Masters Theses

Date of Award

8-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

R. Bruce Robinson

Committee Members

Gregory Reed, Wayne Davis

Abstract

The purpose of this work was to improve iron sequestration in the presence of calcium. This purpose translated into studying the effects of hardness and several process variables on iron sequestration by the silicate-chlorine method. The process variables included testing undiluted N-silicate and sodium orthosilicate to treat iron in the presence of calcium; studying the effect of pH in the presence and absence of hardness; studying the effect of diluting N-silicate in the presence of calcium; exploring the possibility of using activated silica and polysilicates to sequester iron in the presence of calcium; and testing the consistency of results from the performed experimental procedure. Jar tests were performed according to the experimental procedure established previously with a few modifications. Groundwater was modeled by sparging silica-free de-ionized water with N2 gas. Typical amounts of alkalinity and iron were introduced into the model water. The dosage pH was fixed at 7.0 in all the experiments except the one in which the effect of dosage pH was tested. The jars were treated by the silicate-chlorine method and the success of the treatment was measured by monitoring specific parameters such as percent iron filterability, turbidity, color, pH, and total iron concentration, over a period of five days. A high iron filterability (>95%) and a low turbidity (<0.5 NTU) over a period of five days were considered characteristic of good iron sequestration. Results from the performed experiments showed that increased hardness adversely affected iron treatment by undiluted N-silicate; the effect of calcium was greater than that of magnesium. When compared to undiluted N-silicate, sodium orthosilicate failed to stabilize iron. Lower maintained pH (<6.5) favored better iron sequestration in the presence of calcium while a higher maintained pH (>7.5) favored better iron sequestration in the absence of calcium. There was no difference between the performance characteristics of 1/10, 1/50, and 1/100 dilutions of N-silicate at SiO2 concentrations of roughly 10 mg/l. A commercial sample of activated silica did not sequester the iron at all; also, activated silica prepared by partial neutralization with CO2 gas failed to sequester iron. Polysilicates prepared by acid acidification to a low pH gave high iron filterabilities, at dosages of 20 mg/l and above (as SiO2). At similar silica and calcium concentrations, a lower iron content gave higher iron filterability on day 0. A lower dosage pH (<6.5) was found to favor better iron sequestration in the presence of calcium. An experiment conducted to test the consistency of results by the practised experimental procedure showed fairly good consistency between results from identically treated bottles under three unique conditions.

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