Electrokinetic study of calcium carbonate and magnesium hydroxide particles in lime softening

Significant effort has been made to measure and understand the surface charge of CaCO₃ and Mg(OH)₂ particles. Many laboratory experiments have been reported on zeta potential of natural and prepared CaCO₃ and a few have also been published for Mg(OH)₂, however, there are very few reported measurements of zeta potential of CaCO₃ and Mg(OH)₂ particles at conditions relevant to lime softening, despite lime softening being a common and established process for water treatment. The present study aims to understand the interactions and electrokinetic properties of these two particles in lime softening. Effects of various experimental parameters such as pH, temperature, aging, inorganic carbon (CO₃^2-/HCO₃^-), and divalent cations (Ca²⁺/Mg²⁺) on the electrokinetic properties (i.e. zeta potential) of CaCO₃ and Mg(OH)₂ particles were individually studied. The interactions between humic acid (mimicking natural organic matter), silicate (representing silica), clay (mimicking suspended solids) and CaCO₃/Mg(OH)₂ particles were studied, as well as the interactions between CaCO₃ and Mg(OH)₂. Thermodynamic modeling was used to predict precipitates as a function of solution chemistry and assist with data interpretation. The results provide considerable insight into factors that are of importance to lime softening.

Removal of strontium from drinking water by conventional treatment and lime softening in bench-scale studies

The United States Environmental Protection Agency Contaminant Candidate List 3 lists strontium as a contaminant for potential regulatory consideration in drinking water. Very limited data is available on strontium removal from drinking water and as a result, there is an immediate need for treatment information. The objective of this work is to evaluate the effectiveness of coagulation/filtration and lime-soda ash softening treatment methods to remove strontium from surface and ground waters. Coagulation/filtration jar test results on natural waters showed that conventional treatment with aluminum and iron coagulants were able to achieve only 12% and 5.9% strontium removal, while lime softening removed as high as 78% from natural strontium-containing ground water. Controlled batch experiments on synthetic water showed that strontium removal during the lime-soda ash softening was affected by pH, calcium concentration and dissolved inorganic carbon concentration. In all softening jar tests, the final strontium concentration was directly related to the initial strontium concentration and the removal of strontium was directly associated with calcium removal. Precipitated solids showed well-formed crystals or agglomerates of mixed solids, two polymorphs of calcium carbonate (vaterite and calcite), and strontianite, depending on initial water quality conditions. X-ray diffraction analysis suggested that strontium was likely incorporated in the calcium carbonate crystal lattice and was likely responsible for removal during lime softening.