Evaluation of effectiveness of phosphate and zinc as scale inhibitor in delaying precipitation of caco3

Abstract

Precipitation of CaCO3 has long been a concern in water treatment and distribution systems; however, this problem has been identified only recently in some areas (e.g., in Meherpur) of Bangladesh. The potential methods for control of CaCO3 precipitation in the context of Bangladesh include lowering of pH, removal of Ca, and delaying/preventing precipitation using scale inhibitor.Scale inhibitors can prevent or delay precipitation by adsorbing onto the active crystal growth sites of calcite (a polymorph of CaCO3), thereby delaying or reducing precipitation. This study presents an assessment of the potential effectiveness of phosphate and zincas scale inhibitors in reducingthe precipitation of CaCO3 in water. This work involved laboratory batch experiments in electrolyte (NaCl) solutionin deionized water(in order to eliminate possible effects of constituents that could interfere with CaCO3 precipitation, e.g., Mg2+, silica)containing calcium chloride and sodium bicarbonate. Phosphate was added to the solution as potassium dihydrogen phosphate solution, and zinc was added as zinc chloride solution. Subsequently, similar batch experiments were carried out with natural groundwater. In addition, behavior of the experimental systems under “equilibrium” condition was simulated using MINEQL+ chemical equilibrium model. Effect of PO43- and Zn2+ on precipitation of CaCO3 was evaluated as a function of inhibitor concentration (varying from zero to 5 mg/l), mixing time (varying from 30 minutes to 5 hours), and the degree of super-saturation. In electrolyte (NaCl) solutionin deionized water, for a particular degree of super-saturation and mixing time, precipitation of CaCO3 decreased as inhibitor(either phosphate or zinc) concentration increased.The effect of phosphate or zinc in inhibiting precipitation of CaCO3has been found to be more pronounced at lower concentrations of the inhibitors. One interesting observation from theseexperiments was that the inhibition effect of Zn increased with time, while that of phosphate decreased with time. But this trendbecomes less pronounced at higher inhibitor concentrations. However, in experiments with natural groundwater, neither phosphate nor zinc could reduce the precipitation of CaCO3(s); in fact, precipitation slightly increased with addition of phosphate and zinc. From MINEQL+analysis, it appears that this could be due to possible enhanced precipitation of CaCO3(s) [and also dolomite, CaMg(CO3)2(s)] in the presence of silica (which is present in high concentration in natural groundwater), possible precipitation of Ca-PO4 solids, and possible formation of ZnCO3(s) (that would limit availability of Zn2+). Therefore, phosphate or zinc cannot be recommended as inhibitors (of calcium carbonates) in natural groundwater; more studies are needed focusing on the effect of silica on precipitation of calcium carbonate and precipitation of other solids in complex groundwater systems.