Date of Degree

9-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Zhongqi Cheng

Committee Members

Theodore Muth

David Seidemann

Pengfei Zhang

Brian Mailloux

Subject Categories

Environmental Health and Protection | Geochemistry | Water Resource Management

Keywords

ZVI, Arsenic, sulfide, pyrite, SRB, gypsum, arsenite, zero valent iron

Abstract

Bench scale microcosm incubation was first conducted in two stages to examine the removal of arsenic (As) by zero-valent iron (ZVI) in the presence of sulfate reducing bacteria, gypsum as well as organic substrates under anaerobic condition. About 98% of dissolved As was removed from solution within 20 days with concurrent decrease of dissolved sulfate and growth of bacterial population. Mineralogy analysis using SEM-EDS found the formation of iron sulfide and Fourier-transformed infrared (FTIR) spectroscopy analysis indicates that some As is associated with pyrite. The main iron corrosion product was found as ferrihydrite with traces of lepidocrocite. Sequential leaching shows that, on average, 38% of As can be mobilized by V phosphate, the portion of As associated with mixed amorphous phases. More than half of the As is in the crystalized ferric-(oxy)hyroxides and ~10% associated with crystalized sulfide phases. The structurally incorporated As is completely immobilized and not liable to release. Multi-stage column study was also conducted to understand the removal of As under dynamic flow conditions. Four columns were deployed with different flow rates and inflow chemistry. Arsenic removal experiment was performed in two separate stages, followed by leaching with phosphate to test the stability of retained As. The gypsum amended ZVI columns generally show good efficiency in removing As: 33~66% of total As fed into the columns was retained by the column, while 10~26% of retained As were leachable by phosphate. Effluent As and Fe levels suggest that As removal is mainly associated with FeS, in addition to the adsorption of As onto iron corrosion products. Our findings indicate that it is feasible to incorporate gypsum with ZVI and enhance the removal of As from aqueous solutions in either batch reactors or under flow conditions with microbial sulfate reduction. The fast adsorption of As onto Fe (oxy)hydroxides under aerobic condition is followed by precipitation of As with iron sulfides (under reduced condition) in the gypsum-amended ZVI system. The latter is more dominant at reduced conditions and is not subject to the interference of other anions.

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