Experimental screening of some chelating agents for calcium and magnesium removal from saline solutions

摘要

Removal of hardness-causing ions from seawater and desalination brines improves the performance of water treatment units and provides economic and environmental benefits. Chelation of calcium (Ca2+) and magnesium (Mg2+) ions in saline solutions has been experimentally investigated as a stand-alone separation process. In the current study, ethylene glycol bis(2-aminoethyl ether)-N,N,N,N-tetraacetic acid (EGTA), diethylenetriaminepentaacetic acid (DTPA), N-(2-Hydroxyethyl) ethylene diamine-N,N,N-triacetic acid (HEDTA), ethylene diaminetetraacetic acid (EDTA), polyacrylic acid (PAA), boric acid (BA), and sodium tripolyphosphate (STP) were selected as chelating agents. It was found that DTPA exhibited the maximum Ca2+-loading capacity of 128mg/g and minimum Mg2+-loading capacity at pH 3 for the Mediterranean Seawater. The corresponding values for EGTA in Red Seawater were 117 and 2mg/g and were obtained at pH 3 and 6.6, respectively, with an EGTA optimum dose of 1g/L. Maximum Ca2+ and Mg2+ loading capacities (153 and 72mg/g, respectively) were attained using EDTA with reverse osmosis brine. On the other hand, EDTA exhibited the highest calcium and magnesium loadings (158 and 76mg/g) with Red Seawater at pH 7.5. Among the investigated chelating agents, EGTA and EDTA were found to be the most suitable for softening of Red Seawater, whereas EGTA proved to be the most selective for calcium ions. Both calcium and magnesium adsorption onto EGTA have been best described by Langmuir isotherm indicating monolayer coverage of ions on the outer surface of the chelating agent.