Year of Publication

2010

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Engineering

Department

Civil Engineering

First Advisor

Dr. Y.T. (Ed) Wang

Abstract

The removal of arsenic toxicity from water is accomplished by a preliminary preoxidative step transforming the most toxic form, arsenite (As (III)), to the least toxic form, arsenate (As (V)). The potential of As (III) oxidation to As (V) was initially investigated in batch reactors using the chemoautotrophic Thiomonas arsenivorans strain b6 under varying initial As (III) and cell concentrations and at optimal pH and temperature conditions (pH 6.0 and temperature 30°C). The strain b6 completely oxidized As (III) to As (V) during exponential growth phase for lower levels of As (III) concentrations (≤ 100 mg/L) but continued into stationary phase of growth for higher levels (≥ 500 mg/L). Other important factors such as oxygen and carbon limitations during biological As (III) oxidation were also evaluated. The biokinetic parameters of the strain b6 were estimated using a Haldanesubstrate inhibition model with the aid of a non-linear estimation technique.

Microbial As (III) oxidation was further investigated in continuous-flow bioreactors (CSTR and biofilm reactor) under varying As (III) loading rates. Both the reactors achieved As (III) oxidation efficiency exceeding 99% during the steady-state conditions. The reactors were also able to recover from an As (III) overloading phase establishing the resilient nature of the microorganism. The basic mass balance expressions on As (III) and biomass along with the Monod model were used to linearly estimate the biokinetic parameters in the CSTR study. However, in the biofilm study, a steady-state flux model was used to estimate the same parameters. The performance of the model was very good in simulating the transient and steady-state conditions.

Finally, the potential application of one-stage and two-stage reactor systems was investigated for the near complete removal of arsenic. Activated alumina was used as the adsorbent for the As (V) produced by the biological oxidation of As (III). The two-stage reactor process performed better than the one-stage reactor system in lowering the arsenic level below the detection limit (1 mg/L) for at least eight days of operation. However, pH fluctuations and probable competition from ions such as PO43- , SO42-, and Cl- severely impacted the performance of the reactors. Further study is needed to improve the overall efficiency of the reactor systems for achieving complete removal of arsenic for a longer operating time.

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