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

Abstract

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 128 mg/g and minimum Mg²⁺-loading capacity at pH 3 for the Mediterranean Seawater. The corresponding values for EGTA in Red Seawater were 117 and 2 mg/g and were obtained at pH 3 and 6.6, respectively, with an EGTA optimum dose of 1 g/L. Maximum Ca²⁺ and Mg²⁺ loading capacities (153 and 72 mg/g, respectively) were attained using EDTA with reverse osmosis brine. On the other hand, EDTA exhibited the highest calcium and magnesium loadings (158 and 76 mg/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.

Keywords:

• Chelation
• Seawater
• Brine
• Adsorption
• Calcium
• Magnesium
• Isotherm

Acknowledgement

This work was financially supported by the Science and Technology Development Fund (STDF) of Egypt, under grant number [STDF/3991].

Notes

Presented at EuroMed 2015: Desalination for Clean Water and Energy Palermo, Italy, 10–14 May 2015. Organized by the European Desalination Society.