fluorescencecalibrationcurve for humic substances.
Table 6 Quantificationofhurnic substanceby differenttechniques S. Technique No. I
AI-Jubail,A1-Khobar, Jeddah, ppm ppm ppm
UV-visible 1.99 spectrometry (isolated samples)
1.93
1.21
Fluorescence spectrometry (a) Isolatedsample
2.095
2.06
1.39
(b) Seawater (unisolated)
2.7
2.34
1.4
(c) Standard (actual value)
10.3 (10)
10.37 (10)
19.88 (20)
All values mentioned are average of six samples with duplicate determination
4. Conclusions
1. Procedure for the separation of humic substances from seawater in gram quantities has been standardized using XAD-2 and XAD-7 resins. This procedure of preconcentration of humic acid in conjunction with
3. Total acidity in Jeddah sample is 5.0 meq/g whereas in AI-Jubail and A1-Khobar contain 3.36 and 3.23 meq/g, respectively. Furthermore, about 80% content of humic substance is fulvic acid and 20% is humic acid in samples from all three sites. 4. Jeddah samples indicated more favorable conditions for bacterial activity in comparison to eastern province sites sample, viz. AI-Jubail and AI-Khobar. 5.
Infrared spectroscopic studies revealed presence of various functional groups like carboxyl, phenolic, OH groups, derivatives of benzene and sulphur group, etc.
6. A method has been developed for the estimation of humic substances in seawater by UV-visible spectrometry technique after preconcentration. By this method, concentration of humic substances estimated in the samples of A1-Jubail, AI-Khobar and Jeddah are 1.99, 1.93 and 1.2 ppm, respectively. The percentage relative standard deviation of the method is 3.8%. 7. A fluorescence spectrometric method was also developed for the estimation of humic substances in seawater after their preconcentration from seawater by XAD resin. Concentration of humic substances in the three sites of AI-Jubail, A1-Khobar and Jeddah are 2.095, 2.06 and 1.39 ppm, respectively. The percent relative standard deviation of the method is 1.75%.
A.G.1. Dalvi et al. / Desalination 132 (2000) 217-232 .
Fluorescence technique could also be used for the estimation o f humic substances without separation provided a standard is run in parallel and correction factor is applied as seawater impurities could interfere in the estimation.
5. Recommendations
In future, following studies could be carried out: 1. To map out the humic substances along the coasts o f Red Sea and Arabian Gulf. Hence, the method developed is applied to various locations specially where RO plants are situated. Seasonal variations also need to be established. 2. To study the effectiveness o f pretreatment of RO system in removal o f humic substances, each RO plant raw seawater feed and after pretreatment samples have to be evaluated in detail for their humic substances behavior. 3. To study the into smaller fication by arriving at composition
degradation o f humic substances components as well as identiGC/MS for the purpose of the probable structure or o f h u m i c substances.
4. Identification o f carcinogenic compound on chlorination o f humic substances and their carry over to MSF distillate and S W R O permeate are to be studied in detail in the future. References
[1] [2] [3]
J.P. Croue, F. Lefebore, B. Martin and B. Legube, Water Science Techn., 27 (1993) 143. M. Alborzfar, G. Jonson and C. Gron, Water Research, 32 (1998) 2983. K. Ruohom/~ki, P. V~is~inen, S. Mets~nuuronen, M. Kulovaara and M. Nystr6m, Desalination, 118 (1998) 273.
[4] [5] [6]
[7] [8] [9]
[10] [11]
[12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
231
N. Belzile, H.A. Joly and H.C. Li, J. of Chem., 75 (1997) 14. I.V. Permonova and F.H.K. Frimmel, Water Research, 32 (1998) 872. R.M. Gareels, F.T. Mackenzie and C. Hunt, Chemical Cycles and Global Environment, William Kantmann Inc., Los Altos, CA, 1973. C.J. Steelink, J. Chemical Education, 54 (1977) 599. A.P. Black and R.F. Chfistman, J. AWWA, 55 (1963) 897. S.A. Waksman, Humus, Origin, Chemical Composition and importance in Nature. 2nd ed., William & Wilkins, Baltimore, MD, 1938, 526 pp. M.M. Kononova, Soil Organic Matter. Pergamon, Elmsford, NY, (1966) 544 pp. J.M. Sympons, T.A. Bellar, J.K. Carswell, J. DeMenco, K.L. Jrop, G.G. Robeck, D.R. Seegar, C.J. Solcum, B.L. Smith and A.A. Steven, J. AWWA, 67 (1975) 634. National Interim Primary Drinking Water Regulation Fed. Regist., 44 (1979) 6824. A.A. Steven, C.J. Slocum, D.R. Seegar and G.G. Robeck, J. AWWA, 68 (1976) 615. D.B. Bebcock and P.C. Binger, J. AWWA, 71 (1979) 149. S. Muttanara, E.I. Sales and Z. Gazali, Water supply, 13 (1995) 105. G. Mierle and R. Ingram, Water, Air, Soil Pollution, 56 (1991) 349. P.D. Boehm and J.G. Quinn, Geochim., Cosmochim. Acta, 37 (1973) 2459. F.D. Martin and R.H. Fierce, J. Environ. Letter, 1 (1971) 49. S.H. Eberle and K.H. Sheweer, Worn Wasser, 41 (1974) 287. E.T. Gjessing, Environ. Sci. Tech, 4 (1970) 437. J.P. Riley and D. Taylor, Anal Chim. AOa, 46 (1969) 307. R.F.C. Montoura and J.P. Riley, Anal. Chim. Acta, 76 (1975) 97. J.A. Leenher and E.W.D. Huffman, J. Res. US Geol. Surv., 4 (1976) 737. R.L. Gustafson, R.L. Aibright, J. Heister, J.A. Lino and O.T. Reid, lnd Eng. Chem. Prod. Res. Dev., 7 (1968) 107.
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[25] R. Simpson, The Separation of Organic Chemicals from Water. Rohm and Haas, Philadelphia,PA, 1972. [26] E.M. Thurman, R.L. Malcolm and G.R. Aiken, Anal. Chem., 50 (1978) 775. [27] G.R. Aiken, E.M. Thurman and R.L. Malcolm, Anal. Chem., 51 (1979) 1799. [28] R.L. Malcolm and P. MacCarthy, Envir. Sci. Techn., 16 (1982) 20. [29] J.A. Rice and P. MacCarthy, Organic Geochemistry, 17 (1991) 635 [30] G.R. Aiken, D.M. MacKnight, R.L. Wershaw and P. MacCarthy, Humic Substances in Soil, Sediment and Water. Wohn Willing & Sons, 1985. [31] G.H. Wagner and F.J. Steven, Proc., Soil Sci. Soc. Am., 29 (1965) 43. [32] B.K.G. Theng, J.R.H. Wake and A.M. Pasner, Soil Sci., 102 (1966) 70. [33] A.S.R.Juo and S.A. Barber, Soil Sci., 108 (1969) 89. [34] R.L. Warshaw, M.A. Mikita and C. Steelink,Environ. Sci. Techn., 15 (1981) 1461. [35] M. Schnitzer and S.I.M. Skinner, Soil Sci., 105 (1968) 392. [36] J.H. van der Mass, Basic Infrared Spectroscopy. 2nd ed., Heyden & Sons Ltd, London, 1972. [37] E.D. Olsen, Modem Optical Methods of Analysis. McGraw Hill, New York, 1975.
[38] F.J. Stevenson, Humus Chemistry, Genesis, Composition, Reaction. Wiley Interscience, New York, 1982, p. 384. [39] P. MacCarthy and S. O'Cinneide, J. Socil. Sci., 25 (1974) 420. [40] Y. Chen, N. Senesi and M. Schnitzer, Soil Sci. Soc. Am. J., 41 (1977) 352. [41] T.M. Miano, G. Sposito and J.P. Martin, Soil Sci. Soc. Am. J., 52 (1988) 1016. [42] B.K. Seal, K.R. Roy and S.K. Mukherjee, J. Ind. Chem. Soc., 41 (1964) 212. [43] M. Schnitzer, In: Characterization of Humic Constituents by Spectroscopy in Soil Biochemistry, A.M. MacLam, J. Skujins, (eds.), Marcel Dekker, New York, 1971, pp. 60-92. [44] F.J. Stevenson, Humus Chemistry, Genesis, Composition, Reaction. Weiley Inter Science, New York, 1982, p. 443. [45] M. Schnitzer and S.U. Khan, Humic Substances in Environment. Marcel Dekker, New York, 1972. [46] E.M. Filippova, V.V. Fadeev and V.V. Chubarov, Origin and structure of fluorescence and from aquatic humic substances, Proc., 5th Internal Conf. on Laser Application in Life Science, Moscow, Russia, (1995) 203.
Desalination 132 (2000) 217-232 www.elsevier.com/locate/desal
Studies on organic foulants in the seawater feed of reverse osmosis plants of SWCC Abdul Ghani I. Dalvi*, Radwan A1-Rasheed, Mohammad A. Javeed Research and Development Center, Saline Water Conversion Corporation, P. O. Box 8328, Al-Jubai131951, Saudi Arabia Tel. +966 (3) 361-0333; Fax +966 (3) 362-1615; E-mail: [email protected]
Received 3 July 2000; accepted 17 July 2000
Abstract
In the seawater desalination inorganic constituents (40,000-50,000 ppm) pose serious problems to multistage flash (MSF) process and more acutely to seawater reverse osmosis (SWRO) process which is also greatly influenced by organic content of (2-4 ppm). It is well-known that fouling on the RO membrane causes serious problems including (i) gradual decline of membrane flux thereby decreasing permeate production, (ii) an increase in AP thereby increasing the requirement of high pressure pump rating and (iii) degradation of membrane itself. The dissolved organic matter is not a single substance but a mixture of ill-defined aliphatic and aromatic compounds. Humic substance constitute major portion of total organics in seawater, thus to identify the nature, and compositional characteristic of such substance along with its estimation is of utmost importance to overcome the problems associated with fouling of RO membranes. Humic substances from Al-Juball, A1-Khobar (Gulf Coast) and Jeddah (Red Sea Coast) were isolated and analyzed by different techniques, viz elemental analysis, UV-visible speclrometry, IR Spectrometry and fluorescence spectrometry. Method of isolation and purification of humic substance has been standardized to recover gram quantifies. Two methods of quantitative estimation have also been developed. The first method involves estimation of humic substance from seawater by UV-visible spectrometer atler isolation. In the second method, humic substance was determined using fluorescence technique by spectro-photofluorometer either after isolation and pre-concenlration or directly in the seawater without isolation. The precision of methods in terms of percentage relative standard deviation for UV-visible method is 3.8% while for fluorescence method is 1.75%. Elemental composition, nature, IR speclra and concentration of humic substances of A1-Jubail and A1-Khobar are similar. However, humic substances from Jeddah differed in many respects including the composition. The remarkable deviation in elemental composition indicates that perhaps nature and origin of humic substances fi'om Gulf and Red Sea coasts are different. High nitrogen, sulphur and hydrogen to carbon ratio indicate high bacterial activity in the Red Sea region. Keywords: Organic foulants; Humic substances; Fulvic acid; Humic acid; Reverse osmosis; Quantitative estimation; Fluorescence; UV-visible spectrometry; Fluorometry; Infra-red spectroscopy; XAD resins
*Corresponding author. Presented at the Conference on Membranes in Drinking and Industrial Water Production, Paris, France, 3-6 October 2000 International Water Association, European Desalination Society,American Water Works Association, Japan Water Works Association 0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: SO0 l 1 - 9 1 6 4 ( 0 0 ) 0 0 1 5 3 - 3
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I. Introduction
Desalination in the broad sense is a process through which water of low salinity is produced to an extent that it becomes potable. Among the known desalination processes, multistage flash (MSF) distillation and reverse osmosis (RO) membrane filtration are most popular and widely used techniques. Major chemical constituents (40,000-50,000 ppm) of seawater are of inorganic origin plus a minute quantity (2-4 ppm) of organic origin. Though organics are negligible in concentration as compared to inorganic constituents, they pose more acute problems in reverse osmosis desalination process. It is well known that fouling in RO membranes causes serious problems including (i) a gradual decline of membrane flux thereby decrease in permeate production, (ii) an increase in AP thereby increasing requirement of high pressure pump rating and (iii) degradation of membrane itself. All these factors reflect on the cost of water production. Hence, now-a-days attempts are being made to deplete the concentration of organic and some of the inorganic constituents from the feed to RO to overcome these problems by various pretreatment methods. Other than conventional methods such as coagulation [1], filtration and separately passing through activated carbon or clays for decreasing the organic load from the feed of RO, some of the more recent techniques [2,3] are also emerging such as nanofiltation and ultrafiltration which are felt to be quite promising pretreatment method to overcome the said problems and to reduce the overall cost of production. Moreover, these organic contaminants have been found to be the precursors for the formation of organic derivatives, some of which are carcinogenic. The dissolved organic matter is not a single substance but a mixture of many ill-defined aliphatic and aromatic compounds. However, among the total dissolved organic substance (DOS) in seawater
---90% is represented by the humic materials or substances. Much attention has, therefore, been paid to isolate them and study their chemistry. Humic substances account for significant and variable proportions of organic matter in soils, sediments, and they are also found as soluble organic matter in fresh and seawaters [6--8]. Humic substances comprise a general class of ubiquitous substances in terrestrial and aquatic environments. Studies on humic substance in the earlier days [9,10] were confined only to soil scientists. The term 'humus' (Latin equivalent to soil) was initially introduced to describe the dark colored organic matter in soil, and later, the term was modified as 'humus acid' or 'humic acid'. These substances are known to play a role in the food chain, affect the aesthetic quality of water by imparting color and act as a complexing agent for inorganic ions. Thus there has been a growing interest to determine the contribution by humic substances to biological, physical and geochemical processes in natural water systems. Chloroform was found to be ubiquitous in chlorinated drinking water, hence the US Environmental Protection Agency (USEPA) issued a regulation, limiting the concentration of chloroform and its sister trihalomethanes (THMs) to 100 ppb [11,12]. Various authors have supported the hypothesis that it is the na~ral organic matter (humic substance) which is the most common reaction precursor to trihalomethane formation [13-15]. Aquatic humic substances are organic acids that are derived from soil humus and aquatic plants. Generally more than half of the dissolved organic carbon in water is due to humic substances. As there is growing concern over the role of humic substance in the various aspects of water chemistry, more and more efforts are now concentrated to understand the role, structure and chemistry of humic acid in aqueous systems mainly because of their complexation with metal ions, mobilization of toxic trace metal [16], formation of chlorinated
A.G.L Dalvi et al. /Desalination 132 (2000) 217-232
methanes in water treatment, role in bacterial growth after chlorination of feed and interaction with organics like solublization of pesticides and hydrocarbon [17]. To asses the potential for bacterial growth and THMs formation potential due to humic substance in sea water, it is essential to isolate and characterize the humic substances. Further-more, there is a need to develop methods to estimate the humic substances in the seawater and other aqueous systems. The isolation and estimation of humic substances are two difficult tasks to be achieved but they are very important, as various aspects are related to humic sub-stances. Low concentration of humic substances in seawater or aqueous system and non-availability of efficient sorbent to obtain gram quantities, makes the isolation and characterization, a tedious, lengthy and difficult task. The methods once developed will enable to map out the areas of high humic substances content where preventive measures could be taken to avoid biofouling on RO membrane. Humic molecules as such are refractory in nature, not easily assimilable by microorganisms. However, during the chlorination of feed water, humic substances are broken down into smaller fragments thereby loosing their inert nature and become primary source of bacterial nutrient promoting in membrane biofouling. Smaller fragments formed during the chlorination process are also major source for the formation of carcinogenic organic compound such as trihalomethanes, haloacetic acids and other disinfection by-products. Both these problems are of utmost importance in RO process. Thus much attention is being paid recently to better understanding the situation. In this direction research work leading to abatement of organic fouling is actively being pursued. Present study was carried out with a view to standardize the procedure for extraction of dissolved organic substances from seawater, their isolation and characterization, and to study the concentration and distribution of humic
219
substances, in the aquatic systems near the RO desalination plants located in the Eastern Province (Arabian Gulf) and Western Province (Red Sea). To achieve the above said objectives, the project was carried out with the following: • To standardize extraction and purification procedure of humic substances in seawater to get at least gram quantities. • To isolate humic substances from sea near the locations of SWCC plant in (a) Eastern Province (Arabian Gulf) and (b) Western Province (Red Sea). • Characterization and estimation of pure isolated humic substances by physicochemical techniques, viz., UV, IR, TOC and CHNS (carbon, hydrogen, nitrogen and sulphur) analyzer. • Development of a method for the determination of concentration of humic substances in aqueous system by direct or indirect methods using techniques like absorption spectrometry or flourometry.
2. Experimental There are several methods and techniques such as precipitation [18], solvent extraction [19], ultrafiltration [20], for isolation and extraction of humic substances. However, all these methods are tedious, time consuming and involve complicated procedures to isolate and concentrate humic substances from large volumes of water. After the development of macroporous resins for chromatography [21-23], these resins were fully exploited for removing trace organic solutes from water [24-27]. Humic acid concentration is less than 2 ppm whereas the total dissolved organic substance (DOS) is itself only about 2-3 ppm. To get gram quantities of humic substances, large amount of seawater under specific conditions is passed through exchange column of specific resins Amerlite XAD-2
220
A.G.I. Dalvi et al./ Desalination 132 (2000) 217-232
and/or XAD-7 having high efficiency absorption or adsorption for humic substance and then eluted under different conditions to obtain concentrated matter.
ii........................
ii
2.1. Procedure o f isolation
Glass column of 55 cm length and 5 cm diameter fitted with high porosity disc and socket at bottom and with B24/40 female socket at the top end was used. Schematic diagram of experimental set up used for isolation of humic substance from seawater is given in Fig. 1. Unchlorinated seawater from intake was pumped into a cleaned 200 L drum through bolting silk cloth (75 la mesh size) to remove suspended particles. After filtration, the pH of seawater was adjusted to 1.9-2.00 using sulphuric acid and pumped through column with help of master flex pump at the rate of 110-120 ml/min. Thus about 160-170 L of seawater was passed through the column per day. After passage of 2500-3000 L of seawater in about 20 d through the column, it was washed with distilled water, the pH of which was adjusted to 1.9-2.0. Washing of column with distilled water o f p H 2.0 was carried out till the effluent was free of chloride. This indicated that all salts have been removed from column. After thorough washing of column with distilled water of pH = 2.0, humic acid which was adsorbed on the column was eluted with a mixture of 5M NHnOH and methanol in equal proportion. Elution was kept very slow at the rate of 5-10 ml/h. In about 2.0-3.0 L of eluting solution all adsorbed humic substances were eluted which was further concentrated using a Rotavapour assembly consisting of buchi water bath and water condenser coupled with refrigerated constant temperature circulator. Evaporation was carried out under vacuum at a temperature of 40-45°C. When the solution containing concentrated humic substances reduced from 2-3 L down to about 30-40 ml, it was transferred to a
Fig. 1. Schematic diagram for the extraction of humic substances from the seawater.
dish which was kept in desicator in vacuum. Here, the solution was further concentrated by slow evaporation under vacuum and finally obtaining a solid, dark brown colored residue after about 8-10 days. This solid residue was ground into fine powder and stored in a glass bottle in desicator for further studies.
2.2. Instruments
To obtain UV-visible spectrum of humic substance, Shimadzu UV-2100S dual beam spectrometer having wave-length range of 190-800 nm was used. To record the infra red spectra of humic substances, KBr pallets (disc) were used. 5 mg of the humic substance was thoroughly mixed with 0.3 gm of spectroscopic grade KBr and pressed using pelletizer resulting in transparent disc. Disc was mounted on window holder and IR spectra was recorded using Hitachi 270-50 spectrometer in the range of 2504000 cm-l. To obtain the excitation and fluorescence spectrum of humic acid Shimadzu RF1501 was used. This spectrometer is capable of varying the wavelength of both excitation and emission (fluorescence) from 220 to 900 nm. Carlo Erba EA 1108 was used for elemental analysis of carbon (C), hydrogen (H), nitrogen
A.G.1. Dalvi et al. / Desalination 132 (2000) 217-232
(N) and sulphur (S). To estimate total organic carbon (TOC) Shimadzu TOC 500 model was used.
2. 3. Quantification of humic substances For quantification of humic substance a small glass column about 30 cm in length and 1.2 cm diameter filled with XAD-2 resins was used. Two liters of unchlorinated seawater from sites of estimation were taken. The pH value of seawater was adjusted to 2 and passed through the column which was already conditioned at pH = 2. Seawater was passed at the rate of 6-8 drops/ min. After loading 2 L of seawater, column was washed with distilled water whose pH is adjusted to 2 in order to remove all salts. Then humic acid adsorbed on the column was eluted with eluting mixture of equal proportion of 5 M NHnOH and methanol of equal proportion. Elution rate was adjusted to 3--4 drops/min. About 10 times column volume (=300 ml) eluting solution was used for complete removal of humic substances absorbed on the column. This effluent was concentrated at 40°C to 125-130 ml to remove ammonia. Later the eluted concentrate solution was transferred to a 250 ml volumetric flask and made up to mark. Portion of these solution were used for quantification of humic substance by UV-visible and fluorescence techniques.
221
3. Results and discussion
3.1. Estimation and characterization of isolated humic substances It is practically difficult to characterize and estimate any material by means of single technique. In order to get more reliable and dependable result, it is always recommended to use as many techniques as possible to characterize or estimate any material or substance. Hence, various techniques were adopted to characterize and estimate humic substances isolated from seawater at three different locations of Red Sea and Arabian Gulf coasts. The techniques used to characterize and estimate were: (a) Elemental analysis (b) Total organic carbon (TOC) (c) Ultraviolet and visible absorption spectroscopy (d) Infrared spectroscopy (e) Fluorescence spectroscopy Table 1 indicates that a huge amount of seawater was used to obtain humic substance in gram quantities. Last column of the Table 1 gives approximate concentrations of humic substances in seawater based on amount recovered from the total volume of seawater used,
Table 1 Volume of seawater used for isolation ofhumic substance and their concentration Station
Volume of seawater, Amount isolated by resin Concentration ofhumic substances in seawater, ppm L (dry form), g Based on 100% retention on Based on 50% retention
resin AI-Jubail AI-Khobar Jeddah
2400 2600 3100
1.8 2.1 1.77
on resin 0.75 0.80 0.59
1.5 1.6 1.18
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A.G.I. Dalvi et al. / Desalination 132 (2000) 217-232
Table 2 Percentageof fulvic and humicacid ofhumic substances Components Fulvic acid, % Humic acid, % Total acid, meq/g
Stations AI-Jubail AI-Khobar Jeddah 80.4 79.5 84 19.6 20.5 16 3.36 3.229 5.01
The humic substances usually consist of humic and fulvic acids in addition to other components. Humic acid components are soluble at higher pH but insoluble at low pH while fulvic acid component is soluble in both bases and acids [28]. Humic substances isolated from three locations viz. A1-Jubail, A1-Khobar and Jeddah were subjected to analysis, it was found that 80.4, 79.5 and 84% of substances contained fulvic acid component respectively in above said sites. It can be seen from Table 2 that humic acid component in isolated humic substance conrained only 19.5% in AI-Jubail, 20.5% in A1Khobar and 16% in Jeddah. High percentage of fulvic acid in Jeddah sample (=84%) means more aliphatic components in humic substance and this was also confirmed by high hydrogen to carbon ratio compared to other samples. In general, aliphatic components are higher than aromatic component in all the samples from three sites. Furthermore, total acidity was high in Jeddah sample which was 5.01 meq/g compared to 3.36 and 3.23 meq/g for AI-Jubail and AIKhobar samples respectively.
3.2. Elemental analysis
Elemental analysis is probably the most common technique used in characterization of humic substances [29]. In fact, chemical analysis is the cornerstone of all chemical inquiry. Elemental analysis could also be useful in establishing the purity of humic substances
preparations methods. Elemental composition, along with elemental ratios such as C/H, O/C, N/C, are fundamental approaches used in describing and understanding the geochemistry of the isolated substances and often are single most useful indicator of unique nature of a given humic substance. Having stated the importance of elemental data for the interpretation, it is necessary to obtain reliable and accurate results. However, there are limitations in obtaining accurate data for various reasons. Foremost limitation is the availability of very small samples with high purity. The result of elemental analysis for C, H, N, S for all three samples collected from AI-Jubail, A1-Khobar and Jeddah are tabulated in Table 3. However, oxygen values are computed after taking into account the ash content of the sample. A standard of a known composition was also run along with the above samples to ascertain the analysis. It can be seen that C % in AI-Khobar and A1-Jubail were around 42% while in Jeddah it was only 27%. However, hydrogen in Jeddah was 5.46% compared to 6.23% in A1-Jubail and A1-Khobar samples. In other words, percentage ratio of H/C in Jeddah was very high, thereby indicating that aliphatic components were more in Jeddah sample compared to samples from AI-Khobar and AI-Jubail. Nevertheless, H/C ratio in A1-Khobar and A1Jubail plant samples were also high (-1.77) which indicated that aliphatic components in humic substances from these sites were also high but not as high as was in Jeddah sample (which was highest among the samples from all three locations). In addition, ratio of O: C was also above 0.7 in all three samples indicated high proportion of fulvic acid in humic substances. This fact was supported by solubility test in acid as shown in Table 2 where it was indicated that all the three samples from AI-Jubail, AI-Khobar and Jeddah contained 80.4%, 79.5% and 84% of fulvic acid, respectively compared to 19.6, 20.5
A.G.I. Dalvi et al. /Desalination 132 (2000) 217-232
223
Table 3 Elemental analysis Sample Element,% identity C H Khobar HA Jubail HA Jeddah HA Standard Actual value
S
O
Ash
Atom, % C H
N
42.25 6.23 7.19
0
40.43
3.9
3.52
42.19 6.23 6.38
0
37.5
7.7
N
O
Atomratio H/C C/H
0.514 0
2.53
1.77
3.516 6.23 0.456 0
2.34
1.772 0.564 0.666 0.130
6.23
S
O/C
N/C
0.565 0.719 0.146
26.95 5.46 10.28 10.23 38.98 8.1
2.246 5.46 0.734 0.319 2.44 2.131 0.41
30.14 3.8 8.23 9.9 27.85 - (31.35) (4.0) (8.13) (9.3) (27.85) - -
2.512 3.8 2.612 4.0
1.08
0.327
0.588 0.309 1.74 1.513 0.661 0.693 0.237 0.581 0.290 1.74 1.531 0.653 0.666 0.222
Values in brackets are actual value of standard
and 16% of humic acid component respectively in these plants. Hence, it can be stated that aromatic components in all three humic substances were less compared to aliphatic components. However Jeddah sample contains higher aliphatic component compared to other samples. The decrease in aromacity of Jeddah humic substance indicated less refractory or inert nature and thus more viable or assimilable nature o f carbon as a nutrients for microorganisms. This could be one of the important factors for high SDI and biofouling in Jeddah. The difference in nature of humic substance isolated from Jeddah compared to AI-Jubail and A1-Khobar can also be noted from N/C ratios. This indicates a lot of bacterial activity in Jeddah in comparison with other two sites. Secondly relatively high N/C in Jeddah samples indicated that large proportion of humic substances produced were from aquagenic refractory material. Sulphur content in Jeddah sample was around 10% whereas it was absent in A1-Jubail and A1-Khobar samples. High nitrogen and sulphur content may provide an ideal breeding ground for microorganism. However the source and origin of sulphur need further investigation.
Based on elemental analysis the most likely empirical formula for humic substances in A1Jubail and AI-Khobar area is CsH14NO whereas for Jeddah sample it is C4I-I8NOS05. Visual observation of samples also revealed that A1Jubail and A1-Khobar were similar of having dark brown color fine powder which formed lumps while Jeddah sample was very fine crystalline powder with light yellowish brown color. It did not form lump.
3.3. Molecular weight Humic substances molecular weight determination using gel permeation chromatography for AI-Jubail, AI-Khobar and Jeddah lied in the range of 3000-3700. Average molecular weight of three site samples are 3440, 3620, and 3718 for AI-Jubail, A1-Khobar and Jeddah, respectively.
3.4. Temperature dependence o f weight loss Weight loss determination of humic substances has been carried out on three samples
224
A.G.I. Dalvi et al. /Desalination 132 (2000) 217-232
Table 4 Weight loss analysis and ash content Station
Table 5 Total organic carbon of isolated humic substances
% weight loss @ temp.
% ash
55°C Al-Jubail 6.6 AI-Khobar 6.1 Jeddah 2.6
1O0°C
750 °C
content
9.1 8.2 4.0
92.2 96.1 91.9
7.7 3.9 8.1
from different sites of AI-Jubail, AI-Khobar and Jeddah and results are shown in Table 4. All three samples indicated gradual increase of weight loss as temperature increased up to 100°C. This is due to loss of moisture. It has been reported [30] that loss of weight at 5560°C corresponds to water determined by Karl Fisher water content in humic substances. Here in this case also loss of weight at 55°C corresponds to water content in these three samples. Loss in weight beyond the temperature of 80°C is considered as loss due to decomposition o f humic substances. Samples dried at different temperatures up to 80°C regained the weight when again exposed to air whereas the samples heated at temperature 80°C or above, did not regain or regained negligible weight [30] was the basis for the conclusion that the humic substances heated beyond the temperature of 80°C start decomposing. Present samples was heated up to 750°C to determine the ash content and results of ash content are also shown in Table 4. Ash content were 7.5-8.0% in AI-Jubail and Jeddah sample whereas A1-Khobar sample contained only 4%.
3.5. Total organic carbon (TOC)
Total organic carbon of the isolated humic substances from all three sites were determined by dissolving known amount of dry humic substance in water (free of carbon). The results are shown in Table 5. TOC of humic substances
S. No.
Humic substances,ppm
TOC, ppm A1-Jubail AIJeddah Khobar
1 2 3 4
1 5 10 20
0.32 1.8 3.8 7.4
0.36 2.44 3.7 7.9
8,
0.12 1.1 2.2 4.5
0
0
y - 0.386x- 0.08
s
Al-,lubai] &
[ 3
.~36
J~ld~
2
5
~0 IS Hlmb Ikll~Immmllml
20
Fig. 2. Variation of total organic carbon (TOC) as a function ofhumic substances.
obtained from AI-Jubail and AI-Khobar were higher compared to Jeddah. In fact, AI-Jubail and AI-Khobar humic substances as function of TOC can be represented by Eq. (1) while Eq. (2) represents the case for Jeddah, Al-Khobar and AI-Jubail: Y = 0.386 X - 0.0894
(1)
Jeddah: Y = 0.228 X - 0.0636
(2)
where Y --- TOC and X = humic substance both in ppm.
225
A.G.I. Dalvi et al. / Desalination 132 (2000) 217-232
Fig. 2 represents the graphic relation of humic substance with TOC. Both the graph and the equations show that values of TOC obtained were in reasonable agreement with the values of carbon obtained by elemental analysis for these samples as shown in Table 3. For Al-Jubail and AI-Khobar (eastern province) samples carbon values by TOC were about 38% whereas for Jeddah it was 22%. However values of carbon obtained by elemental analysis were 42% and 27% for eastern province location and Jeddah, respectively. As it can be seen in Fig. 2 that two different equations and graphs followed by humic substances obtained from eastern and western provinces indicate that the composition and perhaps nature and origin are different for humic substances isolated from these sites.
too
so % T
7o
~o ao 9o 20 10 i
I
js~o
I
3ooo
' 2.soo
2OOO1 laoo
' 16oo
Wave Nmnb~
' 14oo
I
~
I
;
|coo
a¢o
:
6oo
;
4oo
~.
2.so
~'~
Fig. 3. Infra red spectrum of humic substances isolated from A1-Jubail site.
9c 80 TO ~o
3.6. 1R spectroscopic studies
50
Spectroscopic method like other investigation techniques are severely limited when applied to humic substances. This is because humic substances comprised of very complicated illdefined mixtures of various long chained organic compounds and macromolecules. Thus the spectra of humic substances represent the summation of the responses of many different species and functional groups present in these substances. In some cases only small fraction of total number of molecules or groups contribute to total spectra. Occasionally, spectral contribution due to some groups may be masked partially or totally, thereby causing hindrance, hence no clear understanding which in certain cases could lead to totally different inferences or interpretations. Infrared spectrum in the region of 4000 to 250 cm -1 obtained by KBr pallet are shown in Figs. 3-5. It can be seen that all IR spectra obtained for samples from three sites viz. A1Jubail (Fig. 3), A1-Khobar (Fig. 4) and Jeddah (Fig. 5) had in general similar features and bands.
40 Jo 2 10 , 4000
3~10
)riO0
~
, ~
,
,
a
,
ll0O
1600
1~
12100
, I~
, ~
, a0O
,
,
~
W n v e N g m b ~ m -n
Fig. 4. Infra red spectrum of humic substances isolated from AI-Khobar site.
10o
io
% T
60 SO a
2o
~o0
i 3ga~
loa0
~oo
ao00
i~o
i~
i~a
i~
t0o0
moo
~ .
~
Fig. 5. Infra red spectrum of humic substances isolated from Jeddah site.
226
A. G.I. Dalvi et al. / Desalination 132 (2000) 217-232
The samples of AI-Khobar and AI-Jubail showed the closer resemblance whereas Jeddah sample IR showed distinct difference due to the presence of some additional peaks. Prominent bands which were observed in all samples are 31503500 cm -~, 2800-3050 cm -~, 1550-1725 cm -~, 1375-1450 cm-1, 1220 cm-1, and 1050-1150 cm-1 region. However, in the case of Jeddah additional sharp intense band at 600 cm-1 was observed, in addition to the absence of a band around 1220 which could be clearly observed in samples from other two sites. Bands around 1230 cm-1 due to C-O and OH deformation were more clearly seen in AI-Khobar and A1-Jubail samples which was absent or perhaps merged into the band around 1100 cm-~ in Jeddah sample. The band around 1100 cm-l is very intense in the case of Jeddah sample. In addition, very sharp absorption was noticed at 600 cm-~ with shoulder peak at 460cm -1 in Jeddah sample. However peaks of 600 and 480 cm -1 also observed in AI-Jubail and A1-Khobar samples but spectra were diffused and peaks were broad and less intense. The broad peaks in the region 31503500 cm -1 which was observed in all the three samples corresponds to absorption where OH stretching occurs, [31] and its broadness is generally attributed to hydrogen bonding [32,33]. When Hydrogen is bonded to the more electronegative atoms such as oxygen or nitrogen the bond is polarized, leaving the hydrogen atom with partial positive charge. This partially positive charge hydrogen atom can then interact electrostatically with other atoms which are oppositely charged. This interaction can be intramolecular (occur within the same molecule) or intermolecular (between functional group of the different molecules). Thus hydrogen bonding results in increased separation between the hydrogen atom and the atom to which it is covalently attached or bonded. This brings the change in frequency of absorption and broadening result due to the statistical distri-
bution in the extent of hydrogen bonding in the molecule. All the three samples of humic substances indicated the band around 3400 cm-l due to OH stretching broadened because of hydrogen bonding. A shoulder peak around 2920-2950 cm-1 was also evident in the spectra of all the three humic substances. These bands were attributed to the asymmetric and symmetric stretching vibration of aliphatic C-H bonds in methyl and/or methylene units in the compound [32]. The assignment of this band was confirmed by some worker in this field by methylation of humic substance and corresponding observation of increased absorbance in the spectra [31-34]. Absorbance band observed around 17001710 cm-1 is generally attributed to the C=O stretching vibration mainly due to carbonyl group [31,32]. This band was very sharp and distinct in the spectra of the Jeddah sample compared to AI-Jubail and AI-Khobar samples. This probably points out to the fact that fulvic acid component is high in concentration in case of Jeddah sample compared to AI-Jubail and AIKhobar samples. The band at 1650 cm-1 is assigned to aromatic C=C stretching vibration, i.e., carboncarbon double bonds conjugated C=O or COO [35]. Generally C=C bond in benzene is infrared inactive, however benzene derivatives which decrease the symmetry of the molecule, bond becomes infrared active. This band was more pronounced in AI-Jubail and A1-Khobar samples compared to Jeddah again support the aromatic component in Jeddah is relatively less compared to aliphatic component. Band at 1450 cm-1 (as shoulder) has been attributed to the bending vibration of aliphatic C-H group [33] and band at 1400 cm-x is due to O-H bending vibrations of alcohol or carboxylic acids as occurs with simple model compounds. These two bands were superimposed on each other and were not resolved completely. These bands were present in all three samples. The band at 1200-1230cm -~ has been
227
A.G.1. Dalvi et al. / Desalination 132 (2000) 217-232
assigned to C-O stretching vibration and OH bonding deformation mainly due to carbonyl groups. This band disappears on producing salts [33]. This band 1050--1100 cm-1 was present in all three sites samples but more intense in Jeddah sample. This band is attributed to the C-H bond (single bond) stretching in a branched CH3 unit [36]. More intensity of this band in Jeddah compared to other samples again support the idea that the Jeddah sample is characterized to be more aliphatic in nature. Presence of intense and sharp band at 600 cm -I in Jeddah sample makes the Jeddah sample IR spectra look very different from those of AI-Jubail and A1-Khobar IR spectrum. This band was attributed to C-S bond [36]. As it can be seen from Table 3 that elemental analysis indicates the presence of 10.23% S in the Jeddah samples whereas A1-Khobar and A1-Jubail did not contain any sulphur. Thus the presence of intense band at 600 cm-l only in Jeddah sample was as expected• Further it was reported that treated and untreated sewage is discharged into the Red Sea and this could be one of the source of high sulphur content in seawater and its humic substances.
. . . . .
i
.
.
ii P
.
.
.
.
.
.
.
!
.
.
.
.
.
.
.
" • !
(a) AI-Jubail
L~----JJ
~00
. . . . . . . . .
6OO
i . . . . . . . . .
I
* . . . . . .
8O
~'°"
(b) AI-Kh0bar
400
60O
!
(rm)
II*
(c) Jeddah
3. 7. UV-visible spectroscopic 0
UV-visible spectra of samples generally appear as broad band, thus f'me structural information is lost due to this broadening. Olsen [37] demonstrated that how one can get misleading information due to impurities present in the samples. Considering the ill-defined multicomponent nature of humic substances, UVvisible spectra results from overlap of absorbances of various chromophores or functional groups [38] are very broad and diffused. The UV-visible absorptivities ofhumic substances do vary as a function of pH [39] and these are due to the ionization of carboxylic and phenolic functional groups. Though, UV-visible spectro-
0
0
0
0
0
,
!
•
400 I.
G00
(rim)
Fig. 6. UV-visible spectra of humic substances from a, Al-Jubail; b, A1-Khobar;c, Jeddah.
228
A.G.I. Dalvi et al./ Desalination 132 (2000) 217-232
scopy is a valuable tool in identification of chromophoric functional groups in discrete organic molecules but there are limitations. In spite of the limitation of UV-visible spectroscopy, it has been exploited and used for estimation of degree of humification by Chen et al. 1977 [40]. Fig. 6 shows the UV-visible absorption spectrum obtained with isolated humic substance from three sites. Irrespective of site of sample, spectrum with a broad band having absorption peaks around 300 nm was observed. This absorption band could be utilized for estimation of humic substances in seawater. After isolation of humic substance from seawater by procedure, which is described in experimental part, was subjected to estimation by UV-visible spectral studies. Absorbance of isolated solution containing the humic substance was used to compute the concentration of humic substance. It can be seen from Table 6 that concentration of humic substances estimated using UV-visible spectroscopy technique after separation from 2 L
Absorbance vs Humlc substances
0,3-
0.25,
0.2,
0.15.
0.1.
0.05.
0
5
10 15 Humlc $ubsblncu Conc ppm
20
25
Fig. 7. Calibrationcurveof humic substance (UV-visible region).
of seawater of all three sites, viz. A1-Jubail, AIKhobar and Jeddah were 1.99, 1.93 and 1.2 ppm respectively. The percentage relative standard deviation of the method is 3.8%. Calibration curve used for estimation of humic substances was obtained by dissolving known quantities of humic substances and taking spectrum as shown in Fig. 7. This figure shows a linearity up to 20 ppm and expressed by the following equation: Y = 0.0125 X + 0.0184
(3)
Hence from the above equation and the absorbance (Y) of the unknown humic substance solution, it would be possible to compute concentration (X) of humic substances in the unknown solution.
3. 8. Fluorescence spectroscopy studies
Absorption of radiation by an atom or a molecule raises the molecule from electronic ground state to higher level. Most of the absorbed energy is dissipated in the form of heat and come back to ground state by radiationless transitions depending upon the excitation. However, in some molecules major portion of absorbed energy is emitted as electromagnetic radiation at higher wavelength (low energy as part of it is dissipated in radiationless transition) than the excitation radiation wavelength which depends on allowed electronic transitions according to spectroscopic rules. Relatively few organic compounds exhibit fluorescence mainly in UVvisible region of electromagnetic radiation spectrum. Absorption is a pre-requisite of fluorescence. Some functional groups if present in certain molecules (or compound) may enhance the fluorescence intensity while other functional groups may quench it. Humic substances are known to fluoresce [41-44]. Fluorescence Spectroscopic studies suffer from all the limitations of UV-visible spectroscopy in regard
229
A.G.I. Dalvi et al. / Desalination 132 (2000) 217-232
to obtaining information about functionality in humic substances. It is known that only small fraction of the absorbed radiation is emitted as fluorescence and since humic substances are comprised of multicomponent mixtures, it is likely that fluorescence spectra may represent even small portion or part of the total molecule rather than those represented by UV-visible spectra. These limitations have been highlighted in number of the review [43-45]. In short, it can be concluded that fluorescence spectroscopy can not be used for direct determination of functional groups in humic substances. However, fluorescence technique could be utilized for estimation of humic substance like UV-visible spectroscopy. Initially, it is necessary to determine the absorption maximum wavelength for excitation of molecules to get fluorescence at the optimum intensity. This is obtained by taking excitation spectrum i.e. observing the fluorescence at a fixed wavelength and varying excitation wavelength. The excitation spectra for humic substances in water are shown in Fig. 8. It can be seen from the figure that there is broad bands absorption which peaks at 256 nm and 661 nm in addition to direct emission from excitation source at 421 nm and 842 nm which are sharp in nature. Broad bands at 256 nm and 661 nm are due to molecular absorption levels which are diffused and overlap with molecular orbital levels of longer life time compared to atomic levels which are sharp and have short life time [46]. Excitation spectra of any molecule, in fact reveals their absorption bands or level. Fig. 8 shows the fluorescence spectrum of isolated humic substance. It was evident from the fluorescence spectrum that there is a broad band emission at 462 nm. This fluorescence emission at 462 nm is a characteristic of humic substance and could be used for quantification. Pure humic substance isolated from seawater was used for obtaining fluorescence calibration curve. Calibration was found to be linear up to 20 ppm and can be seen in Fig. 9. Using this calibration
500
I;
o,o
220
i'i
A.:"i Wavelength (nm)
900
ExcitationSpectrum
00
0.0
, 220
, Wave length (nm) Fluorescence Spectrum
Excitation spectrum (1) 256 (broad) (2) 421 (sharp) (3) 661 (broad) (4) 843 (sharp)
900
Fluorescence Spectrum 256(sharp) 462 (broad 514 (sharp) 706 (broad)
Fig. 8. Excitation and fluorescence spectrum.
curve, the concentration of humic substance could be obtained directly either in seawater by obtaining fluorescence intensity at excitation wavelength of 256 nm or from eluted humic substance solution obtained after separating it from 2 L of seawater by XAD-2/XAD-7 resin as discussed in detail in experimental section. The results obtained by fluorescence spectroscopic technique using two type of samples i.e. seawater as such or isolated humic solution from seawater are shown in Table 6. It can be seen that humic substance concentration estimated after isolation were 2.09, 2.06 and 1.39 ppm whereas amount estimated directly in seawater prior to separation were 2.7, 2.34 and 1.4 ppm in Al-Jubail, A1-Khobar and Jeddah, respectively. The percentage relative standard deviation of the method is 1.75% for isolated samples. Amount estimated prior to separation is higher compared to isolated samples. This is due to interference of impurities in seawater. Perhaps these impurities or other species present in seawater also fluoresce at the same wavelength as humic substance thereby enhancing the estimation.
230
A.G.I. Dalvi et al. / Desalination 132 (2000) 217-232
other techniques could also be used for their quantitative estimation. 2. Elemental analysis of isolated humic substances from all three sites indicated that humic matter isolated from eastern and western provinces is of two different nature and compositions. Their empirical formula (CsHlnNO and C4I-IsNOS0.5) and degree of aromacity were different.
Fluorescence Intensity v$ Humlc substances 400
y = 16.701X + 10.412 300
i
200,
J
100.
0
5
10 15 Huml¢ eull~tanmm Cone. ppm
20
25
Fig. 9. Fluorescencecalibrationcurve for humic substances.
Table 6 Quantificationofhurnic substanceby differenttechniques S. Technique No. I
AI-Jubail,A1-Khobar, Jeddah, ppm ppm ppm
UV-visible 1.99 spectrometry (isolated samples)
1.93
1.21
Fluorescence spectrometry (a) Isolatedsample
2.095
2.06
1.39
(b) Seawater (unisolated)
2.7
2.34
1.4
(c) Standard (actual value)
10.3 (10)
10.37 (10)
19.88 (20)
All values mentioned are average of six samples with duplicate determination
4. Conclusions
1. Procedure for the separation of humic substances from seawater in gram quantities has been standardized using XAD-2 and XAD-7 resins. This procedure of preconcentration of humic acid in conjunction with
3. Total acidity in Jeddah sample is 5.0 meq/g whereas in AI-Jubail and A1-Khobar contain 3.36 and 3.23 meq/g, respectively. Furthermore, about 80% content of humic substance is fulvic acid and 20% is humic acid in samples from all three sites. 4. Jeddah samples indicated more favorable conditions for bacterial activity in comparison to eastern province sites sample, viz. AI-Jubail and AI-Khobar. 5.
Infrared spectroscopic studies revealed presence of various functional groups like carboxyl, phenolic, OH groups, derivatives of benzene and sulphur group, etc.
6. A method has been developed for the estimation of humic substances in seawater by UV-visible spectrometry technique after preconcentration. By this method, concentration of humic substances estimated in the samples of A1-Jubail, AI-Khobar and Jeddah are 1.99, 1.93 and 1.2 ppm, respectively. The percentage relative standard deviation of the method is 3.8%. 7. A fluorescence spectrometric method was also developed for the estimation of humic substances in seawater after their preconcentration from seawater by XAD resin. Concentration of humic substances in the three sites of AI-Jubail, A1-Khobar and Jeddah are 2.095, 2.06 and 1.39 ppm, respectively. The percent relative standard deviation of the method is 1.75%.
A.G.1. Dalvi et al. / Desalination 132 (2000) 217-232 .
Fluorescence technique could also be used for the estimation o f humic substances without separation provided a standard is run in parallel and correction factor is applied as seawater impurities could interfere in the estimation.
5. Recommendations
In future, following studies could be carried out: 1. To map out the humic substances along the coasts o f Red Sea and Arabian Gulf. Hence, the method developed is applied to various locations specially where RO plants are situated. Seasonal variations also need to be established. 2. To study the effectiveness o f pretreatment of RO system in removal o f humic substances, each RO plant raw seawater feed and after pretreatment samples have to be evaluated in detail for their humic substances behavior. 3. To study the into smaller fication by arriving at composition
degradation o f humic substances components as well as identiGC/MS for the purpose of the probable structure or o f h u m i c substances.
4. Identification o f carcinogenic compound on chlorination o f humic substances and their carry over to MSF distillate and S W R O permeate are to be studied in detail in the future. References
[1] [2] [3]
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