Removal of Mg²⁺, K⁺, SO₄^-2Ions from Seawater by Precipitation Method

Abstract

Removal of Mg 2+ , K +, and SO4 -2 ions in seawater has been successfully done by precipitation in a mixing tank method. This study aims to remove the content of magnesium ions (Mg + ), potassium (K + ) and sulfate (SO4 -2 ) in sea water with the addition of chemicals disodium phosphate (1.2 % volume), calcium chloride (2 % volume) and sodium hydroxide (2% volume). Stirring is performed at 100 rpm and the pH solution is adjusted to 9. Disodium phosphate serves to bind magnesium ions and potassium, CaCl2 serves to bind the sulfate ion, while sodium hydroxide is used to adjust the pH of the solution mixture and also reacted with magnesium ions. In total, the removal efficiencies of Mg 2+ , K + and SO4 -2 ions in seawater were 97%, 96%, and 92%, respectively. The precipitated solids contains component of PO4 -(14.5%), Mg 2+ (13.8%), SO4 2-(28.2%), Ca 2+ (24.1%) and K + (1.9%) ions.Keywords: sea water, disodium phosphate, calcium chloride, precipitation, removal ions INTRODUCTIONThe average waters of the world's oceans contain 3.5% dissolved salts, 96.5% water and average salinity is about 35 g/kg. When salts dissolve in water they usually react with water and dissociate (break apart) into ions, positively and negatively charged atoms or groups of atoms. These ions are either negatively charged anions, or positively charged cations. Six major ions make up > 99% of the salts dissolved in seawater: four cations sodium (Na + ), magnesium (Mg 2+ ), calcium (Ca 2+ ), and potassium (K + ); and two anions: chloride (Cl -) and sulphate (SO4 2-). These six ions and the five next most common ones make up the major constituents of seawater and comprise 99.99% of dissolved materials. Sodium chloride (NaCl), makes up 86% of dissolved ions in seawater. Many other elements (trace elements) are dissolved in seawater in concentrations less than one part per million (ppm). The molar concentration of magnesium in sea water is about five times higher than calcium. In fact, both magnesium and calcium are in a delicate equilibrium where slight charge in alkalinity and carbon dioxide tension may cause precipitation (Irving, 1926). Jijun et al, (2012) developed the precipitation of calcium and magnesium from seawater using CO2. Mg ion can rarely incorporate into the solid precipitates, compared, for example, to calcium salts. The rate of Mg phosphates nucleation is several orders of magnitude lower than that of Ca phosphates, at the same values of pH and total phosphate concentration (Golubeva, 2001). The removal rate of Ca is higher than the removal rate of Mg in pH value between 8.0 and 9.0. In the Mg-free seawater system the removal rate is high enough in pH value between 8.0 and 9.0. Meanwhile, in the Ca-free seawater system there is no precipitation between pH 8.0 and 8.3 (Jijun et al, 2012). Precipitation of phosphate is high enough in pH range from 8 to 10. However, it decreased as pH is increased from 10 to 12, probably because of competition between phosphate and hydroxyl OH − ions (Liu and Warmadhewanti, 2009

A Novel Technique for Water Hardness Removal by Using Polystyrene

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