An exploration of the removal of iron (fe3+) from aqueous solutions using ?-dicalcium silicate and ?-monocalcium silicate as alternative precipitants to calcium hydroxide

Abstract

The present work has studied the removal of iron (Fe3+) ions from solution using ?-dicalcium silicate (C2S), ?-monocalcium silicate (CS) and calcium hydroxide. The merits of these precipitants were analyzed by studying their effectiveness for removing iron (Fe3+) from solutions of ferric chloride and the properties of the flocs formed. The initial concentration of iron in solution was varied between 10 and 1000 mg l?1 Fe3+ for different additions of precipitant. The residual silicate particles upon hydrolysis appear to enhance the overall removal process in a manner different from the coagulating effect of calcium ions. Considerable enhancement in the rate of removal of iron resulted from use of more finely ground silicates. It is suggested that the reaction can proceed either by the specific adsorption of polynuclear ferric hydroxo complexes on to the surface of negatively charged silica rich layers of C2S, formed by loss of Ca2+ upon hydrolysis or by direct exchange of Fe3+ with Ca2+ in the charged layer. At the end of the second minutes of the settling period, remaining iron concentrations were 12.0 mg l?1 and 525.0 mg l?1, respectively when 3.90 g l?1 of C2S and 2.10 g l?1 of calcium hydroxide were applied to solutions containing 1000 mg l?1 Fe3+. By the end of second minutes of settling period, the remaining iron concentrations were 40 mg l?1 and 78 mg l?1, respectively when 0.39 g l?1 of C2S and 0.21 g l?1 of calcium hydroxide were applied to solutions containing 100 mg l?1Fe3+ in a sedimentation tank designed on the basis of a surface loading rate of 48 m3m?2d?1. In the case of C2S, the specific resistance to filtration and the Capillary Suction Time (CST) values of the sludge produced were lower than that when calcium hydroxide was used thus indicating better dewaterability characteristics. The filter yields of the sludges from C2S and calcium hydroxide treatment of 10, 100 and 1000 mg l?1 iron solutions were estimated as 27.0, 15.8, 23.2 kg m?2h?1 and 12.8, 6.5, 7.8 kg m?2h?1, respectively. © Publications Division Selper Ltd., 1996.