Fate of heavy metals and inorganic compounds during sediment resuspension

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1 Fate of heavy metals and inorganic compounds during sediment resuspension Investigators: Mason B. Tomson, Louis J. Thibodeaux, and Amy T. Kan Institution: Rice University, Houston, TX and Louisiana State University, Baton Rouge, LA Project period: Jan. 1, 22-Dec. 31, 23 Current Project amount and period: $86,25, Oct 1,21 to Dec.31, 22 Research category: Fate and transport of heavy metals Goals The objective of this study is to understand the dynamics and fate of heavy metals during resuspension event. Five specific objectives are: 1. To test the hypothesis that sorption and desorption of heavy metals from natural sediments are not the reverse of each other, that is, sorption/desorption of toxic metals exhibits hysteresis. 2. To test the hypothesis that though there is a number of unique combinations of sediments, it is possible to identify a few key parameters that can be used to predict heavy metal release and thereby the potential for ecological exposure. Simulate resuspension events in the laboratory, study the kinetics of heavy metal release during resuspension and identify a few key parameters controlling contaminant transportation; and 3. Heavy metal release has been found to be multiphasic, with one set of process controlling the early resuspension and a completely different set of processes important at times in excess of a few hours. Both rate processes need further study. 4. Particle fines are released upon initial resuspension. These fines do ot resettle rapidly yet they have been found to contain many times the heavy metal load on a mg/g basis. These fines will facilitate the migration of many of the most dangerous heavy metals. Furthermore, it is just these fines that will impact the existing organisms. Understanding these issues will be a priority. 5. Lastly, the possibility to identify a potential remediation plan that can be used to prevent heavy metals release during resuspension events and dredging will also be investigated. Attention has been and will continue to be focused on the impacts of resuspension on fines versus settleable particles and short versus long time frames. Rationale In resuspension of contaminated sediments heavy metals, such as Pb, Cd, Cu, and As, represent several special challenges and will be the focus of this HSRC research. For example, DiGiano, et al.(1) observed that the dredging elutriate test (DRET) protocol corresponds reasonably well to field dredge tests for PCBs and probably for other organic contaminants during dredging, but not for the heavy metals. This notion was further explained by Myers et al.(2): but this approach is not recommended for application to release of dissolved metals during dredging because the rapid and pronounced change in redox and the complicated environmental chemistry of metals make equilibrium

2 approaches highly unreliable and uncertain. During resuspension generally the largest physical-chemical effect, with respect to heavy metal sorption, is the change in redox of the freshly disturbed sediments. At the point of dredging the sediments are suspended in the river bottom and there is an immediate increase in solid surface area and corresponding immediate change in the physical chemical parameters that characterize the water. Following these immediate changes there will be several time scales that are applicable, 1. the slower redox processes; 2. the desorption kinetics; and 3. the relative rates of redeposition of the sediment particles. The objective of this study is to understand the dynamics and kinetics of heavy metal release processes and to derive methods to reduce heavy metal release during dredging and resuspension. Approach Methodology Adsorption and desorption experiments Studies of the adsorption/desorption of cadmium, lead and zinc by Utica and Lula sediments were carried out in batch experiments and the reversibility of cadmium, lead and zinc adsorption on these sediments were investigated by comparing observed desorption isotherms and edges to their respective adsorption isotherms and edges. Cd, Pb and Zn solutions (4 ml) at various concentration levels (15-5 mm) were transferred to 5-ml plastic tubes and 1 g of dry Utica sediment was added. The sediment-water mixtures were then sealed and placed on a slowly rotating rack that provided gentle (4 rpm) end-over-end mixing during the reaction period. At the end of each experiment, reaction tubes were centrifuged and the supernatant solutions were withdrawn with a syringe and filtered through.2 mm Nalgene syringe filters (SFCA). The filtered samples were then acidified with 1% nitric acid analyzed for solution metal concentrations. Desorption experiments were conducted in two different ways; by replacing 3 ml supernatant with background electrolyte solution, and by lowering the solution ph. Desorption experiments were also conducted by replacing supernatant solutions with EDTA-containing electrolyte solution to investigate the desorption mechanisms. The effect of the washing procedure (removing colloidal materials) on heavy metal adsorption reversibility has been studied by prewashing the sediments before initiating adsorption and subsequent desorption experiments. Sediment analysis For determination of heavy metal binding, we used a modification of the sequential extraction method of Tessier et al. (1979). The fraction of each metal in each sediment fraction (1. Exchangeable; 2. Carbonates; 3 iron and manganese oxides; and 4 organic matter and pyrite) were determined. Reactive metal concentrations were determined following a 24-hour extraction in 1N HCl at a solid/solution ratio of 1:5 (Huerta-Diaz and Morse, 199). The acid volatile sulfide (AVS) and simultaneously extracted metal (SEM) concentrations were determined following the method of Simpson (21). Total organic and inorganic carbon were determined by combustion and coulometric titration. Pore water and suspended particulate matter from Trepengier sediment was also analyzed for heavy metal concentration. The heavy metal in suspended particulate matter was collected on a preweighte filter paper (.45 mm) and extracted with 1 N HCl for 24 hours to obtain the reactive metal concentration.

3 Resuspension experiments Resuspension experiments were conducted on two sediments, Trepangier bayou and Lake Charles sediments at various particle concentrations (2 and 4 g/l). Trepangier bayou sediment was collected in the summer of 22 and kept frozen to maintain its anoxic character. Lake Charles sediment was collected in 1995 and kept refrigerated till present. These two sediments were resuspended in aerated.1m NaCl solution by an overhead propeller stirrer at room temperature. The experiments were monitored for 1 to 4 weeks. The solution ph and Eh were measured regularly during the experiments. Water samples were collected as a function of time (15, 3 min followed by 1, 3, 5, 24, 72 h) and episodically over the next few weeks (1, 2, 3, 4 weeks) for the determination the concentration of 25 inorganic compounds including Fe, Mn, Al, Ca, Mg, Zn, Pb, Cd, Cu, As, P, Cr and S. In subsequent samplings, the samples were taken from the supernatant solution (~ 2 ml) and the amount of solution withdrawn was replaced by an equivalent volume of.1 M NaCl solution to maintain a constant solid/solution ratio. The stirrer was stopped for 2 to 3 minutes prior to sampling to allow solids to settle. Dissolved heavy metal and the inorganic compound concentrations were measured by either ICP/OES (major elements) or ICP/MS (trace elements). Two methods have been used to prevent heavy metal release, CaCO 3 addition for ph control and polymer addition for fines control. Resuspension experiments were also conducted on the Trepangier sediment with the addition of CaCO 3 (12%). The effect of polyacrylic acid (PAA) on heavy metal desorption from natural sediments Utica and Trepangier bayou sediments were used as sorbents in these desorption experiments. Adsorption experiments were initiated by adding ~1 ml of Cd stock solution (2.5mM) into 2 g of sediments in a 1-ml plastic bottle. The bottle was shaken by hand to mix the solution and sediment and left overnight to reach adsorption equilibrium. At the end of adsorption, 2 g of slurry was transferred into 5-ml plastic bottles. Different amounts of a polyacrylate stock solution (45, MW, 9.47 g/l, ph 7.3) were then added into each bottle and left overnight. To begin the desorption experiment, 4 ml of electrolyte solution was added to each bottle and the bottles were tumbled at 3 rpm for 24 hours. The polylacylate concentrations of the solutions were 23, 115, 227, 1137, and 1867 mg/l, respectively. At the end of each experiment, reaction tubes were centrifuged and the supernatant solutions were filtered and then analyzed for solution metal and polyacrylate concentrations. Accomplishments 1) Reversibility of heavy metal adsorption on natural sediments. The percent adsorption of cadmium, zinc and lead on Utica sediment after desorption by lowering the solution ph are shown in Fig. 1. The extent of metal desorption is minimum at high ph and it increases significantly with decreasing solution ph. The desorption edges of cadmium, zinc and lead are almost identical to their respective adsorption edges suggesting that heavy metal adsorption on Utica sediment is completely reversible. In addition, more than 9% of adsorbed cadmium and lead are released from Utica and Lula sediment after four successive desorption steps with 2mM

4 EDTA solution suggesting that cadmium and lead are readily desorbable from both sediments. % adsorbed desorption starting point desorption adsorption ph % adsorbed desorption adsorption starting point ph % adsorbed desorption adsorption starting point ph Figure 1 Percent adsorption of Cd, Pb and Zn on Utica sediment (25g/l) in.1 M NaNO M NaN 3 solution as a function of ph after desorption by lowering solution ph (o). a) Cd on Utica at [Cd] init = 1 mm; b) Zn on Utica at [Zn] init = 1 mm; c) Pb on Utica at [Pb] init = 36 mm. For comparison, Cd, Pb and Zn adsorption edges are also present as solid circles. Cadmium adsorption /desorption experiments on "washed" Utica and Lula sediments were conducted following the same procedure as the "unwashed" sediments, where the "washed" sediments were washed several times before the initiation of adsorption experiment to remove the suspended colloidal matter. The amount of cadmium sorbed by "washed" sediments is higher than that observed on unwashed Utica at ph 4-7 (Fig.2). With each additional sediment washing, higher cadmium adsorption is observed. The lower adsorption of cadmium on "unwashed" sediments may be explained by the presence of colloidal materials in the sediments. Apparent unconformity of adsorption and desorption isotherms was observed by the replaced supernatant method. However, the agreement between adsorption and desorption isotherms has been improved significantly after washing sediments several times (Fig. 3), indicating the important role that colloidal materials play on heavy metals mobility in sediments. Cd ads (mmol/kg) washed 3 washed 1 unwashed Cdads (mmol/kg) Cd in solution (mmol/l) Cd in solution (mmol/l) Figure 2: A comparison of adsorption isotherms of adsorption and Cd on prewashed and unwashed Utica sediment prewashed and Figure 3: A comparison of desorption isotherms of Cd on

5 3 times prewash; prewash;s: adsorption prewash. unwashed Utica =: adsorption, o: desorption, three times no prewash; D: desorption, no 2) Impact of Dredging of Anaerobic Sediments Table 1a (shown in tomson2.pdf) shows the change of solution ph and redox potential during the resuspension of 4 g/l anaerobic sediment from Trepangier bayou. ÂSEM/AVS of this sediment is about.4, indicating that it is a safe sediment according to the standards proposed by EPA for five metals (Cu, Cd, Ni, Pb and Zn). Nevertheless, after exposing the anoxic suspension to atmosphere for 4 weeks, redox potential increases constantly from about 24 to +56 mv and the solution ph decreases from 7.3 to 1.8. The dramatic decrease of the solution ph is mainly due to the oxidation of sulfide and it increases the mobility of heavy metals. As shown in Table 1a, metal concentrations in the solution increase up to 2 fold. After 5 days resuspension, the water become highly contaminated with respect to cadmium, lead, zinc, copper and arsenic (76, 447, 173, 31, 14 mg/l, respectively). However, the short-term release of both Fe and heavy metal in a few hours have a different release pattern when compares to the long term release over weeks, indicating different mechanisms affect heavy metal release in short versus long time frame. For resuspension experiment conducted on Trepangier sediment at 2g/l, a similar trend is found, but heavy metals were not released to the same extent. Similar resuspension experiments were conducted on the Trepangier sediment with the addition of 12% CaCO 3. As shown in Table 1b (shown in tomson2.pdf) and Figure 4, with the addition of the calcite, solution Eh and ph do not change as dramatically. The addition of calcite does not have a strong impact on the short term heavy metal release, yet has a large stabilization effect on sediment over the long term. Eh increased from about 255 to -97 mv and the solution ph stays near neutral ( ) during the resuspension for 8 days. In this case, heavy metal release to the overlying water is negligible (open symbols), while the release of heavy metal from the same sediment (filled symbols) could be several orders of magnitude higher without the amendment of calcite. For resuspension experiments conducted on Lake Charles sediments, little change in ph occurred and metals release was negligible, because Lake Charles sediments are partially oxidized

6 a. Eh (mv) b Time (hrs) ph Eh w/calcite Eh w/o calcite ph w/calcite ph w/o calcite Heavy Metal Conc. (ug/l) Cu, w/o calcite Cu, w/calcite Pb, w/o calcite Pb, w/calcite Cd w/o calcite Cd w/calcite As w/o calcite As w/ calcite Time (hrs) Figure 4. Comparison of ph, Eh, and heavy metal concentrations of a Trepengier sediment resuspension experiment with 12% (w/w) calcite added versus the control Trepengier resuspension experiment where no calcite was added. The filled symbols are data from Trepengier sediment and open symbols are from calcite amended Trepengier sediment. 3) Effect of polyelectrolyte on sediment resuspension and heavy metal desorption Heavy metal release during dredging and resuspension is largely due to the resuspension of fine colloidal particles. The fine colloidal particles typically contain high surface area and heavy metal concentrations. Polyelectrolytes are widely used as flocculants in solid liquid separation and sludge dewatering process and as soil amendment in agriculture to improve the retention of nutrients. Polyelectrolyte is also a strong adsorbent for heavy metals. It is proposed that such polymers may be useful in dredging or capping to aid both the dewatering and settling of sediments and to slow the release of heavy metal. Preliminary data indicate that the polymer may have significant impact on the sedimentation and heavy metal desorption. However, the condition of polymer application needs to be carefully designed. For example, both the amount of

7 polymer and aqueous ph will have strong impact on the process. Solution ph is critical toward all four processes, (1) the amount of polyacrylate adsorption to sediments, (2) heavy metal complexation to polyacrylate, (3) heavy metal adsorption/desorption from sediment and (4) heavy metal adsorption to surface bound polyacrylate. Preliminary research has been done to test the different classes of polymers (anionic, cationic, neutral polymers and surfactants) on sedimentation. Cd desorption from polyacrylate (45, MW, ph 7.3) amended sediments were also tested. As shown in Figure 5, the desorption of Cd from both Utica and Trepengier sediments are approximately 1 fold lower in the presence of 226 mg/l polyacrylate. However, much more Cd is desorbed in the presence of 1137 mg/l polyacrylate at ph 7.3, as expected. a. Cd Conc. (mg/l) Trepengier Utica 1 2 Polyacrylate conc. (mg/l) b. Cd Conc. (mg/l) Trepengier Utica Polyacrylate conc. (mg/l) Figure 5. Cd desorption concentrations from two sediments in the presence of various concentrations of polyacrylate at ph 7.3, where plot a is from -22 mg/l polymer and plot b is from -12 mg/l polymer concentrations. Conclusions: 1. The adsorption of cadmium, lead and zinc on natural sediments, using the current experimental methods, appears to be reversible. This supports "total metal" in sediments as a sediment quality criteria, unless it can be shown that only a fraction of the sediment-bound metals are available during resuspension. 2. Washed sediments appear to have greater adsorption capacities for heavy metals than unwashed sediments, suggesting that colloidal materials play an important role in metal adsorption and transportation. Future remediation studies should include investigations on how to control colloidal materials in the water systems. 3. The mobility of heavy metals in contaminated sediments is strongly affected by the solution ph. The risk of heavy metal release during resuspension event is determined by the balance of the acid producing capacity (APC) and acid neutralization capacity (ANC) of the sediment. In poorly buffered sediments, resuspension events may lead to a significant decrease of the solution ph, resulting in the enhanced mobility of most heavy metals. In contrast, if the sediment is well buffered (e.g., 12% calcite), the release of heavy metals from the contaminated sediment would be significantly reduced. ÂSEM/AVS is proposed by the EPA as a measure of toxicity of the sediments. However, if anoxic sediments are exposed to atmosphere, APC and ANC should also be measured.

8 Proposed Efforts over the Next Year: 1) Conduct resuspension experiments on more fresh sediments with distinct variation in sediment character such as AVS, carbonate content, etc. 2) Model the mobility of heavy metal release during resuspension event using a few predominant mechanisms. 3) Investigate the interactions between polymers and priority contaminants sediments (heavy metals and organic pollutants) and test the potential use of polymers to reduce the mobility of organic and inorganic pollutants during sediment resuspension events. Supplemental Keywords Adsorption, desorption, resuspension, heavy metal, ph, redox potential, polymer. Student Supported Yan Gao, research associate; Lili Cong, Ph.D. student; Xuekun Cheng, Ph. D. student. Publications and Presentations 1. More Realistic Soil Cleanup Standards with Dual-Equilibrium Desorption, Chen, W., Kan, A.T., Newell, C.J., Moore, E., and Tomson, M.B., Ground Water, Vol. 4, No. 2, Pp , Critical Evaluation of Desorption Phenomena of Heavy Metals from Natural Sediments. Gao, Y., Kan, A.T. and Tomson, M.B. (in progress). 3. Evaluating the Effect of Dual-Equilibrium Desorption on Soil/Groundwater Remediation Using Numerical Approach: A decision-support Model; Chen, W., Lakshmanan, K., Kan, A.T., and Tomson, M.B., Submitted for Publication to the National Groundwater Association, October, Heavy Metal Desorption from Natural Sediment, Gao, Y., Kan, A.T., and Tomson, M.B., Presentation at the 224 th ACS Fall National Meeting, to be held in Boston, Massachusetts, August 18-22, A Rapid Method to Measure the Desorption Resistant Fraction in Sediment Sorbed Contaminants; Cong, Lili, Kan, A.T., and Tomson, M.B., 224 th American Chemical Society National Meeting, Boston, Massachusetts, August 18-22, Adsorption/Desorption Characteristics of Naphthalene onto/from C 6 Surface, Cheng, X., Kan, A.T., and Tomson, M.B., NanoSpace 22 Future Technology Frontiers, The Fifth International Conference on Integrated Nano/Micro/Biotechnology for Space and Medical and Commercial Applications, Presentation at the Moody Gardens Hotel on Galveston Island, Texas, June 24-28, Nanomaterial Transport in the Environment, Tomson, M.B., School of Continuing Studies, Rice University, Houston, Texas, October 23, 22.

9 8. Aquatic Chemistry of Natural Waters; Rice University Brine Chemistry Consortium, Tomson, M.B., Conoco, Ponca City, OK, August 14, Environmental Impact of Carbon Nanomaterials; Cheng, X., Kan, A.T. and Tomson, M.B., Presented at the NanoDays 22 Conference for the Center for Biological and Environmental Nanotechnology (CBEN); October 14-15, 22 at Rice University. 1. Interaction of Heavy Metal with Inorganic Nanomaterials and Their Environmental Impact., Gao, Yan, Kan, A.T., Wahi, R., Colvin, V., and Tomson, M.B. Poster Presentation at the NanoDays 22 Conference for the Center for Biological and Environmental Nanotechnology (CBEN); October 14-15, 22 at Rice University. 11. Adsorption/Desorption Characteristics of Organic Compounds onto/from Nanomaterial Surfaces, Cheng, X., Kan, A.T., and Tomson, M.B., Presentation at the 224 th ACS Fall National Meeting, Boston, Massachusetts, August 18-22, 22.