Preparation and evaluation of some anionic starch derivatives as flocculants

Six types of anionic starch derivatives, viz. carboxymethyl starch, carboxyethyl starch, poly(acrylic acid)-starch graft copolymer, starch-2-hydroxylpropylcitrate, starch-2-hydroxypropylphosphate and starch-2-hydroxypropylsulfate, were prepared and evaluated as flocculants. The flocculation efficiency was monitored via estimation of the optimal dose, transmission % and weight removal %. The different factors affecting the flocculation were studied. These factors include the type of acidic group, the length of the chain bearing the acidic group, the chemical formula and molar mass of the flocculant, the acid content and the pH. The flocculation efficiency of various anionic starches depends on (a) the acidic group type and follows the order: sulfate > phosphate > carboxyl; (b) the length of the alkyl chain bearing the carboxyl group in the order: starch-2-hydroxypropylcitrate > carboxyethyl starch > carboxymethyl starch; (c) the chemical formula which indicate that the flocculants prepared via grafting have a higher flocculation efficiency than those prepared by etherification. The flocculation efficiency increases with increasing the acid content and/or decreasing the molar mass of the flocculant. Flocculation efficiency at pH 8 is higher than tat pH 6.