Interactions of silver with wastewater constituents

The interactions of silver ion with wastewater constituents including chloride, sludge particulates, and dissolved organic matter (DOM) were investigated using fresh and processed sludge samples from various municipal wastewater treatment plants. Fresh sludge samples were used to examine the silver removal characteristics in the presence of chloride. Processed sludge samples that were subjected to a deionized-water washing procedure to remove the chloride ion were used to examine the interactions among Ag(I), sludge solids, and DOM. Results showed that, under field conditions, most of the silver can be removed through precipitation with chloride and adsorption by sludge particulates. Results obtained using washed sludge samples showed that Ag(I) adsorption by sludge particulates increases with the increase of pH in the acidic pH range, but decreases with the increase of pH in the alkaline pH range. Since the DOM concentration increases significantly with the increase of pH in the alkaline pH region, it can be concluded that the formation of Ag(I)-DOM complexes reduces the Ag(I) adsorption by sludge. Fitting of the Langmuir isotherm to the adsorption data indicated that adsorption constants have a slight increase with the increase of pH in the acidic pH range, but decrease significantly with the increase of pH in the alkaline pH range. The decrease of the Ag(I) adsorption constants in the alkaline pH range can be attributed to the formation of Ag(I)-DOM complexes which weaken the Ag(I) binding strength by sludge surface sites. The fitted isotherms also showed that the Ag(I) adsorption capacity increases with the increase of pH in the entire pH range.