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Removal of dissolved metals in wetland columns filled with shell grits and plant biomass
Accepted Manuscript Removal of Dissolved Metals in Wetland Columns Filled with Shell Grits and Plant Biomass Firoozeh Bavandpour, Yuanchun Zou, Yinghe He, Tanveer Saeed, Yong Sun, Guangzhi Sun PII: DOI: Reference:
S1385-8947(17)31453-5 http://dx.doi.org/10.1016/j.cej.2017.08.112 CEJ 17563
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
9 August 2017 22 August 2017 23 August 2017
Please cite this article as: F. Bavandpour, Y. Zou, Y. He, T. Saeed, Y. Sun, G. Sun, Removal of Dissolved Metals in Wetland Columns Filled with Shell Grits and Plant Biomass, Chemical Engineering Journal (2017), doi: http:// dx.doi.org/10.1016/j.cej.2017.08.112
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Removal of Dissolved Metals in Wetland Columns Filled with Shell Grits and Plant Biomass
Firoozeh Bavandpour1, Yuanchun Zou2, Yinghe He3, Tanveer Saeed4, Yong Sun1, Guangzhi Sun1,2,3*
1
School of Engineering, Edith Cowan University, Joondalup, WA 6027, Australia.
2
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and
Agroecology, Chinese Academy of Sciences, Changchun 130102, China. 3
College of Science and Engineering, James Cook University, Townsville, 4811, Australia.
4
Department of Civil Engineering, University of Asia Pacific, Dhaka, Bangladesh.
*Correspondence. Tel: +61 8 63045423; Email: [email protected]
ABSTRACT
Two lab-scale wetland systems were studied for the removal of dissolved Cu, Mn, Fe, Pb and Zn. Vegetated with Typha domingensis, each system consisted of two units, a vertical and a horizontal flow wetland column, which were filled with either crushed sea shell grits or composted green waste as main media. A synthetic acidic wastewater was prepared by dissolving H2SO4, Pb(CH3COO)2, MnCl2, FeSO4, CuSO4 and ZnSO4 in a distilled water. As it passed through each column, metal concentrations, pH and conductivity were monitored. The pH value of the wastewater increased in the shell grit columns, where dissolved metals were almost completely (>99%) removed. In the wetland columns filled with the green waste, 1
the average percentage removals were 90, 77, 27, 98 and 75% for Cu, Mn, Fe, Pb and Zn, respectively. Scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS) analysis showed that the surface characteristics of the shell grits remained largely unchanged before and after being used in the columns; but the mass compositions of carbon increased, whereas calcium and oxygen decreased. Infrared spectroscopy (IR) and x-ray diffraction (XRD) were used to further analyse the chemical compositions and functional groups of the surfaces of the shell grits. Keywords: Constructed wetland; heavy metals; nonhazardous waste; wastewater treatment.
1. INTRODUCTION
Mining and mineral processing activities often generate acidic wastewaters, such as drainage water from mine sites and seepage water from tailings and waste rock piles [1,2]. Metals in these acidic wastewaters tend to be in soluble forms, capable of penetrating to the groundwater or travelling long distances in surface waters [3]. These wastewaters are typically stored in open pits or dams for natural evaporation, a containment strategy that does not remove contaminants. Effective and cost efficient onsite treatment to remove the contaminants is a critical issue of water environment protection and risk mitigation.
Various active or passive technologies have been investigated for removing dissolved metals from polluted waters. Active treatment systems include sulfidogenic bioreactors [4], precipitation tanks [5], adsorption columns/beds [6], membrane filtration [7] and electrochemical systems [8]. Recent developments in nanotechnology [9] and biodegradable materials [10] have improved the competitiveness of some active systems, but they still have
2
the disadvantages of relatively higher energy input and operating costs, and environmental incompatibility [11]. Passive technologies (such as constructed wetland, anoxic limestone drain, and permeable reactive barrier) are typically associated with lower operation and maintenance costs [12,13], which somewhat offset their disadvantage of large land requirement, in particular when the systems are in remote locations. Among the passive systems, constructed wetlands are known to be an effective ecological system.
Constructed wetlands have been studied for the removal of heavy metals from domestic wastewater [14,15], industrial effluents [16], leachate [17], storm runoff [18] and acid mine drainage (AMDs) [2]. In general, considerable percentage metal removals have been reported in the treatment of AMDs [19]. To date, most studies of acidic wastewater treatment using constructed wetlands have been conducted in the surface flow systems, sized largely based on hydraulic loading [20,21]. In subsurface flow systems, wastewaters filter through the packed media, instead of flowing over them. In theory, higher removal rates of dissolved metals (in mass removal per m2 system surface) from the wastewaters are obtainable in the subsurface flow wetlands due to greater contact between the pollutants and wetland media, provided that adequate hydraulic conductivity is maintained [22,23].
The predominant mechanisms of metal removal in the constructed wetlands are either biotic or abiotic, depending on system design and environmental condition. In a newly constructed subsurface flow wetland, the abiotic routes (e.g. sedimentation, flocculation, adsorption, ion exchange, precipitation and complexation) can remove the bulk of various pollutants within short retention time [14,24,25]. However, a wetland only has limited capacity to sustain the abiotic mechanisms; it may need extensive maintenance, or complete reconstruction, should its media become saturated with immobilised metals [26]. Biotic removal mechanisms (such
3
as microbial transformations/immobilisation, plant uptake and harvesting) are sustainable routes of metal removal [4,27], provided that carbon, nutrients and suitable environmental conditions required by the microorganisms and plants are available [28].
The biotic and abiotic routes are both affected by the physio-chemical characteristics and quantity of wetland media [29]. The media supply the alkalinity and adsorption sites required for metal oxidation, precipitation and adsorption, and the surfaces and carbon sources for microorganisms (such as sulphur-reducing bacteria) to grow. The selection of wetland media is a critical factor that determines the effectiveness and life span [30] of a subsurface flow wetland when it is used to remove dissolved metals from wastewaters.
Gravel and sand are the traditional media employed in subsurface flow constructed wetlands [30,31]. However, these materials are unsuitable for removing dissolved metals, due to limited ability to increase alkalinity and facilitate precipitation. A variety of nonhazardous solid wastes have been studied in passive wastewater treatment systems; with mixed results reported in the literatures [32,33]. Examples of these solid wastes include alum sludge from drinking water treatment works [34], furnace steel slag [35], mining processing residue [34] construction waste [36], sugar cane bagasse [37], oyster-shell [21], coir husk chip [23], wood mulch [25,30], rice straw [38] and fragmented limestones [39]. Major advantages of using the nonhazardous wastes are two-folds: (a) enhanced pollutant removal efficiency due to suitable characteristics of selected materials, and (b) transformation of the solid wastes to resources. This study focused on the effectiveness of two nonhazardous solid wastes, crushed sea shells and composted urban green waste (primarily threaded woody plants), to remove dissolved metals.
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2. MATERIALS AND METHODS
2.1. Synthetic wastewater The chemical compositions of acidic wastewaters (such as mine drainage) vary significantly from site to site. In this study, a synthetic wastewater was prepared based on the characteristics of an effluent at a coal mining site [40], but with higher concentrations of copper and lead to allow the study of their removal. Measured amounts of Pb(CH3COO)2, MnCl2, FeSO4, CuSO4 and ZnSO4 powders, and H2SO4 solution was added in a distillate water, to produce a synthetic wastewater with target pollutant concentrations shown in Table 1. Under acidic condition, the metals were found to be primarily in dissolved forms.
Table 1. Chemical compositions and target metal concentrations in the synthetic wastewater Theoretical concentration
Reagent used
Amount (g) per L water
Cu
4 mg/L
CuSO4(H2O)5
0.01572
Fe
200 mg/L
FeSO4(H2O)4
0.80287
Mn
18 mg/L
MnCl2(H2O)7
0.08262
Pb
2 mg/L
Pb(CH3COO)2
0.00314
Zn
12 mg/L
ZnSO4(H2O)7
0.05275
pH
2-3
H2SO4
2.2. The lab-scale constructed wetlands Two lab-scale wetland systems (operated in parallel) were installed outdoors (sheltered from rain). These two systems (namely, A and B) had identical configurations; each consisting of a cylindrical vertical flow column (A1 or B1) as the first treatment stage, followed by a rectangular horizontal flow column (A2 or B2) as the second stage, as shown in Figure 1.
5
Manual Feed
Cylindrical column diameter = 8.5 cm
60 cm
Column A1 or B1 Column A2 or B2
Rectangular column width = 10 cm
60 cm
10 cm
Typha domingensis
40 cm
Effluent
Figure 1: A schematic diagram of the hybrid wetland columns
The bottom of each vertical flow column was filled with 15-30 mm round gravel to a depth of 10 cm, as a drainage layer. The same gravel was used in the water inlet and outlet sections (each being 50 mm long) of the horizontal flow columns to form inflow and outflow zones. The main spaces in the columns were filled with crushed sea shell grits (in system A) or composted urban green waste (in B). Some general characteristics of these media are described in Table 2.
6
Table 2. Main media used in the wetland columns Descriptions
Source of supply
Crushed sea shell
Crushed shells of oysters, mussels, and various other farmed
A commercial product
grits
shellfish, being sold as a calcium source for poultry. Sizes
(shell grits) from Laucke
range from <1mm to 12mm. Measured pack porosity is 48%.
Mills, Australia.
Composted urban
Composted mixture of shredded plants (tree, shrub, etc.) and
Ayr waste transfer
green waste
humus materials (lignin, rotted wood, etc.); measured pack
station, Burdekin Shire
porosity 52%.
Council, Australia.
Mature plants of Typha domingensis were collected from a creek on James Cook University’s Townsville campus and re-planted in these columns. This species was selected due to its ability to tolerate low pH and adaptability to the local tropical climate. The plants were given two weeks to establish in the columns, prior to the commencement of experiments.
2.3. System Operation, Sampling and Analysis From a feed storage tank, two litres of the synthetic wastewater were collected and dosed manually to the top of the vertical flow column of each system; the effluent flowed by gravity towards the inlet zone of the horizontal flow column, before overflowing into an effluent collection tank. The manual dosing was done four days per week (Monday to Thursday). In total, eight litres of wastewater were dosed into each system per week; giving an average hydraulic loading rate of 0.252 m3/m2d (m2 represents surface area) on each vertical flow column, or 0.036 m3/m2d on each horizontal flow column.
Water samples were collected on a weekly basis, every Thursday from the feed tank and outlets of the vertical flow columns, and every Friday from the effluent tanks. Immediately after collection, the samples were filtered through 0.45μm syringe filters. The temperature, 7
conductivity and pH values of the filtered samples were measured using a sensION MM374 measuring kit (Hach). The analyses of dissolved Cu, Fe, Mn, Pb, and Zn concentrations were carried out in the Advanced Analytical Centre at James Cook University using an inductively coupled plasma mass spectrometer (ICP-MS). Unused shell grits, together with used grits from the upper part of column A1, were sent to Edith Cowan University for SEM-EDS, XRD and IR analyses, using a scanning electron microscope (JCM-6000, JOEL), X-Ray Powder Diffraction apparatus (PANAnalytical), and Spectrum Two IR Spectrometer (Perkin Elmer), respectively.
3. RESULTS
3.1. Overall performance A total of 40 water samples (including 8 influent samples from the feed tank, and 32 effluent samples from the first and second stages of the two systems) were collected and analysed. Table 3 presents the average data from each treatment stage, and overall removal percentages of dissolved metals. Higher dissolved pollutant removal percentages (>99%) were obtained in system A than in system B, primarily due to the high removal efficiency obtained in the first stage VF column (A1).
Temperature, pH and conductivity all three parameters of the synthetic wastewater were stable in the sample collection period, as indicated by their low standard deviation values in. However, as the synthetic wastewater passed through the systems, the change of pH values were significantly different in systems A and B; this being the primary factor for the discrepancies in metal removal efficiencies in the two systems.
8
Table 3. Mean dissolved pollutant concentrations (±standard deviations) in the two lab-scale wetland systems
System B
System A
Parameter (unit)
Influent
First stage
Second stage
Effluent
Removal %
Effluent
Removal %
Overall Removal %
Cu (μg/L)
3998±365
24.3±25.0
99.4
6.06±1.59
75.1
99.8
Fe (mg/L)
160±40
1.37±1.23
99.1
1.24±1.11
9.5
99.2
Mn (μg/L)
18788±1718
8351±2900
55.6
3.53±2.69
93.7
100.0
Pb (μg/L)
1574±490
0.99±0.78
95.7
0.16±0.21
99.5
100.0
Zn (μg/L)
12088±1225
318±242
97.4
14.7±4.9
95.4
99.9
pH
2.59±0.19
7.79±0.19
8.11±0.25
Conductivity (µS/cm)
1893±531
1038±226
1153±226
Water temperature (0C)
24.7±0.9
25.0±0.9
24.9±0.8
Cu (μg/L)
3737±2013
6.5
400±341
89.3
90.0
Fe (mg/L)
84.8±39.2
47.0
37.1±14.4
56.3
76.8
Mn (μg/L)
19695±4697 -4.8
13685±3945
30.5
27.2
Pb (μg/L)
976±694
37±22
96.2
97.6
75.3
75.0
38.0
Same as above Zn (μg/L)
12263±5089 -1.4
3024±3358
pH
3.11±1.11
5.80±0.89
Conductivity (µS/cm)
1495±441
1093±103
Water temperature (0C)
24.9±0.5
25.0±0.9
3.2. Pollutant mass removal rates Across each treatment stage, the mass removal rates (MR) of dissolved metals were calculated based on their concentration changes and the superficial volumes of the packed media. The
9
MR value (in g/m3d or mg/m3d, where m3 represents the packed volume of main media) was calculated as:
MR
Q C in C out Vmedia
(1)
where Cin (mg/L) represents the concentration of Cu, Fe, Mn, Pb or Zn in the inflow; Cout (mg/L) is their concentrations in the outflow; Q is the mean daily flow rate of the synthetic wastewater (1.143 L/d); and Vmedia (m3) is the packed volume of the main media (excluding drainage layer, inlet and outlet zones) in each wetland column. Figure 2 illustrates the profile of mass removal rates vs. time. Figure 3 shows the average removal rates in individual stages of the two systems.
12000
260
180
160
1000
8000
mg/m3d
mg/m3d
mg/m3d
200
6000 4000 2000
140 120
1
2
3
4
5
6
7
8
9
800 600 400 200
0
0 0
Mn Removal Rate
1200
10000
220
0
1
2
3
Week
4
5
Week
6
7
8
9
0
1
2
3
4
5
6
7
Week
1000
150
Pb Removal Rate
Zn Removal Rate 800
90
600
mg/m3d
120
mg/m3d
1400
Fe Removal Rate
Cu Removal Rate
240
60 30
─○─ System A ─∆─ System B
400 200
0
0 0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Week
Week
Figure 2: The mass removal rates of dissolved metal vs. time (m3 represents the total volume of packed media in each hybrid wetland system).
10
8
9
Figure 3: Mean mass removal rates of dissolved metals in individual vertical flow (upper) and horizontal flow (lower) columns. Error bars represent standard deviations.
3.3. Surface image and chemical compositions of the sea shell grits Figure 4 shows the SEM images of shell grits before and after being used as the media in column A1. Both images show relatively smooth surface, having very limited micro-pores compared with common adsorbents (such as activated carbon). The surface textures and shapes of the shell grits appear largely unchanged during the experiment. Figure 5 present the results of XRD scans of the unused shell grits, and estimated chemical components in their surface layers. The profile of IR scan of the shell grits is given in Figure 6.
11
Unused grit
Used grit
Figure 4: SEM images of the sea shell grits.
Figure 5: XRD scan image of the surface layers of unused sea shell grits.
12
As shown in Figure 5, the shell grits primarily consist of quartz (mass percentage 56%) and calcite-type materials (14%), having significant peaks. Sodium, iron, and copper compounds are also indicated by the database associated with XRD instrument.
100
95
Unused Media
1443 cm-1 1083 cm-1
90
Used Media
%T
712
cm-1
85 856 cm-1
80 1416 cm-1
75 0
500
1000
1500
2000
2500
3000
3500
4000
cm-1 Figure 6: IR diagrams of unused and used shell grits
The IR scan (Figure 6) of shell grit media produces three major peaks at 712, 856 and 1083 cm-1. The peaks at 712 and 856 cm-1 are likely related to chemical bond between either C and O (in form of CO3) or between Ca and CO3. The peaks at 1083 cm-1 are possibly caused by the bonds between Si and O, or Ca and CO3. After being as media in the wetland columns, the peak at 1416 cm-1 (unused grit) appeared to have shifted to 1443 cm-1 (used grit), but both peaks have been connected to Si-O in a previous study [41]. Overall, the similarity of peaks
13
produced by the IR scans indicates that treatment of the acidic wastewater had no significant impact on the chemical bonds of the shell grit media.
4. DISCUSSION
4.1. Correlations between pH changes and metal removal efficiencies Figure 7 presents the plot of pH change vs. the logarithmic values of influent to effluent mass concentration ratios (i.e. Ci/Co) of the dissolved metals, when the synthetic wastewater passed through the first stages of the two wetland systems (i.e. columns A1 and B1). As shown in Figure 7, reduced acidity (increase in pH) resulted in greater removal efficiencies for all five metals. One-way ANOVA analysis showed that for all five metals, there are significant (p<0.05) correlation between log(Ci/Co) and increases in pH values. Both axes in Figure 7 are in fact concentration ratios in logarithmic scale; hence, the percentage reductions of the molar concentration of hydrogen ions are correlated to C1/C0 ratios. Although it does not provide direct evidence of metal removal route, such correlation can exclude any zero-order process (such as steady assimilation to plant biomass) being the primary route of metal removal in the wetland columns.
Relatively, the removals of lead and copper appear more sensitive to pH than manganese and iron. In columns A1 and B1, strong correlations between pollutant concentration reductions (Ci/Co) and pH values demonstrate that precipitations are likely to have taken place in the vertical flow columns. In A1, the removal percentage of Mn was significantly lower than those of Cu, Fe, Pb and Zn, suggesting that Mn precipitation is less sensitive to acidity than the other metals in vertical flow columns.
14
Figure 7: Log (Ci/Co) vs. the change of pH values in the vertical flow columns. An increase in the pH is represented by a positive x-axis value.
Figure 8: Log (Ci/Co) vs. the change of pH values in the horizontal flow columns.
In the horizontal flow columns, no correlation was found between the change of pH and removals of the metals, as shown in Figure 8, indicating that either: (a) the pH range of 5.88.1 was adequate for further chemical precipitations of remaining metals in the effluents from 15
A1 and B1, or (b) in addition to precipitations, other mechanisms (e.g. adsorption) were making a simultaneous and substantial contribution.
4.2. Chemical compositions of shell grits Slightly increase number of minor peaks and lower intensity of major peaks in Figure 7 suggest that the compositions of the shell grit surfaces may have become more diverse. Neither XRD nor IR analysis can accurately identify the chemical compositions. These scans only indicate the likely compositions, and result interpretations require some assumptions of what is likely to be found.
Table 4. Distributions of major elements (mass percentage) in the surface layers of the unused and used shell grit media, according to interpretation by the SEM-EDS analyses
Element
% Mass of unused
% Mass of used
media
media
C
65.81
86.00
O
17.45
8.49
Na
0.40
_
Al
2.44
_
Si
7.63
1.08
Ca
6.26
2.05
Mn
0.03
0.47
Fe
_
0.43
Cu
_
0.22
16
Table 4 present the EDS analysis results of unused and used shell grits. The main elements in the shell grits were C, O, Si and Ca. Increased mass compositions of C, Mn, Fe and Cu on the surfaces of used shell grits may be caused by the deposits of metals or carbon deposits from the environment (such as dust and fragments from plants).
4.3. Mechanisms of dissolved metal removals in the wetland columns The shell grit wetland system demonstrated excess metal removal capacity under the current pollutant loading. Virtually all the dissolved metals were removed from the wastewater; the bulk of removal taking place in the first stage (A1). In comparison, system B demonstrated limited pollutant removal capacity, especially in terms of Fe and Mn removal. System B was expected to support metal removal via microbial routes, facilitated by the organic media to provide carbon for heterotrophic microorganisms [28]. However, at the start of experiments the seeding of bacteria into columns B1 and B2 was not carried out. It appears that this factor, combined with limited operation period, did not allow sufficient microbial populations to establish in system B.
The mechanisms of metal transformations in a subsurface flow wetland are complex. Although the removal of dissolved metals in system A was significant and stable during the experiment, no concrete conclusion can be drawn regarding its long-term efficiency. It is likely that the removal of dissolved metals in column A1 proceeded via abiotic routes, such as precipitations to insoluble hydroxide or sulphide compounds, as the pH value increased. Relatively greater ability of dissolved oxygen transfer of the vertical flow column can also benefit the abiotic routes. The shell grits may have either supplied the chemical substances (such as HCO3-) for metal precipitations on their surfaces, or provided active sites for
17
adsorption, or both. In a wetland system, possible precipitation reactions for iron and manganese are given in equations 2-6 [40,42,43].
Fe 2 0.25O2( aq ) H Fe 3 0.5 H 2 O
(2)
Fe 3 3H 2 O Fe( OH )3( s ) 3H
(3)
2 Fe 3 2 H 2 S FeS 2( s ) Fe 2 4 H
(4)
Mn 2 0.5O2( aq ) 2 H Mn 4 H 2 O
(5)
Mn 4 2 H 2 O MnO2( s ) 4 H
(6)
Theoretically, a wetland only has limited capacity to reduce acidity and/or retain the metals via abiotic routes. Once their full capacity is reached, the amount of metals removed from the wastewater can rapidly decrease. Biotic routes of metal removal (especially microbe-induced oxidation and sulphate reduction) are desirable. A generalised representation of the microbial sulphate reduction process is given in equations 7-8, where CH2O represents a small organic molecule, and M represents a heavy metal species [2]. In columns B1 and B2, although organics are available, metal removals may have taken place primarily via abiotic routes, as the pH value increased.
SO4 2CH 2 O H 2 S 2 HCO3
(7)
H 2 S M 2 MS 2 H
(8)
In this study, plant uptake is believed to have a negligible impact on the metal removals, especially in the first stages of the wetland systems. Significant uptake of dissolved metals into plant tissues would have made pollutant removal rate to be steady, regardless of loading, 18
which was not observed from the experiment data. The role of vegetation in constructed wetlands has always been debatable. Some researchers indicated that dissolved metals can be absorbed to the anionic sites of plant cell walls, such that plant tissues can absorb up to 200,000 times of heavy metals relative to their surrounding environment [44], metal immobilization into wetland media can be higher in vegetated systems [45], and plant roots may buffer the impact of toxic pollutants on microbial communities [46]. On the contrary, other researchers have suggested that the plants may in fact have a detrimental effect, due to increased mobility of metals within the rhizosphere [47].
5. CONCLUSIONS
Lab-scale experiments demonstrated that wetland columns employing with crushed shell grit media have significant capacity for removing dissolved metals from acidic wastewater. Over 99% percentage removals were obtained for Cu, Fe, Mn, Pb and Zn, under mass loading rates of 0.223, 9.72, 1.038, 0.089 and 0.644 g/m3d, respectively. SEM-EDS and IR analyses of the grits indicated slight changes of chemical compositions, after the grits were used as wetland media. Linear correlations between pH changes and percentage reductions of the dissolved metals indicated that they were predominantly removed via abiotic routes.
ACKNOWLEDGEMENTS
The authors thank Mr Cameron Wycherley and Mrs Ruilan Liu at James Cook University who assisted the experiment and water sample collection. Mr. Zhe Jia at Edith Cowan University is thanked for assisting XRD analyses of the shell grits. This study was supported
19
by CAS/SAFEA International Partnership Program for Creative Research Teams and ECU Collaboration Enhancement Scheme.
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[16] D. Sukumaran. Phytoremediation of heavy metals from industrial effluent using constructed wetland technology. Applied Ecology Environ. Sci., 1 (2013) 92-97. [17] R. Bakhshoodeh, N. Alavi, A.S. Mohammadi, H. Ghanavati, H. Removing heavy metals from Isfahan composting leachate by horizontal subsurface flow constructed wetland. Environ. Sci. Pollution Res., 23 (2016) 12384-12391. [18] L.W. Gill, P. Ring, B. Casey, N.M Higgins, P.M. Johnston. Long term heavy metal removal by a constructed wetland treating rainfall runoff from a motorway. Sci. Total Environ., 601 (2017), 32-44. [19] P. Kongroy, N. Tantemsapya, Y.F. Lin, S.R. Jing, W. Wirojanagud. Spatial distribution of metals in the constructed wetlands. Int. J Phytoremediation, 14 (2012) 128-141. [20] A. Pedescoll, R. Sidrach-Cardona, M. Hijosa-Valsero, E. Bécares. Design parameters affecting metals removal in horizontal constructed wetlands for domestic wastewater treatment. Ecol. Eng., 80 (2015) 92-99. [21] R.S. Yam, C.C. Hsu, T.J. Chang, W.L. Chang. A preliminary investigation of wastewater treatment efficiency and economic cost of subsurface flow oyster-shellbedded constructed wetland systems. Water, 5 (2013) 893-916. [22] S. Buddhawong, P. Kuschk, J. Mattusch, A. Wiessner, U. Stottmeister, Removal of arsenic and zinc using different laboratory model wetland systems. Eng. Life Sci., 5 (2005) 247-252. [23] Lizama Allende, K., Fletcher, T. and Sun, G., The effect of substrate media on the removal of arsenic, boron and iron from an acidic wastewater in planted column reactors. Chemical Engineering Journal. 2012. 179: 119-130. [24] R. Connolly, Y.Q. Zhao, G. Sun, S. Allen, Removal of ammoniacal-nitrogen from an artificial landfill leachate in downflow reed beds, Process Biochem., 39 (2004) 19711976.
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[25] T. Saeed, G. Sun, A comparative study on the removal of nutrients and organic matter in wetland reactors employing organic media. Chem. Eng. J., 171 (2011) 439-447. [26] R.H. Kadlec, S.D. Wallace, Treatment Wetlands, 2nd ed., CRC Press, Boca Raton, FL, USA, 2009. [27] L. Marchand, M. Mench, D.L. Jacob, M.L. Otte, Metal and metalloid removal in constructed wetlands, with emphasis on the importance of plants and standardized measurements: a review, Environ. Pollution, 158 (2010) 3447-3461. [28] A.L. Dann, R.S. Cooper, J.P. Bowman, Investigation and optimization of a passively operated compost-based system for remediation of acidic, highly iron- and sulfate-rich industrial waste water, Water Research, 43 (2009) 2302-2316. [29] A.K. Yadav, R. Abbassi, N. Kumar, S. Satya, T.R. Sreekrishnan, B.K. Mishra, The removal of heavy metals in wetland microcosms: effects of bed depth, plant species, and metal mobility, Chem. Eng. J., 211-212 (2012) 501-507. [30] J. Herrera-Melián, A. González-Bordón, M. Martín-González, P. García-Jiménez, M. Carrasco, J. Araña, J. Palm tree mulch as substrate for primary treatment wetlands processing high strength urban wastewater. Journal of Environmental Management, 139 (2014) 22-31. [31] G. Sun, Y. Zhu, T. Saeed, G. Zhang, X. Lu, Nitrogen removal and microbial community profiles in six wetland columns receiving high ammonia load, Chem. Eng. J., 203 (2012) 326-332. [32] L. Batty, P. Younger, The use of waste materials in the passive remediation of mine water pollution. Surveys in Geophysics, 25 (2004) 55-67. [33] A.I. Stefanakis, V.A. Tsihrintzis, Effect of loading, resting period, temperature, porous media, vegetation and aeration on performance of pilot-scale vertical flow constructed wetlands, Chem. Eng. J., 181-182 (2012) 416– 430.
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[34] T. Hua, R. Haynes, Y.F. Zhou, A. Boullemant, I. Chandrawana. Potential for use of industrial waste materials as filter media for removal of Al, Mo, As, V and Ga from alkaline drainage in constructed wetlands–adsorption studies. Water Research, 71 (2015) 32-41. [35] I. Blanco, P. Molle, L.E.S. de Miera, G. Ansola. Basic oxygen furnace steel slag aggregates for phosphorus treatment. Evaluation of its potential use as a substrate in constructed wetlands. Water Res., 89 (2016) 355-365. [36] J. Wang, P. Zhang, L. Yang, T. Huang. Adsorption characteristics of construction waste for heavy metals from urban stormwater runoff. Chinese J. Chem. Eng., 23 (2015) 1542-1550. [37] T. Saeed, G. Sun, A lab-scale study of constructed wetlands with sugarcane bagasse and sand media for the treatment of textile wastewater. Bioresource Technol., 128 (2013) 438-447. [38] W. Cao, Y. Wang, L. Sun, J.Y. Jiang, Y. Zhang. Removal of nitrogenous compounds from polluted river water by floating constructed wetlands using rice straw and ceramsite as substrates under low temperature conditions. Ecol. Eng, 88 (2016) 77-81. [39] D.M.R. Mateus, M.M.N. Vaz, H.J.O. Pinho, Fragmented limestone wastes as a constructed wetland substrate for phosphorus removal, Ecol. Eng., 41 (2012) 65-69. [40] R. Geremias, R.C. Pedrosa, J.C. Benassi, V.T. Favere, J. Stolberg, C.T. Menezes, M.C. Laranjeira, Remediation of coal mining wastewaters using chitosan microspheres. Environ. Technol., 24 (2003) 1509-1515. [41] N. V. Chukanov, IR spectra of minerals and reference samples data. In: Infrared spectra of Mineral Species. Springer. 2014. [42] H. L. Ehrlich, Geomicrobiology. 4th ed., CRC Press, Boca Raton, FL, USA, 2002.
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[43] W. Stumm, J.J. Morgan, Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, John Wiley and Sons, NJ, USA, 1996. [44] E.L. Edroma, Copper pollution in Rwenzori National Park, Uganda. J. Applied Ecology, 2 (1974) 1043-1056. [45] T.Y. Yeh, Removal of metals in constructed wetlands: review, Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, 12 (2008) 96-101. [46] T. Weber, T. Allard, E. Tipping, M.F. Benedetti, Modeling iron binding to organic matter. Environ. Sci. Technol., 40 (2006) 7488-7493. [47] D.L. Jacob, M. L. Otte, Conflicting processes in the wetland plant rhizosphere: metal retention or mobilization? Water, Air Soil Pollution: Focus, 3 (2003) 91-104.
25
FIGURE CAPTIONS
Figure 1: A schematic diagram of the hybrid wetland columns
Figure 2: The mass removal rates of dissolved metal vs. time (m3 represents the total volume of packed media in each hybrid wetland system).
Figure 3: Mean mass removal rates of dissolved metals in individual vertical flow (upper) and horizontal flow (lower) columns. Error bars represent standard deviations.
Figure 4: SEM images of the sea shell grits.
Figure 5: XRD scan image of the surface layers of unused sea shell grits.
Figure 6: IR diagrams of unused and used shell grits.
Figure 7: Log (Ci/Co) vs. the change of pH values in the vertical flow columns. An increase in the pH is represented by a positive x-axis value.
Figure 8: Log (Ci/Co) vs. the change of pH values in the horizontal flow columns.
26
Shell grits proved to be effective wetland media for treating acidic wastewater
Over 99% removals of dissolved Cu, Mn, Fe, Pb and Zn were obtained
The metals were removed primarily via abiotic routes
The mass compositions of shell grits changed marginally in wetland columns
S1385-8947(17)31453-5 http://dx.doi.org/10.1016/j.cej.2017.08.112 CEJ 17563
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
9 August 2017 22 August 2017 23 August 2017
Please cite this article as: F. Bavandpour, Y. Zou, Y. He, T. Saeed, Y. Sun, G. Sun, Removal of Dissolved Metals in Wetland Columns Filled with Shell Grits and Plant Biomass, Chemical Engineering Journal (2017), doi: http:// dx.doi.org/10.1016/j.cej.2017.08.112
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Removal of Dissolved Metals in Wetland Columns Filled with Shell Grits and Plant Biomass
Firoozeh Bavandpour1, Yuanchun Zou2, Yinghe He3, Tanveer Saeed4, Yong Sun1, Guangzhi Sun1,2,3*
1
School of Engineering, Edith Cowan University, Joondalup, WA 6027, Australia.
2
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and
Agroecology, Chinese Academy of Sciences, Changchun 130102, China. 3
College of Science and Engineering, James Cook University, Townsville, 4811, Australia.
4
Department of Civil Engineering, University of Asia Pacific, Dhaka, Bangladesh.
*Correspondence. Tel: +61 8 63045423; Email: [email protected]
ABSTRACT
Two lab-scale wetland systems were studied for the removal of dissolved Cu, Mn, Fe, Pb and Zn. Vegetated with Typha domingensis, each system consisted of two units, a vertical and a horizontal flow wetland column, which were filled with either crushed sea shell grits or composted green waste as main media. A synthetic acidic wastewater was prepared by dissolving H2SO4, Pb(CH3COO)2, MnCl2, FeSO4, CuSO4 and ZnSO4 in a distilled water. As it passed through each column, metal concentrations, pH and conductivity were monitored. The pH value of the wastewater increased in the shell grit columns, where dissolved metals were almost completely (>99%) removed. In the wetland columns filled with the green waste, 1
the average percentage removals were 90, 77, 27, 98 and 75% for Cu, Mn, Fe, Pb and Zn, respectively. Scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS) analysis showed that the surface characteristics of the shell grits remained largely unchanged before and after being used in the columns; but the mass compositions of carbon increased, whereas calcium and oxygen decreased. Infrared spectroscopy (IR) and x-ray diffraction (XRD) were used to further analyse the chemical compositions and functional groups of the surfaces of the shell grits. Keywords: Constructed wetland; heavy metals; nonhazardous waste; wastewater treatment.
1. INTRODUCTION
Mining and mineral processing activities often generate acidic wastewaters, such as drainage water from mine sites and seepage water from tailings and waste rock piles [1,2]. Metals in these acidic wastewaters tend to be in soluble forms, capable of penetrating to the groundwater or travelling long distances in surface waters [3]. These wastewaters are typically stored in open pits or dams for natural evaporation, a containment strategy that does not remove contaminants. Effective and cost efficient onsite treatment to remove the contaminants is a critical issue of water environment protection and risk mitigation.
Various active or passive technologies have been investigated for removing dissolved metals from polluted waters. Active treatment systems include sulfidogenic bioreactors [4], precipitation tanks [5], adsorption columns/beds [6], membrane filtration [7] and electrochemical systems [8]. Recent developments in nanotechnology [9] and biodegradable materials [10] have improved the competitiveness of some active systems, but they still have
2
the disadvantages of relatively higher energy input and operating costs, and environmental incompatibility [11]. Passive technologies (such as constructed wetland, anoxic limestone drain, and permeable reactive barrier) are typically associated with lower operation and maintenance costs [12,13], which somewhat offset their disadvantage of large land requirement, in particular when the systems are in remote locations. Among the passive systems, constructed wetlands are known to be an effective ecological system.
Constructed wetlands have been studied for the removal of heavy metals from domestic wastewater [14,15], industrial effluents [16], leachate [17], storm runoff [18] and acid mine drainage (AMDs) [2]. In general, considerable percentage metal removals have been reported in the treatment of AMDs [19]. To date, most studies of acidic wastewater treatment using constructed wetlands have been conducted in the surface flow systems, sized largely based on hydraulic loading [20,21]. In subsurface flow systems, wastewaters filter through the packed media, instead of flowing over them. In theory, higher removal rates of dissolved metals (in mass removal per m2 system surface) from the wastewaters are obtainable in the subsurface flow wetlands due to greater contact between the pollutants and wetland media, provided that adequate hydraulic conductivity is maintained [22,23].
The predominant mechanisms of metal removal in the constructed wetlands are either biotic or abiotic, depending on system design and environmental condition. In a newly constructed subsurface flow wetland, the abiotic routes (e.g. sedimentation, flocculation, adsorption, ion exchange, precipitation and complexation) can remove the bulk of various pollutants within short retention time [14,24,25]. However, a wetland only has limited capacity to sustain the abiotic mechanisms; it may need extensive maintenance, or complete reconstruction, should its media become saturated with immobilised metals [26]. Biotic removal mechanisms (such
3
as microbial transformations/immobilisation, plant uptake and harvesting) are sustainable routes of metal removal [4,27], provided that carbon, nutrients and suitable environmental conditions required by the microorganisms and plants are available [28].
The biotic and abiotic routes are both affected by the physio-chemical characteristics and quantity of wetland media [29]. The media supply the alkalinity and adsorption sites required for metal oxidation, precipitation and adsorption, and the surfaces and carbon sources for microorganisms (such as sulphur-reducing bacteria) to grow. The selection of wetland media is a critical factor that determines the effectiveness and life span [30] of a subsurface flow wetland when it is used to remove dissolved metals from wastewaters.
Gravel and sand are the traditional media employed in subsurface flow constructed wetlands [30,31]. However, these materials are unsuitable for removing dissolved metals, due to limited ability to increase alkalinity and facilitate precipitation. A variety of nonhazardous solid wastes have been studied in passive wastewater treatment systems; with mixed results reported in the literatures [32,33]. Examples of these solid wastes include alum sludge from drinking water treatment works [34], furnace steel slag [35], mining processing residue [34] construction waste [36], sugar cane bagasse [37], oyster-shell [21], coir husk chip [23], wood mulch [25,30], rice straw [38] and fragmented limestones [39]. Major advantages of using the nonhazardous wastes are two-folds: (a) enhanced pollutant removal efficiency due to suitable characteristics of selected materials, and (b) transformation of the solid wastes to resources. This study focused on the effectiveness of two nonhazardous solid wastes, crushed sea shells and composted urban green waste (primarily threaded woody plants), to remove dissolved metals.
4
2. MATERIALS AND METHODS
2.1. Synthetic wastewater The chemical compositions of acidic wastewaters (such as mine drainage) vary significantly from site to site. In this study, a synthetic wastewater was prepared based on the characteristics of an effluent at a coal mining site [40], but with higher concentrations of copper and lead to allow the study of their removal. Measured amounts of Pb(CH3COO)2, MnCl2, FeSO4, CuSO4 and ZnSO4 powders, and H2SO4 solution was added in a distillate water, to produce a synthetic wastewater with target pollutant concentrations shown in Table 1. Under acidic condition, the metals were found to be primarily in dissolved forms.
Table 1. Chemical compositions and target metal concentrations in the synthetic wastewater Theoretical concentration
Reagent used
Amount (g) per L water
Cu
4 mg/L
CuSO4(H2O)5
0.01572
Fe
200 mg/L
FeSO4(H2O)4
0.80287
Mn
18 mg/L
MnCl2(H2O)7
0.08262
Pb
2 mg/L
Pb(CH3COO)2
0.00314
Zn
12 mg/L
ZnSO4(H2O)7
0.05275
pH
2-3
H2SO4
2.2. The lab-scale constructed wetlands Two lab-scale wetland systems (operated in parallel) were installed outdoors (sheltered from rain). These two systems (namely, A and B) had identical configurations; each consisting of a cylindrical vertical flow column (A1 or B1) as the first treatment stage, followed by a rectangular horizontal flow column (A2 or B2) as the second stage, as shown in Figure 1.
5
Manual Feed
Cylindrical column diameter = 8.5 cm
60 cm
Column A1 or B1 Column A2 or B2
Rectangular column width = 10 cm
60 cm
10 cm
Typha domingensis
40 cm
Effluent
Figure 1: A schematic diagram of the hybrid wetland columns
The bottom of each vertical flow column was filled with 15-30 mm round gravel to a depth of 10 cm, as a drainage layer. The same gravel was used in the water inlet and outlet sections (each being 50 mm long) of the horizontal flow columns to form inflow and outflow zones. The main spaces in the columns were filled with crushed sea shell grits (in system A) or composted urban green waste (in B). Some general characteristics of these media are described in Table 2.
6
Table 2. Main media used in the wetland columns Descriptions
Source of supply
Crushed sea shell
Crushed shells of oysters, mussels, and various other farmed
A commercial product
grits
shellfish, being sold as a calcium source for poultry. Sizes
(shell grits) from Laucke
range from <1mm to 12mm. Measured pack porosity is 48%.
Mills, Australia.
Composted urban
Composted mixture of shredded plants (tree, shrub, etc.) and
Ayr waste transfer
green waste
humus materials (lignin, rotted wood, etc.); measured pack
station, Burdekin Shire
porosity 52%.
Council, Australia.
Mature plants of Typha domingensis were collected from a creek on James Cook University’s Townsville campus and re-planted in these columns. This species was selected due to its ability to tolerate low pH and adaptability to the local tropical climate. The plants were given two weeks to establish in the columns, prior to the commencement of experiments.
2.3. System Operation, Sampling and Analysis From a feed storage tank, two litres of the synthetic wastewater were collected and dosed manually to the top of the vertical flow column of each system; the effluent flowed by gravity towards the inlet zone of the horizontal flow column, before overflowing into an effluent collection tank. The manual dosing was done four days per week (Monday to Thursday). In total, eight litres of wastewater were dosed into each system per week; giving an average hydraulic loading rate of 0.252 m3/m2d (m2 represents surface area) on each vertical flow column, or 0.036 m3/m2d on each horizontal flow column.
Water samples were collected on a weekly basis, every Thursday from the feed tank and outlets of the vertical flow columns, and every Friday from the effluent tanks. Immediately after collection, the samples were filtered through 0.45μm syringe filters. The temperature, 7
conductivity and pH values of the filtered samples were measured using a sensION MM374 measuring kit (Hach). The analyses of dissolved Cu, Fe, Mn, Pb, and Zn concentrations were carried out in the Advanced Analytical Centre at James Cook University using an inductively coupled plasma mass spectrometer (ICP-MS). Unused shell grits, together with used grits from the upper part of column A1, were sent to Edith Cowan University for SEM-EDS, XRD and IR analyses, using a scanning electron microscope (JCM-6000, JOEL), X-Ray Powder Diffraction apparatus (PANAnalytical), and Spectrum Two IR Spectrometer (Perkin Elmer), respectively.
3. RESULTS
3.1. Overall performance A total of 40 water samples (including 8 influent samples from the feed tank, and 32 effluent samples from the first and second stages of the two systems) were collected and analysed. Table 3 presents the average data from each treatment stage, and overall removal percentages of dissolved metals. Higher dissolved pollutant removal percentages (>99%) were obtained in system A than in system B, primarily due to the high removal efficiency obtained in the first stage VF column (A1).
Temperature, pH and conductivity all three parameters of the synthetic wastewater were stable in the sample collection period, as indicated by their low standard deviation values in. However, as the synthetic wastewater passed through the systems, the change of pH values were significantly different in systems A and B; this being the primary factor for the discrepancies in metal removal efficiencies in the two systems.
8
Table 3. Mean dissolved pollutant concentrations (±standard deviations) in the two lab-scale wetland systems
System B
System A
Parameter (unit)
Influent
First stage
Second stage
Effluent
Removal %
Effluent
Removal %
Overall Removal %
Cu (μg/L)
3998±365
24.3±25.0
99.4
6.06±1.59
75.1
99.8
Fe (mg/L)
160±40
1.37±1.23
99.1
1.24±1.11
9.5
99.2
Mn (μg/L)
18788±1718
8351±2900
55.6
3.53±2.69
93.7
100.0
Pb (μg/L)
1574±490
0.99±0.78
95.7
0.16±0.21
99.5
100.0
Zn (μg/L)
12088±1225
318±242
97.4
14.7±4.9
95.4
99.9
pH
2.59±0.19
7.79±0.19
8.11±0.25
Conductivity (µS/cm)
1893±531
1038±226
1153±226
Water temperature (0C)
24.7±0.9
25.0±0.9
24.9±0.8
Cu (μg/L)
3737±2013
6.5
400±341
89.3
90.0
Fe (mg/L)
84.8±39.2
47.0
37.1±14.4
56.3
76.8
Mn (μg/L)
19695±4697 -4.8
13685±3945
30.5
27.2
Pb (μg/L)
976±694
37±22
96.2
97.6
75.3
75.0
38.0
Same as above Zn (μg/L)
12263±5089 -1.4
3024±3358
pH
3.11±1.11
5.80±0.89
Conductivity (µS/cm)
1495±441
1093±103
Water temperature (0C)
24.9±0.5
25.0±0.9
3.2. Pollutant mass removal rates Across each treatment stage, the mass removal rates (MR) of dissolved metals were calculated based on their concentration changes and the superficial volumes of the packed media. The
9
MR value (in g/m3d or mg/m3d, where m3 represents the packed volume of main media) was calculated as:
MR
Q C in C out Vmedia
(1)
where Cin (mg/L) represents the concentration of Cu, Fe, Mn, Pb or Zn in the inflow; Cout (mg/L) is their concentrations in the outflow; Q is the mean daily flow rate of the synthetic wastewater (1.143 L/d); and Vmedia (m3) is the packed volume of the main media (excluding drainage layer, inlet and outlet zones) in each wetland column. Figure 2 illustrates the profile of mass removal rates vs. time. Figure 3 shows the average removal rates in individual stages of the two systems.
12000
260
180
160
1000
8000
mg/m3d
mg/m3d
mg/m3d
200
6000 4000 2000
140 120
1
2
3
4
5
6
7
8
9
800 600 400 200
0
0 0
Mn Removal Rate
1200
10000
220
0
1
2
3
Week
4
5
Week
6
7
8
9
0
1
2
3
4
5
6
7
Week
1000
150
Pb Removal Rate
Zn Removal Rate 800
90
600
mg/m3d
120
mg/m3d
1400
Fe Removal Rate
Cu Removal Rate
240
60 30
─○─ System A ─∆─ System B
400 200
0
0 0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Week
Week
Figure 2: The mass removal rates of dissolved metal vs. time (m3 represents the total volume of packed media in each hybrid wetland system).
10
8
9
Figure 3: Mean mass removal rates of dissolved metals in individual vertical flow (upper) and horizontal flow (lower) columns. Error bars represent standard deviations.
3.3. Surface image and chemical compositions of the sea shell grits Figure 4 shows the SEM images of shell grits before and after being used as the media in column A1. Both images show relatively smooth surface, having very limited micro-pores compared with common adsorbents (such as activated carbon). The surface textures and shapes of the shell grits appear largely unchanged during the experiment. Figure 5 present the results of XRD scans of the unused shell grits, and estimated chemical components in their surface layers. The profile of IR scan of the shell grits is given in Figure 6.
11
Unused grit
Used grit
Figure 4: SEM images of the sea shell grits.
Figure 5: XRD scan image of the surface layers of unused sea shell grits.
12
As shown in Figure 5, the shell grits primarily consist of quartz (mass percentage 56%) and calcite-type materials (14%), having significant peaks. Sodium, iron, and copper compounds are also indicated by the database associated with XRD instrument.
100
95
Unused Media
1443 cm-1 1083 cm-1
90
Used Media
%T
712
cm-1
85 856 cm-1
80 1416 cm-1
75 0
500
1000
1500
2000
2500
3000
3500
4000
cm-1 Figure 6: IR diagrams of unused and used shell grits
The IR scan (Figure 6) of shell grit media produces three major peaks at 712, 856 and 1083 cm-1. The peaks at 712 and 856 cm-1 are likely related to chemical bond between either C and O (in form of CO3) or between Ca and CO3. The peaks at 1083 cm-1 are possibly caused by the bonds between Si and O, or Ca and CO3. After being as media in the wetland columns, the peak at 1416 cm-1 (unused grit) appeared to have shifted to 1443 cm-1 (used grit), but both peaks have been connected to Si-O in a previous study [41]. Overall, the similarity of peaks
13
produced by the IR scans indicates that treatment of the acidic wastewater had no significant impact on the chemical bonds of the shell grit media.
4. DISCUSSION
4.1. Correlations between pH changes and metal removal efficiencies Figure 7 presents the plot of pH change vs. the logarithmic values of influent to effluent mass concentration ratios (i.e. Ci/Co) of the dissolved metals, when the synthetic wastewater passed through the first stages of the two wetland systems (i.e. columns A1 and B1). As shown in Figure 7, reduced acidity (increase in pH) resulted in greater removal efficiencies for all five metals. One-way ANOVA analysis showed that for all five metals, there are significant (p<0.05) correlation between log(Ci/Co) and increases in pH values. Both axes in Figure 7 are in fact concentration ratios in logarithmic scale; hence, the percentage reductions of the molar concentration of hydrogen ions are correlated to C1/C0 ratios. Although it does not provide direct evidence of metal removal route, such correlation can exclude any zero-order process (such as steady assimilation to plant biomass) being the primary route of metal removal in the wetland columns.
Relatively, the removals of lead and copper appear more sensitive to pH than manganese and iron. In columns A1 and B1, strong correlations between pollutant concentration reductions (Ci/Co) and pH values demonstrate that precipitations are likely to have taken place in the vertical flow columns. In A1, the removal percentage of Mn was significantly lower than those of Cu, Fe, Pb and Zn, suggesting that Mn precipitation is less sensitive to acidity than the other metals in vertical flow columns.
14
Figure 7: Log (Ci/Co) vs. the change of pH values in the vertical flow columns. An increase in the pH is represented by a positive x-axis value.
Figure 8: Log (Ci/Co) vs. the change of pH values in the horizontal flow columns.
In the horizontal flow columns, no correlation was found between the change of pH and removals of the metals, as shown in Figure 8, indicating that either: (a) the pH range of 5.88.1 was adequate for further chemical precipitations of remaining metals in the effluents from 15
A1 and B1, or (b) in addition to precipitations, other mechanisms (e.g. adsorption) were making a simultaneous and substantial contribution.
4.2. Chemical compositions of shell grits Slightly increase number of minor peaks and lower intensity of major peaks in Figure 7 suggest that the compositions of the shell grit surfaces may have become more diverse. Neither XRD nor IR analysis can accurately identify the chemical compositions. These scans only indicate the likely compositions, and result interpretations require some assumptions of what is likely to be found.
Table 4. Distributions of major elements (mass percentage) in the surface layers of the unused and used shell grit media, according to interpretation by the SEM-EDS analyses
Element
% Mass of unused
% Mass of used
media
media
C
65.81
86.00
O
17.45
8.49
Na
0.40
_
Al
2.44
_
Si
7.63
1.08
Ca
6.26
2.05
Mn
0.03
0.47
Fe
_
0.43
Cu
_
0.22
16
Table 4 present the EDS analysis results of unused and used shell grits. The main elements in the shell grits were C, O, Si and Ca. Increased mass compositions of C, Mn, Fe and Cu on the surfaces of used shell grits may be caused by the deposits of metals or carbon deposits from the environment (such as dust and fragments from plants).
4.3. Mechanisms of dissolved metal removals in the wetland columns The shell grit wetland system demonstrated excess metal removal capacity under the current pollutant loading. Virtually all the dissolved metals were removed from the wastewater; the bulk of removal taking place in the first stage (A1). In comparison, system B demonstrated limited pollutant removal capacity, especially in terms of Fe and Mn removal. System B was expected to support metal removal via microbial routes, facilitated by the organic media to provide carbon for heterotrophic microorganisms [28]. However, at the start of experiments the seeding of bacteria into columns B1 and B2 was not carried out. It appears that this factor, combined with limited operation period, did not allow sufficient microbial populations to establish in system B.
The mechanisms of metal transformations in a subsurface flow wetland are complex. Although the removal of dissolved metals in system A was significant and stable during the experiment, no concrete conclusion can be drawn regarding its long-term efficiency. It is likely that the removal of dissolved metals in column A1 proceeded via abiotic routes, such as precipitations to insoluble hydroxide or sulphide compounds, as the pH value increased. Relatively greater ability of dissolved oxygen transfer of the vertical flow column can also benefit the abiotic routes. The shell grits may have either supplied the chemical substances (such as HCO3-) for metal precipitations on their surfaces, or provided active sites for
17
adsorption, or both. In a wetland system, possible precipitation reactions for iron and manganese are given in equations 2-6 [40,42,43].
Fe 2 0.25O2( aq ) H Fe 3 0.5 H 2 O
(2)
Fe 3 3H 2 O Fe( OH )3( s ) 3H
(3)
2 Fe 3 2 H 2 S FeS 2( s ) Fe 2 4 H
(4)
Mn 2 0.5O2( aq ) 2 H Mn 4 H 2 O
(5)
Mn 4 2 H 2 O MnO2( s ) 4 H
(6)
Theoretically, a wetland only has limited capacity to reduce acidity and/or retain the metals via abiotic routes. Once their full capacity is reached, the amount of metals removed from the wastewater can rapidly decrease. Biotic routes of metal removal (especially microbe-induced oxidation and sulphate reduction) are desirable. A generalised representation of the microbial sulphate reduction process is given in equations 7-8, where CH2O represents a small organic molecule, and M represents a heavy metal species [2]. In columns B1 and B2, although organics are available, metal removals may have taken place primarily via abiotic routes, as the pH value increased.
SO4 2CH 2 O H 2 S 2 HCO3
(7)
H 2 S M 2 MS 2 H
(8)
In this study, plant uptake is believed to have a negligible impact on the metal removals, especially in the first stages of the wetland systems. Significant uptake of dissolved metals into plant tissues would have made pollutant removal rate to be steady, regardless of loading, 18
which was not observed from the experiment data. The role of vegetation in constructed wetlands has always been debatable. Some researchers indicated that dissolved metals can be absorbed to the anionic sites of plant cell walls, such that plant tissues can absorb up to 200,000 times of heavy metals relative to their surrounding environment [44], metal immobilization into wetland media can be higher in vegetated systems [45], and plant roots may buffer the impact of toxic pollutants on microbial communities [46]. On the contrary, other researchers have suggested that the plants may in fact have a detrimental effect, due to increased mobility of metals within the rhizosphere [47].
5. CONCLUSIONS
Lab-scale experiments demonstrated that wetland columns employing with crushed shell grit media have significant capacity for removing dissolved metals from acidic wastewater. Over 99% percentage removals were obtained for Cu, Fe, Mn, Pb and Zn, under mass loading rates of 0.223, 9.72, 1.038, 0.089 and 0.644 g/m3d, respectively. SEM-EDS and IR analyses of the grits indicated slight changes of chemical compositions, after the grits were used as wetland media. Linear correlations between pH changes and percentage reductions of the dissolved metals indicated that they were predominantly removed via abiotic routes.
ACKNOWLEDGEMENTS
The authors thank Mr Cameron Wycherley and Mrs Ruilan Liu at James Cook University who assisted the experiment and water sample collection. Mr. Zhe Jia at Edith Cowan University is thanked for assisting XRD analyses of the shell grits. This study was supported
19
by CAS/SAFEA International Partnership Program for Creative Research Teams and ECU Collaboration Enhancement Scheme.
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FIGURE CAPTIONS
Figure 1: A schematic diagram of the hybrid wetland columns
Figure 2: The mass removal rates of dissolved metal vs. time (m3 represents the total volume of packed media in each hybrid wetland system).
Figure 3: Mean mass removal rates of dissolved metals in individual vertical flow (upper) and horizontal flow (lower) columns. Error bars represent standard deviations.
Figure 4: SEM images of the sea shell grits.
Figure 5: XRD scan image of the surface layers of unused sea shell grits.
Figure 6: IR diagrams of unused and used shell grits.
Figure 7: Log (Ci/Co) vs. the change of pH values in the vertical flow columns. An increase in the pH is represented by a positive x-axis value.
Figure 8: Log (Ci/Co) vs. the change of pH values in the horizontal flow columns.
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Shell grits proved to be effective wetland media for treating acidic wastewater
Over 99% removals of dissolved Cu, Mn, Fe, Pb and Zn were obtained
The metals were removed primarily via abiotic routes
The mass compositions of shell grits changed marginally in wetland columns