Stabilization of Mercury in River Water and Sediment Using Biochars

by

Peng Liu

A thesis

presented to the University of Waterloo

in fulfillment of the

thesis requirement for the degree of

Doctor of Philosophy

in

Earth and Environmental Sciences

Waterloo, Ontario, Canada, 2016

© Peng Liu 2016 Author’s Declaration I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners.

I understand that my thesis may be made electronically available to the public.

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Abstract 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

iii 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

2- methylation of Hg. The analyses show elevated concentrations of anions (e.g., for SO4

-1 - up to 1000 mg L 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 car