Treatment of oil well “produced water” by waste stabilization ponds: Removal of heavy metals

water research 43 (2009) 4258–4268

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Treatment of oil well ‘‘produced water’’ by waste stabilization ponds: Removal of heavy metals R. Shpiner, S. Vathi, D.C. Stuckey* Department of Chemical Engineering and Chemical Technology, Imperial College, Prince Consort Road, London SW7 2AZ, United Kingdom

article info

abstract

Article history:

Oil well produced water (PW) can serve as an alternative water resource for restricted

Received 4 February 2009

halotolerant agricultural purposes if the main pollutants, hydrocarbons and heavy metals,

Received in revised form

can be removed to below the irrigation standards. In this work, the potential removal of

29 May 2009

cadmium(II), chromium(III) and nickel(II) from PW by chemical precipitation in biological

Accepted 4 June 2009

treatment was evaluated. Precipitation as a sulphide salt was found to be a very effective

Published online 11 June 2009

mechanism, which together with biosorption, biological metal uptake, precipitation as hydroxides and carbonates could remove heavy metals down to below irrigation standards.

Keywords:

The existence and capability of these various mechanisms was demonstrated in the

Produced water

performance of a continuous artificial pond followed by intermittent sand filter, achieving

Oil well

removals of around 95% for nickel(II) and even higher removal rates for cadmium(II),

Heavy metals

chromium(III) from artificial PW after the installation of an anaerobic stage. The treated

Precipitation

effluent quality was higher than that required by current European standards. ª 2009 Elsevier Ltd. All rights reserved.

Oxidation pond Biological uptake

1.

Introduction

‘Produced water’ (PW) is used to describe the water from an oil well after its separation from oil in API separators. It is a large volume wastewater constituting, on average, 5 times the volume of the oil produced (Tellez et al., 2002). In desert regions like the Middle East, where fresh water is scarce and costly, it may be economically viable to re-use the PW for both domestic and agriculture purposes. However, PW contains pollutants such as organics (dispersed and dissolved oil), total dissolved solids (TDS-salts), ammonia, boron, and heavy metals, and hence must be treated before use (Funston et al., 2002). PW can be treated using a variety of technologies, however, the choice depends on its cost, the quality of water required, and local legislation. Among the technologies that have been tested are separation by hydrocyclones (Lohne, 1994), microfiltration (Campos et al., 2002) ultrafiltration

through polymer membranes (Cheryan and Rajagopalan, 1998; Tansel et al., 1995; Farnand and Krug, 1989) and more recently electrodialysis (Dallbauman and Sirivedhin, 2005). Biological treatment systems that were examined include activated sludge (Tellez et al., 1995, 2002, 2005), air lift attached growth (Campos et al., 2002) aerated lagoons (Beyer et al., 1979), wetlands (Ji et al., 2002; Rambeau et al., 2004), and the Upflow Anaerobic Sludge Blanket (UASB) (Rincon et al., 2003). In biological systems heavy metals are removed either by direct mechanisms such as biosorption or uptake as micronutrients, or by indirect mechanisms such as precipitation as sulphide salts following the reduction of sulphate to sulphide under anaerobic conditions (Mara and Pearson, 1998). Waste stabilization ponds (WSPs) are large constructed basins for low cost wastewater treatment, and use the activity of phototrophic, autotrophic, and heterotrophic microorganisms to

* Corresponding author. Tel.: þ44 207 594 5591; fax: þ44 207 594 5638. E-mail address: [email protected] (D.C. Stuckey). 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.06.004

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remove pollutants (Rittmann and McCarty, 2001). Due to its varied redox potentials and microorganism population, all the mechanisms described above for metal removal exist in WSPs. Precipitation as a sulphide salt is an effective way to achieve high degrees of separation of various heavy metals from industrial wastewater (Bhattacharyya et al., 1979), and is considered superior to hydroxide precipitation as it results in lower effluent concentrations and less interference from chelating agents (Veeken et al., 2003). Bhattacharyya et al. (1979) also found that the settling rates obtained with conventional hydroxide precipitation were considerably lower than those with sulphide, and overall separation was optimum with about 60% of the stoichiometrical dosage and a pH greater than 8.0 (Bhattacharyya et al., 1979). The high efficiency of sulphide precipitation stems from the very low solubility product of some of the metal salts (such as CdS and NiS), although removal of the precipitates formed depends strongly on the resulting particle size distribution (Peters et al., 1984). Some work has already been carried out on heavy metal precipitation in semi-natural systems such as ponds and wetlands. Kaplan et al. (1987) only found a slight decrease in total metal concentrations during the various stages of wastewater treatment with stabilization ponds. The distribution among dissolved (free and chelated) and particulate fractions changed towards solubilisation of most of the particulate fraction. A considerable decrease in the free cations within the soluble fraction occurred due to a proteinaceous chelating agent(s) released by the microbial population in the ponds. Toumi et al. (2000) found that the anaerobic pond achieved the highest heavy metal removal in waste stabilization. The facultative and High Rate Algal Pond (HRAP) stages of treatment removed part of the remaining metals, resulting in removal rates of around 90% for Zn and Cu, and 50–70% for Pb. In other work, Toumi et al. (2003) observed that HRAP and maturation ponds contributed significantly to the removal of heavy metals. The Cd and Cr concentrations decreased in the HRAP from an average of 1.9–0.9 mg L1, and 31–12.9 mg L1, respectively, and after the second maturation pond to 0.7 mg L1 and 11 mg L1, respectively. It was stated that chemical precipitation and sedimentation with suspended matter were the principal factors responsible for metal removal. Groudeva et al. (2001) treated waters contaminated with crude oil and toxic heavy metals (Cd, Cu, Pb, Mn, Fe) with constructed wetlands and found that the concentrations of the heavy metals decreased to below the relevant permissible levels. The removal of cadmium, copper and lead and, to some extent iron, was achieved by microbial dissimilatory sulphate reduction taking place in the anoxic zone of the wetland, and biosorption. In PW systems there is no evidence currently available on the potential of removing heavy metals to low levels using WSPs, although there is some evidence of heavy metals precipitating after discharge to the environment, therefore controlled precipitation before discharge seems promising. Also, there are considerable differences between wastewater and PW in which the medium has a high ionic strength, and both the initial and target concentrations are as low as 100 mg L1 and 10 mg L1, respectively, for cadmium and chromium.

In this study the potential for removing nickel, cadmium and chromium in WSPs from oilfield produced water down to European Water quality limits for reuse and discharge (100 mg L1, 10 mg L1and 10 mg L1, respectively) were examined. Experiments were carried out to assess the potential for removal by precipitation using a series of serum bottles tests and a lab-scale artificial pond. Past experiments in our lab (results not shown) had shown limited success in removing heavy metals down to the required concentrations by other metals removal mechanisms (Shpiner, 2007). The experimental conditions and the solutions used were chosen to simulate the conditions at a known well site in the Gulf.

2.

Materials and methods

2.1.

Artificial produced water medium

The medium used in this work was Marmulþ, a synthetic PW similar in composition to the water from a well in the Gulf, with additional essential nutrients (N, K, P) as NaNO3, K2SO4 and Na2HPO4. The nutrients were necessary to improve the growth conditions for both algae and bacteria, and the medium composition is shown in Table 1. The Marmulþ medium was filtered to remove any undissolved salts, and the pH of all Marmul media was between 8.6 and 8.8. However, the composition of the medium was changed in some parts of the work; Marmulþ without the heavy metals (Cd(C2H3O2)2$2H2O and NiSO4$6H2O) was used for the metal experiments in order not to interfere with metal mass balances. Finally, NaNO3 was replaced with NH4HCO3, keeping

Table 1 – Composition of media. Chemicals

CaCl2$2H2O NH4Cl Ba(OH)2 H3BO3 Cd(C2H3O2)2$2H2O CuSO4$5H2O NaF FeSO4–EDTA complexa Pb(C2H3O2)2$3H2O MgSO4 MgCl2 NiSO4$6H2O NaCl (s) NaHCO3 ZnSO4 (NH4)6Mo7O24$7H2O Na2SiO3$4H2O NaNO3 (s) Na2HPO4 (s) K2SO4 (s)

Media concentrations (mM) Marmul

Marmulþ

Marmulþ enriched

400 110 0.7 550 0.27 0.31 10 38 0.27 300 580 0.5 79103 11103 1 10103 330 – – –

400 110 0.7 550 0.27 0.31 10 38 0.27 300 580 0.5 79103 11103 1 10103 330 1.18103 0.141103 4.88103

400 110 0.7 550 0.27 0.31 10 38 0.27 300 580 0.5 79103 11103 1 10103 330 7.646103 0.704103 4.88103

a For the preparation of EDTA–FeSO4 complex 345 mg FeSO4$7H2O and 465 mg of Na2EDTA were dissolved in 1 L of deionised water.

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the amount of nitrogen constant, in order to prevent anoxic conditions in the anaerobic zone at the front of the pond which will suppress the growth of Sulphate Reducing Bacteria (SRBs).

2.2.

Microorganism source and cultivation

Bacteria were received from PRI (Plant Research International, Wageningen, The Netherlands) after a selection of species capable of growing in PW, and potentially able to degrade oil. Five different species of bacteria were used to seed the reactor; Rhodococcus rhodochrous, Rhodococcus ruber, Ralstonia sp, Acinetobacter venetianus and Paenibaccilus naphtalenovorans. All the bacteria were cultivated in 2 L bioreactors in a constant temperature room (30  C). The medium used was Marmulþ enriched with yeast extract and glucose, autoclaved before inoculation, while the inoculation of the bacteria took place under sterile conditions (over a Bunsen burner). Yeast extract and glucose were added on a daily basis through a filter (0.45 mm). The bioreactors were aerated using artificial stone diffusers, and the pH of the media was adjusted to around neutral by adding small quantities of HCl (2 M) or NaOH (2 M). In addition, local strains from an experimental wetland at the well site were also used for seeding. Three samples of the wetland soil were brought to the lab from the inlet, centre and outlet parts of the wetland and cultivated in a mixture of Marmulþ enriched with Yeast extract, glucose and crude oil as their carbon source. We used different biological seeds to determine whether ‘‘natural’’ or selected pure strains would be capable of similar removal efficiencies. Algae were also received from PRI after the selection of strains capable of growing in PW: two different strains of Chlorella (Chlorella kessleri, Chlorella vulgaris) and three of Scenedesmus vaculatus. Algae were cultivated in 2 L Erlenmeyer flasks in a 30  C constant temperature room on top of a transparent bench illuminated underneath by white fluorescent lamps (10,000 lux at the bottom of the flask). The flasks were aerated and mixed by air introduced through stone diffusers. In cases of slow growth, carbon dioxide (CO2) from a pure CO2 supply was introduced through a silicon rubber tube. Deionised water (DIW) was added occasionally to compensate for evaporation, and the flasks were shaken manually at least once a day to resuspend any settled algae.

2.3.

Heavy metal precipitation as a sulphide salt

The medium used in this experiment was Marmulþ excluding the metals to be tested: cadmium, chromium and nickel. The experiments were carried out using 30 ml serum bottles (crimp cap) that were initially gassed with pure N2 at a flow rate of 0.1 L min1 for 15 min in order to purge oxygen from the bottle. To 1 L of medium, deoxygenated by bubbling N2 through it using a stone diffuser, cadmium, chromium and nickel were added from a stock solution (manufacturer BDH) to achieve concentrations of 100 mg L1 of chromium(III), 100 mg L1 of cadmium(II) and 500 mg L1 of Nickel(II): these concentrations were slightly higher than those found in the PW in order to represent an increase in concentration due to evaporation, and to ensure their treatability in a worse case scenario. The experiments were carried out at concentrations

of 8–130 mg L1 S, being the amount of S found as sulphate in the PW of the oil well in the Gulf (25–400 mg L1). 15 ml of the medium with the metals was added to the bottles while continuously flushing with N2, and 1 ml of a sodium sulphide solution (3.9103 M–6.2102 M) in deoxygenated DIW was added. After addition, the bottles were sealed with a septum and aluminum cap, and then shaken vigorously by hand for 1 min, and then left on the orbital shaker for another 30 min at 120 rpm to allow for any precipitate growth and flocculation to occur. Samples were taken from the shaker and left to settle for at least 1 h before being analysed. Some of the samples were filtered using a 0.22 mm or 0.45 mm filter (Millipore Millex MCE, 0.45 mm) to evaluate the sizes of the precipitates. All samples were acidified to a pH lower than 4 with 1% Nitric acid prior to injection into the ICP. Furthermore, the experiments with heavy metals were carried out in DIW as well, buffered with solutions of low ionic strengths (I ¼ 0.01) at a pH of 6.95, as described by Perrin and Dempsey (1974). This was done in order to observe the effect of low ionic strength on the removal of heavy metals by sulphide precipitation.

2.4.

Performance of artificial pond and sand filter

2.4.1.

Artificial pond (photobioreactor)

A small (w10 L) open, rectangular (30 cm length and 24 cm width) photobioreactor simulating an oxidation pond was designed and constructed in order to treat the artificial PW continuously at laboratory scale. The depth of the reactor was 15 cm, which was shallower than common oxidation ponds, resembling the top part of a full scale oxidation pond. An oil suspension of around 500 mg L1 made from a combination of Marmulþ, oil and Softanol 90 (surfactant), was fed to the reactor. The suspension was prepared by introducing the oil, drop by drop, to the intensively mixed Marmulþ/surfactant solution under continuous stirring by magnetic stirrer. The mixture was stirred until homogenization was achieved with an average oil droplet size of around 2 mm, which was similar to the size typically found in this type of PW of 5–10 mm. The pond was operated at 30  C with 15 h of light and 9 h of dark, using white light fluorescent lamps with a light intensity of 7500 lux (at the water surface). The pond volume was divided by 2 vertical baffles into 3 channels, and the flow was serpentine. The PW was introduced to the head of the first channel while the outlet was built into the end of the third channel. The outlet was comprised of a small structure which included horizontal and vertical baffles to minimize the entrance of suspended solids into the outlet tube. Based on the work of Nurdogan and Oswald (1996), the exit settling tube had a 1 cm internal diameter, a length of 14.1 cm, and was inclined at 45 , The entrance to the tube was 5 cm above the bottom of the reactor. A schematic of the pond is given in Fig. 1. The ponds were operated under various modes as was described by Shpiner et al. (2007).

2.4.2.

Slow sand filter

A slow sand filter was used after the bioreactor in order to remove particles and serve as a biological filter; the total volume of the column was 5 L, with a diameter of 8 cm, and the sand height in the column was 90 cm. The uniformity coefficient of the sand was 3.9 and d10 was 1.18 mm. Every

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24 cm 3rd channel 8cm

Effluent

2.7.

2nd channel 8cm

Feed st

8cm

1 channel

Anaerobic cell Potential recycle

Fig. 1 – Photobioreactor system.

15 min around 1% of the daily effluent was pumped into the filter, achieving an average hydraulic load of 0.182–0.91 m/ day, depending on the mode of operation. After five and a half months of operation the top 3 cm were removed, and the sand was washed with acid which was analysed by the ICP.

2.5. Analytical methods: inductively coupled plasma–optical emission spectrometry All samples were analysed with a Perkin Elmer Inductively Coupled Plasma–Optical Emission Spectrometer (Optima 200 DV) with a personal computer and the appropriate software (Win Lab 32). Each sample was analysed using the appropriate background as blank. Calibration curves were prepared with the relevant metals (cadmium, chromium and nickel) and were calculated using the software based on standards of a known concentration with a minimum (STD1) and a maximum (STD2) concentration. Two kinds of calibration curves were used for analysis of the multi metal solutions:  One calibration curve made with Marmulþ PW and the three metals: Chromium (STD1 50 mg L1, STD2 100 mg L1), Cadmium (STD1 50 mg L1, STD2 100 mg L1) and Nickel (STD1 100 mg L1, STD2 1000 mg L1).  One calibration curve was made with DIW and the three metals: Chromium (STD1 50 mg L1, STD2 100 mg L1), Cadmium (STD1 50 mg L1, STD2 100 mg L1) and Nickel (STD1 100 mg L1, STD2 1000 mg L1). The second curve was used when a low ionic strength buffered solution was used instead of the PW.

2.6.

Analysis for which results are reported here, were conducted in accordance with the method described by Lake et al. (1985). Both filtered and non-filtered samples of sludge were used.

Sludge analysis

Some of the sludge accumulated during the reactor operation before, during and after heavy metals precipitation was collected after the reactor was drawn off. In order to examine the distribution of metals in the sludge, a procedure of sequential washing of the sludge was performed. The process was first developed by Stover et al. (1976) and later modified by Lake et al. (1985) and used by Aquino and Stuckey (2007).

Statistics

Metal Precipitation Tests: the metal precipitation tests were initially repeated in 20 replicates in order to study the distribution of the removal rates. In the next experiments 5 replicates were used and high deviations were encountered in some experiments, especially the ones in buffered water. Due to the relatively small number of replicates, the Mann–Whitney (U-test) test was selected for comparison between values of two series differing by one parameter (sulphide dose, filter size, ionic strength), and was performed using Stata software (version 8). The median values will be presented in the discussion.

3.

Results and discussion

3.1.

Metal precipitation as sulphide salts

In order for a metal salt to precipitate from solution the product of the activities of the ions comprising it should exceed the solubility product of the salt, and this leads to precipitation and in a few cases crystallization (So¨hnel and Garside, 1992). In the precipitation process coagulation with other material, rather than the pure crystal, also occurs which leads to further growth of the particles to a settleable size, and this is what can be expected in wastewater and PW biological treatment systems where the chemical matrix is diverse and bioflocculants excreted by the bacteria are present. The solubility product of a salt is the product of the activities of its components. For example in the case of cadmium sulphide: o  n    2þ þ S2 4 CdSðSÞ ; Cd

log KSP ¼ 28:85

(1)

However, there is convincing evidence from the literature that a value of pKa(HS) ¼ 17–19 is appropriate, whereas a value of 13 is widely accepted; the higher value of 19, corresponds to DG (S2) ¼ 120.5 kJ mol1, rather than DG (S2) ¼ 86.31 kJ mol1 quoted in the most recent, critically assessed source of such thermodynamic data (Zhdanov, 1985). The uncertainties in pKa(HS) described by Kelsall and Thompson (1993) cast doubt on the KSP values of many metal sulphides quoted in the literature. As shown in Figs. 2 and 3, the chemical behaviour of metal sulphide–water systems depends on pH and electrode (redox) potential. Metal sulphides are especially unstable to oxidation, normally driven by the reduction of dissolved oxygen; metal ion concentrations would then be controlled by the solubility of e.g. the metal hydroxide, rather than the normally much less soluble sulphide. The solubility product can be represented by ion concentrations rather than the activities: h

2þ

Cd

i  S2 ¼

KSP > KSP gCd2þ gS2

(2)

where gi ¼ the activity coefficient of the ion ‘‘i’’ (dimensionless). Since ion activities depend on the ionic strength of the

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1.0 Ni(OH)3

Electrode potential (SHE) / V

0.8 0.6 0.4

NiSO4.4H2O

0.2 0.0

NiO

NiS2

-0.2

Ni2+

-0.4 NiS -0.6 -0.8 -1.0

Ni3S2

NiH0.68 -2

0

2

4

6

NiH0.59 8

10

12

Ni 14

16

pH Fig. 3 – Potential–pH diagram for the Ni–S–H–O system at 298 K and for dissolved nickel(II) and sulphide activities of 10L4 and 10L5, respectively. (Calculated using HSC with data from its database: www.outotec.com).

Fig. 2 – Potential–pH diagram for the Cd–S–H–O system at 298 K and for dissolved cadmium(II) and sulphide activities of 10L4 (Boxall and Kelsall, 1992).

solution, we can calculate them based on the Davis formula (Benjamin, 2002): log gDavis ¼ Az2

I1=2  0:2I 1 þ I1=2

! (3)

where, I ¼ ionic strength of the solution involved, z ¼ ionic charge, A ¼ Parameter related to the dielectric constant (equals 0.51 for water, 25  C). In terms of concentration, the actual solubility product will be the result of a division of the solubility product by the product of the activity coefficients of the ions involved, which means that in high ionic strength solutions like PW, the solubility of each ion is higher than in low ionic strength solutions. The activity coefficient of divalent ions in the PW was calculated to be 0.34. In Table 2, the theoretical solubility and concentration products at saturation are documented compared to the actual concentration in the tests. As can be seen, in all cases the actual concentration product of the metal salts in solution is many orders of magnitude higher than the solubility product and the product concentration at saturation, which means that theoretically most of the metals should be removed (or at least undergo precipitation/crystallization) from solution. Since PW is a complex system comprised of many potential ligands, an attempt was made to predict the removal of heavy

metals by the use of VisualMinteq ver. 2.32, chemical equilibrium software. The system was fed with the Marmul medium and the sulphur concentration, and was run under a redox potential of 0.4 V (Standard Hydrogen ElectrodeSHE), in the range in which Fig. 2 predicts CdS to be stable, and in order to resemble the conditions in the anaerobic part of the reactor. The output of the software is presented in Table 3. The software predicted well the pH of the system to be above 8.5, and as can be seen from Table 3 the cadmium(II) and nickel(II) are expected to be totally precipitated and the chromium(III) is nearly totally removed. With regards to cadmium(II), CdS was not a compound found in the software output which raises a question regarding the chemical reactions database of this software. Even so, total precipitation of nickel(II) and cadmium(II) was predicted which will be tested experimentally.

3.2.

Effect of sulphide concentration on precipitation

In all the precipitation experiments, the metal ion concentration in the blank (bottles shaken without addition of sulphide and filtered) was lower by ca. 10–15% for nickel, and ca. 25–30% for cadmium(II) compared to the stock solutions which were added to the test bottles (acidified and non filtered value), showing the formation of metal-hydroxide or metalcarbonate precipitates in solution. With regards to chromium(III), the blank showed that the settling capabilities of the chromium(III) hydroxide seem to be related to the pH of the solution, rather than to the sulphide concentration (Cr(III) sulphide is not stable in aqueous solutions). In Marmul medium a filtered sample did not contain any chromium(III), even before the addition of the sulphide, which proves that hydroxide or carbonate precipitation was dominant. The results of the metal ion precipitation tests are illustrated in Figs. 4–6; the results presented are the average ones with the number of replicates mentioned. The error bars in Fig. 4 represent the standard deviation values above and below the average in the experiment held with 20 replicates

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Table 2 – Solubility and concentration products of potential precipitates. Solubility product

Concentration product

28.8

CdS NiSa NiSb NiSg

27.8

10 1019.4 1024.9 1026.6

10 1018.5 1023.9 1025.7

Me (mole/L)

Low S (mole/L)

3.6

10 105.1 105.1 105.1

Particle size distribution

Since the results did not concur with the predictions of the theoretical study and model, the possibility of the presence of smaller particles was examined. The results in Fig. 5 reveal that the depletion of nickel(II) was substantially higher when a 0.22 mm filter was used with the low sulphide concentration (median values of 63.3% and 77.6% for the 0.45 mm and 0.22 mm

Table 3 – Visual Mintecq results for percentage of precipitation. Name

SO2 4 1

Cl NHþ1 4 PO3 4 Naþ1 CO2 3 NO1 3 Kþ1 HS1 H 2O E1 Hþ1 Niþ2 Cr(OH)þ1 2 Cdþ2 H4SiO4 Caþ2 Mgþ2

Dissolved Mol/kg

Percent

5.18E03 8.11E02 1.10E04 4.68E04 0.10023 1.10E02 7.65E03 9.76E03 2.53E04 5.95E06 0 1.10E02 3.24E16 4.89E08 1.97E11 1.20E04 6.10E06 8.03E04

100 100 100 66.42901 100 100 100 100 97.13962 100 0 100 4.76E09 3.633514 3.16E03 36.29867 1.525324 91.22771

2.4

10 103.6 103.6 103.6

for each metal. Figs. 5 and 6 present average values of 5 replicates. Due to the lower number of replicates, the whole range of results is presented by the error bars instead of the standard deviation. As can be seen from the low sulphide dose columns in Fig. 4, nearly 100% removal of the chromium(III) (median value 99.38%) and cadmium(II) (median value 98.99%) was observed from the Marmul solutions using a 0.45 mm filter, down to the detection limits of the ICP-OES. However, when high concentrations of sulphide were used, cadmium(II) removal was significantly lower ( p < 0.0001) and slightly lower values were obtained for chromium removal under high sulphide concentrations ( p < 0.02). With regards to nickel the removal rates were lower; nearly 60% removal of Ni was observed when a low dose of Na2S was added (median ¼ 59.7%), and when higher doses of sulphide were used removals were even poorer ( p < 0.09).

3.3.

High S (mole/L)

Actual concentration product Low

6.1

Precipitated

10 102.4 102.4 102.4

High

9.7

108.5 107.5 107.5 107.5

10 108.7 108.7 108.7

respectively; p ¼ 0.009) and also with the high sulphide concentration (median values of 53.3% and 81.0% for the 0.45 mm 0.22 mm; p ¼ 0.0086). There was little evidence of a difference ( p ¼ 0.17) of higher removal of nickel by the 0.22 mm filter with the higher sulphide dose in comparison to the lower sulphide dose, but significantly higher removal by the 0.45 mm filter with the lower dose of sulphide ( p ¼ 0.0086). It can be concluded that a high concentration of sulphide led to smaller particles, of which almost 20% by weight lay between 0.22 mm and 0.45 mm. This probably stems from higher supersaturations resulting in a higher driving force for precipitation, which gave rise to a greater number of nucleation sites and smaller crystals. This observation is supported by results published by Veeken et al. (2003), who noted that precipitation by sulphide resulted in colloidal metal sulphide precipitates that were poorly separable from the water phase by sedimentation or filtration. These authors suggested that the production of colloidal precipitates could be prevented when a high specific surface area for crystal growth is offered, and nucleation is minimised by the use of a membrane reactor in which the solids are retained in combination with control by a pS-electrode to keep the supersaturation low. Results were similar in the case of cadmium(II) precipitation. Higher removal rates were recorded with the 0.22 mm filter in comparison to the 0.45 mm filter with the low sulphide dose (median values of 100% and 92.6% for the 0.22 mm and 0.45 mm respectively; p ¼ 0.0080), and with the high sulphide dose (median values of 89.6% and 76.2% for the 0.22 mm and 0.45 mm respectively; p ¼ 0.0088). However, with cadmium(II), the lower sulphide dose resulted in higher removal rates also

Mol/kg 0 0 0 2.36E04 0 0 0 0 7.44E06 0 0 0 6.81E06 1.30E06 6.23E07 2.10E04 3.94E04 7.72E05

0 0 0 33.571 0 0 0 0 2.86038 0 0 0 100 96.36649 99.99684 63.70133 98.47468 8.772291

Metal Removal(%)

Salt

110 100 90 80 70 60 50 40 30 20 10 0 Low dose of S Cadmium Removal

High dose of S Nickel Removal

Chromium Removal

Fig. 4 – Removal of heavy metals in Marmul by a 0.45 mm filter under two Na2S doses (average results of 20 replicates).

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110 100 90 80 70 60 50 40 30 20 10 0 Low dose of S, 0.45 µm

Low dose of S, 0.22 µm Cadmium Removal

High dose of S, 0.45 µm

Nickel Removal

High dose of S, 0.22 µm

Chromium Removal

Fig. 5 – Removal of heavy metals in Marmul under two Na2S doses and two filter sizes (average results of 5 replicates).

when samples were filtered by a 0.22 mm filter in comparison with results of experiments using the higher sulphide dose ( p ¼ 0.0135).

3.4.

Effect of ionic strength

The effect of ionic strength is illustrated when comparing the results in Fig. 5 with those in Fig. 6. The removal rates of nickel(II) in a buffered solution of 0.01 ionic strength were significantly lower than in Marmulþ medium of high ionic strength (I ¼ 0.12) for high sulphide concentrations and

a 0.45 mm filter (median values of 53.3% and 30.8% for Marmul and for the buffered solution respectively; p ¼ 0.0086) and for the low sulphide concentration (median values of 63.3% and 40.9% for Marmul and buffered solution respectively; p ¼ 0.009) filtered through a 0.45 mm filter. The removal rates of cadmium(II) in a buffered solution of 0.01 ionic strength were significantly lower than in Marmulþ medium of high ionic strength (I ¼ 0.12) for the high sulphide dose (median values of 76.2% and 24.7% Marmul and for buffered solution respectively; p ¼ 0.088) and for the low sulphide concentration (median values of 92.6% and 29.4% for Marmul and buffered

100 90 80 70 60 50 40 30 20 10 0 Low dose of S, 0.45 µm

Low dose of S, 0.22 µm Cadmium Removal

High dose of S, 0.45 µm Nickel Removal

High dose of S, 0.22 µm

Chromium Removal

Fig. 6 – Removal of heavy metals from a buffered solution (pH [ 6.95) under two Na2S doses and two filter sizes (average results of 5 replicates).

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Concentration (µgL^-1)

600

Feed nickel Effluent F0.22 nickel

Effluent nickel Filtrate Nickel

500 400 300 200 100 0 0

10

20

30

40

50

60

70

80

Day Fig. 7 – Concentration of nickel vs time at various locations in the treatment process.

solution respectively; p ¼ 0.088) when filtered through a 0.45 mm filter. The results were similar when filtered through a 0.22 mm filter with a high sulphide dose as nickel(II) removal in Marmul was significantly higher than in the buffered solution (median of 81% in Marmul comparing with 60.9% in buffered solution; p ¼ 0.009) but only borderline significance was encountered in the low sulphide dose (median of 77.5% in Marmul comparing with 67.9% in buffered solution; p ¼ 0.076). Cadmium(II) removal with the high sulphide dose was significantly higher than from the buffered solution (median of 89.6% in Marmul comparing with 84.3% in buffered solution; p ¼ 0.044), and also with the low sulphide dose (median of 100% in Marmul comparing with 84.3% in buffered solution; p ¼ 0.0078). A possible explanation for these results, which contradict the theoretical increase in solubility at high ionic strength solutions, as was described in equation (2), is that the electrolyte causes a compression of the diffuse part of the double layer around the particles, which led to a reduction in the double-layer repulsive interaction, and enabled particles to approach close enough for van der Waals forces to predominate (Shaw, 1992). The chromium(III) removal rates were lower in the buffered solution as hydroxide precipitation is much more pH

dependent than sulphide precipitation (Peters et al., 1984). With the low sulphur dose, no formation of particles above the filter size was encountered, so the higher sodium sulphide levels were probably just enough to exceed the critical coagulation concentration (c.c.c) of the electrolyte, leading to coagulation and removal of particles or amorphous phase (Rai et al., 2004), which was not the case with the lower concentration of sodium sulphide (Shaw, 1992). The experiments with the low ionic strength were characterised by much higher experimental variations than those with the Marmul medium, probably due to the lower number of particle collisions in dilute solutions.

3.5. Heavy metals removal in artificial pond and slow sand filter Removal of heavy metals in the photobioreactor and the slow sand filter was also analysed by taking the following samples and analysing them by ICP:  The prepared medium prior to the introduction of the oil;  The effluent, and the effluent filtered through a 0.22 mm filter;  The filtrate from the slow sand filter.

120 Feed cadmium Effluent F0.22 cadmium

Concentration (µgL^-1)

100

effluent cadmium Filtrate cadmium

80 60 40 20 0 0

10

20

30

40

50

60

70

80

-20

Day Fig. 8 – Concentration of cadmium vs time at various locations in the treatment process.

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water research 43 (2009) 4258–4268

Concentration (µgL^-1 )

120 effluent chromium Filtrate chromium

Feed chromium

100

Effluent F0.22 chromium

80 60 40 20 0 0

10

20

30

40

50

60

70

80

-20

Day Fig. 9 – Concentration of chromium vs time at various locations in the treatment process.

The initial samples of the filtrate were filtered through a 0.22 mm filter, but since there was no significant difference between the filtered filtrate and the filtrate itself, no further filtration was carried out. Sampling and analysis started two weeks after the introduction of a baffle in the first channel of

the reactor. The baffle created a small cell at the front of the reactor with a high volumetric organic load, and was designed to achieve anaerobic conditions in order to enable sulphate reducing bacteria (SRB) to grow. In addition, nitrate containing compounds in the Marmulþ medium (NaNO3) were replaced

a 14.94, 14%

5.03, 14%

25.50, 25%

2.03, 6%

13.19, 37% 5.84, 6%

6.92, 20%

4.63, 4% 53.01, 51% 7.92, 23%

b 1.60, 4%

5.93, 16%

0.93, 6%

1.19, 3%

3.10, 20%

0.30, 2%

11.96, 33% 2.83, 18% 8.38, 54%

15.43, 44%

c

0.37, 1%

0.00, 0%

0.31, 1%

10.49, 26%

1.40, 3% 16.68, 35% 15.99, 34%

1.45, 4% 28.16, 69% 12.43, 27%

Exchangeable

Adsorbed

Organically bound

Carbonate

Sulphide

Fig. 10 – Distribution of heavy metals in the filtered sludge (left) and non-filtered (right) in weight (mg) and percentage of whole: a. nickel (top); b. cadmium (middle); c. chromium (bottom).

water research 43 (2009) 4258–4268

three weeks after the introduction of the baffle with NH4HCO3 keeping the amount of N constant, in order to prevent anoxic conditions at the beginning of the pond, which might suppress the growth and activity of the SRBs. Sulphate concentration in the feed was around 120 mg L1. Results of the nickel(II), cadmium(II) and chromium(III) concentrations at various sampling points are shown in Figs. 7–9, respectively. From Figs. 7 and 8, a similar pattern was evident for nickel(II) and cadmium(II). While their concentrations in the feed was almost constant, their concentration in the effluent decreased gradually, probably following the growth of the sulphate reducing bacteria, resulting in values of less than 10 mg L1 of cadmium(II) (close to the practical detection limit of the ICP), and around 100 mg L1 of nickel(II). However, the rate of the cadmium(II) concentration depletion was greater than that of nickel. This can probably be explained by the higher sulphide concentration needed to precipitate the nickel(II), and also by the slower precipitation reaction of nickel(II) and sulphide (Veeken et al., 2003). Filtering the effluent through a 0.22 mm membrane resulted in further removal of the two metal sulphide precipitates, but the residual concentrations were still higher than those in the sand filter filtrate, especially in the case of nickel(II) where it was down to 30 mg L1 in the filtrate. This was probably due to further floc growth and removal mechanisms such as straining, precipitation or even adsorption in the sand filter bed. ICP analysis of an acid wash of the sand that was removed from the top of the filter revealed that a substantial fraction of the metal ions that had passed through the pond accumulated in the top 3–4 cm of the filter, exhibiting behaviour of a strainer rather than a deep bed filter. The case of chromium(III) was different; as discussed above; chromium(III) was probably removed by the formation of its hydroxide and carbonate salts rather than sulphide. As hydroxide precipitation had occurred already in the feed in an uncontrolled way, chromium(III) concentrations in the feed varied considerably, were very low in the filtered feed, and apparently zero (or at least within the detection limits of the ICP) in the effluent throughout the experiment, independent of the redox conditions. Chromium(III) concentrations in the feed varied due to two possible reasons. Firstly, the experimental method that included the addition of chromium(III) to the medium together with manual uncontrolled mixing could not guarantee that the number of collisions in each case or that the mixing energy will be maintained. Also, small variations in the pH of the solution can affect the solubility of chromium hydroxide resulting in different degrees of precipitation. The high removal rates found in the reactor, even compared to the precipitation tests described earlier in the case of the nickel(II), showed that floc growth in the pond was probably enhanced by: bio-flocculants excreted by the biomass; coagulation with hydrocarbon materials; with the biomass itself; or, due to a very low supersaturation of the metal sulphide(s) as sulphide concentrations depend on rates of sulphate reduction by bacteria. The contribution of the other removal mechanisms, such as biosorption and uptake, seemed to be lower as the removal of cadmium and nickel was

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much lower prior to the introduction of the anaerobic zone, as can be seen in Figs. 7 and 8. Sulphide precipitation probably followed some initial metal-hydroxide and carbonate formation during the addition of the metal ions to the medium, as described earlier.

3.6.

Heavy metals ions in the sludge

The metals were extracted sequentially from a sludge that had accumulated in the bottom of the reactor during its operation. While nickel(II) and cadmium(II) were part of the medium, as can be seen in Table 1, chromium(III) was added only during the period of study of metal removal in the pond that lasted no more than 100 days. As this pond sludge differed substantially from a municipal wastewater treatment sludge for which the method was developed (containing high amounts of tar), the process was not expected to be too accurate, but just to give an estimate of the sludge composition. Some other mechanisms, besides precipitation, leading to other forms of metals in the sludge, such as biosorption and metals uptake as micronutrients, were also examined (Shpiner, 2007) during the study, and although their results are not presented in this paper their influence on the metal forms in the sludge is evident. As can be seen in Fig. 10, the amount of soluble/ exchangeable metals was negligible in the case of chromium(III), low in the case of cadmium(II) (up to 6%), and up to 25% in the case of nickel(II). The highest percentage of adsorbed metals was found with nickel, which was congruous with the results of biosorption found by Shpiner (2007). The adsorbed nickel(II) was up to 6% of the total nickel in the sludge, while a substantial amount of the cadmium(II) was found to be organically bound (up to 54%). Assuming that most of the cadmium(II) during the operation of the anaerobic cell was removed as sulphides, apparently the major removal mechanism when an anaerobic stage was not applied was by metal uptake/complexation with organic compounds as was found by metal ion uptake tests which were conducted in the study (results are not presented) (Shpiner, 2007). Finally, the sequential extraction procedure did not distinguish between metal hydroxides and other phases, and this is probably the reason why the majority of the chromium(II) was found to be in other forms, especially sulphides, which was unexpected since chromium, based on theory, should not form sulphides under these conditions.

4.

Conclusions

Cadmium(II), chromium(III) and nickel (II) present in PW were removed by various processes in a WSP via several mechanisms. The most effective mechanism for the removal of cadmium(II) and nickel(II) was precipitation as a metal sulphide, which was especially effective due to the high ionic strength of the PW. These various mechanisms resulted in very effective overall performance of the bioreactor and the filter, as the concentrations of the heavy metals after treatment were much lower than the stringent standards required.

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Acknowledgment We would like to express our gratitude to Shell-Gamechanger for financial help and support during this project. We would also like to thank PRI (Plant Research Institute, Wageningen) for their help and support in this work.

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