Stabilization of Mercury in River Water and Sediment Using Biochars
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Mercury (Hg) is a common contaminant in air, oceans, lakes, rivers, soils, and sediments as elemental, inorganic and organic forms. Organic Hg (e.g., methylmercury (MeHg)), which is much more toxic than other forms and can cause central nervous system defects, and can be converted from inorganic or elemental forms by microbes. Efforts have been made to decrease the production of MeHg by dredging Hg-contaminated sediment, in situ capping, or by converting Hg to stable forms using reactive media to decrease its bioavailability. However, current remediation techniques are commonly burdened by high capital costs or by secondary contamination. The application of biochar, which is an alternative to activated carbon and can promote Hg stabilization, may be a cost-effective reactive material for managing Hg-contaminated sites. This thesis describes laboratory batch and anaerobic microcosm experiments for evaluating the addition of biochar for Hg stabilization in water and sediment. Laboratory batch experiments were conducted to evaluate the treatment of Hg in aqueous solution at environmental concentrations using 36 biochar samples. The biochars were prepared from various feedstocks (wood, agricultural residue, and manure) pyrolyzed at different temperatures (300, 600, and 700oC). The results indicate >90% removal of total Hg (THg) aqueous concentrations was achieved in systems containing biochars produced at 600oC and 700oC (high T) and 40-90% removal for biochars produced at 300oC (low T). Sulfur (S) X-ray absorption near edge structure (XANES) spectra obtained from biochars with adsorbed Hg were similar to those of washed biochars. Micro-X-ray fluorescence (μ-XRF) mapping results indicate that Hg was heterogeneously distributed across biochar particles. Extended X-ray absorption fine structure (EXAFS) modeling indicates Hg was bound to S in biochars with high S contents and bound to O and Cl in biochars with low S contents. These experiments provide information on the effectiveness and mechanisms of Hg removal in aqueous solutions. Components released from the biochars during these batch experiments include anions, cations, alkalinity, organic acids (OAs), dissolved organic carbon (DOC), and nutrients. These components may influence the speciation of Hg (e.g., complexation with Hg), facilitate the transport of Hg, promote the growth of organisms, and stimulate the methylation of Hg. The analyses show elevated concentrations of anions (e.g., for SO42- up to 1000 mg L-1 from manure-based biochars) and nutrients (NO3-, PO4-P, NH3-N, and K) were observed in the majority of aqueous solutions reacted with the biochars. The release of alkalinity OAs and DOC was highly variable and dependent on the feedstock and pyrolysis temperature. Alkalinity released from wood-based biochar was significantly lower than from others. Concentrations of OAs and DOC released from low-T biochars were higher than from high-T biochars. The carbon (C) in the OAs represented 1-60% of the DOC released, indicating the presence of other DOC forms. The C released as DOC represented up to 3% (majority <0.1%) of the total C in the biochar. The modeling analyses of Hg-dissolved organic matter (DOM) complexes suggest that the majority of Hg was likely complexed with thiol groups. Long-term microcosm experiments were carried out by co-blending Hg-contaminated sediment, biochar, and river water under anaerobic conditions, followed by monitoring for more than 500 days. Factors that may control the evolution of THg and MeHg in aqueous solution were evaluated. Furthermore, the Hg spatial distribution and speciation associated with biochar particles were investigated. The results indicate aqueous concentrations of 0.2-µm THg for co-blended systems were less than for sediment controls during the experimental period; 0.45-µm THg concentrations of co-blended systems decreased by 20-92% compared with the controls. Two peaks in MeHg concentrations were observed at early (~40 days) and late (~400 days) times. These peaks corresponded to the onset of iron and sulfate reducing conditions (early peak) and methanogenic conditions (late peak). The MeHg concentrations in the amended systems were less than those observed in the controls, except for late peaks in the microcosms containing a high-T oak wood biochar and switchgrass (600oC) biochars. Pyrosequencing analyses showed shifts in percentages of microbial sequences associated with fermenters, iron-reducing bacteria (FeRB), sulfate-reducing bacteria (SRB), and methanogens that were correlated to changes in C sources (DOC, OAs, and alkalinity) and electron acceptors (NO3-, Fe, and SO42-). Twelve mercury methylators grouped into SRB, FeRB, methanogens, and fermenters based on the pyrosequencing results were detected in the systems. The fate of the removed Hg was studied using X-ray absorption spectroscopy. Hg was observed to co-occur with S, Cu, Fe, Mn, and Zn on the surface and inside the biochar particles as indicated by µ-XRF mapping. EXAFS modeling showed that Hg was present in an oxide form on the surface of an iron (hydro)oxide particle from fresh sediment and in Hg-sulfide forms for the Hg-rich areas of biochar particles after co-blending with sediment. Sulfur XANES showed the presence of sulfide in these biochar particles. After the addition of biochars, a fraction of the Hg in unstable forms (e.g., dissolvable, HgO, colloidal, nano) in the sediment was likely converted to Hg-sulfide forms. These more stable forms bound on and within the biochar particles are anticipated to have decreased mobility and decreased bioavailability relative to the Hg in the unamended sediment. The detected fluorescence intensity in conventional XRF mapping represents the sum of information along the path of microbeam penetration into the sample, usually limited by the thickness of thin sections. To overcome this limitation, confocal X-ray micro-fluorescence imaging (CXMFI) was applied to delineate the three-dimensional spatial distribution of elements accumulated within switchgrass biochar particles. The maps indicate that Hg, Fe, Ti, Cr, Mn, Co, Ni, Cu, Zn, and As were distributed within the structural material of low-T switchgrass biochar, whereas these elements were preferentially observed on the surfaces of high-T switchgrass biochar. These observations suggest that the accumulations of Hg and other elements in biochars may be through an early-stage diagenetic process, rather than a simple surface adsorption reaction. Hard-wood biochar was sulfurized using CaSx (CPS-CL2) and a dimercapto-related (DMC-CL2) compound to further promote Hg removal. With an initial THg concentration of 17,800 ng L-1, final THg concentrations were 40 and 7.0 ng L-1 using CPS-CL2 and DMC-CL2, and 370 ng L-1 using unmodified hard-wood biochar; for an initial THg concentration of 245,000 ng L-1, final concentrations were 74 and 110 ng L-1 using CPS-CL2 and DMC-CL2, and 5,700 ng L-1 using the unmodified form; for an initial concentration of 4,960 µg L-1, final concentrations were 10 and 29 µg L-1 using CPS-CL2 and DMC-CL2, and 170 µg L-1 using the unmodified biochar. The THg removal percentages were >99.5% using the modified biochars for these three initial concentrations. A suite of synchrotron-based techniques was applied to characterize biochars loaded with Hg. S XANES analyses indicates polysulfur- and thiol-like structures were present in the modified biochars. The accumulated Hg was distributed primarily on the edges of the modified biochar particles as indicated by µ-XRF mapping and CXMFI techniques. Hg EXAFS analysis shows Hg was bound to S in sulfurized biochars. This study demonstrates that biochars may be effective reactive media for Hg removal from aqueous solution and for Hg stabilization in amended sediment. The extent of Hg removal was further enhanced after sulfurization of the biochar. The results also suggest that the Hg stabilization process may be effective for years or longer through strong binding with biochar particles.
Cite this work
Peng Liu (2016). Stabilization of Mercury in River Water and Sediment Using Biochars. UWSpace. http://hdl.handle.net/10012/11081