Date of Award

Fall 1980

Project Type

Dissertation

Program or Major

Chemistry

Degree Name

Doctor of Philosophy

Abstract

The metal ion complexing chemistry of organic ligands commonly found in drinking water sources is the object of the studies in this dissertation. These ligands, called humics as a class, were modelled in some cases by a soil-derived humic material called fulvic acid. The role of natural humics in the transport and speciation of dissolved metal ions in drinking water sources, particularly health hazards like Cd('2+), is investigated. Some properties of soil-derived fulvic acid (SFA) were also elucidated.

One study modelled a drinking water clarification and decolorizing process using aluminum hydroxide coagulation to measure Cd('2+), Zn('2+) and Cu('2+) removal from solution by the coagulation treatment. The effect of SFA on these metal ion removals was determined in a statistically designed experiment where the relative effects of changing metal ion concentration, Al('3+) concentration, solution pH, and SFA concentration on metal ion removal were revealed. Over the variable levels used in the experiment, increasing pH from 5 to 7, and increasing Al('3+) dosage caused the greatest overall increase in Cu('2+), Cd('2+) and Zn('2+) removal. Fulvic acid, representing naturally-occurring humics, aided metal ion removal when enough aluminum coagulant was used to remove all dissolved fulvic acid. When insufficient coagulant was used and some fulvic acid remained in solution, the humic material stabilized dissolved metal ions by formation of a soluble metal ion complex and resolubilized metal ions ordinarily coprecipitated with the aluminum hydroxide floc. Cu('2+) is very effectively removed on average (> 90%) under the experimental conditions, but Cd('2+) and Zn('2+) are not (< 20%).

To perform the metal ion removal experiment, and to filter natural water samples with no substantial metal ion losses due to wall effects or contaminations, a microfilter apparatus was assembled and evaluated. The apparatus has all plastic surfaces for solution contact and allows vacuum filtration directly into a polypropylene receiving vessel, thereby eliminating the need for a filter flask. Cu('2+) losses and contaminations during filtration of standard Cu('2+) solutions through the plastic filter apparatus or a glass filter support, with cellulose acetate or polycarbonate filter membranes, were measured in a statistically designed experiment. The results show small Cu('2+) losses with the plastic filter support and either membrane (0-13% losses) compared to the glass support with either membrane (25-79% losses). The use of an all plastic filter apparatus for the filtration of natural water samples is recommended to minimize changes to the native levels of dissolved metal ions.

A dialysis separation technique, where complexed and unbound metal ions are distinguished in the determination of natural freshwater metal ion binding capacities, was evaluated in another study. The dialysis technique metal ion binding results could not be distinguished from potentiometric titration results in a statistical comparison of the two methods. Nomenclature to describe metal ion binding in natural water systems was suggested. A brief literature survey of metal ion binding studies was described. The dialysis technique was applied to the measurement of 6.25 (mu)M EDTA, 15.5 (mu)M SFA and seven natural water sample capacities of Cu('2+) and Cd('2+) complexation. The metal ion binding capacities of SFA increased with pH and Cu('2+)-SFA capacities were greater than Cd('2+)-SFA capacities at the same pH. The Cu('2+) binding ability of the natural water samples correlates negatively with their alkalinity, pH, hardness and conductance. Cd('2+) binding capacities did not correlate well with any water sample characteristic.

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