Adsorption coagulation

An evaluation of the fate of trace metals in surface and sub-surface waters requires more detailed consideration of complexation, adsorption, coagulation, oxidation-reduction, and biological interactions. These processes can affect metals, solubility, toxicity, availability, physical transport, and corrosion potential. As a result of a need to describe the complex interactions involved in these situations, various models have been developed to address a number of specific situations. These are called equilibrium or speciation models because the user is provided (model output) with the distribution of various species. 

Treatment of dye wastewater involves physical, physico-chemical, chemical, and biological methods. Physical processes are dilution, filtration, and gamma radiation. Physico-chemical includes adsorption, coagulation, flocculation, precipitation, reverse osmosis, ion exchange, etc. 

Chemical and physical methods including adsorption, coagulation-flocculation, advanced oxidation and electrochemical methods are very efficient in color removal. These methods are quite expensive and have operational problems. High sludge formation, regeneration requirement and cost of adsorbent make adsorption an unattractive method for decolorization purposes. So biodegradation begins to play an important role in decolorization of azo dyes. 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

The colloidal silica dispersions are destabilized with hydrolyzed Al(III) primarily because of adsorption of polyhydroxo aluminum cations on the colloid surface which reduces the incipiently negative surface potential of Si02 colloids (adsorption coagulation). The pertinent solution variables describing the destabilization reaction are pH, total aluminum concentration Ct, and the ratio of aluminum dosage to the colloid surface concentration S. 

L. K. Wang, Adsorption coagulation and filtration make a useful treatment combination, part I. Water and Sewage Works, 123(12), 42 7 (1976). 

Healy, T.W. and lellet. V.R., Adsorption-coagulation reactions of Znfll) hydrolyzed species at the zinc oxide-water interface, J. Colloid Interf. Sci., 24, 41,1967. 

Since As(V) can be adsorbed more strongly onto adsorbents than As(III), the oxidation of As(III) can be exploited and integrated with UF and MF systems at low pressure and adsorption/coagulation media for effective and low cost As removal. 

Heneghan, K. S., and Clark, M. M. (1991). Surface water treatment by combined ultraliltration/PAC adsorption/coagulation for removal of namral organics, turbidity and bacteria. In Proceedings of the Seminar on Membrane Technologies in the Water Industry. American Water Works Association, Orlando, FL. 

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