Removal of Divalent Cations from Brine Using Selective Adsorption onto Magnetic Nanoparticles

Abstract

Abstract Divalent cations, especially calcium (Ca2+), are known to significantly affect the performance of anionic surfactants and polymers used in enhanced oil recovery (EOR) processes. An efficient technique to remove Ca2+ from brine is reported, which is based on selective adsorption of Ca2+ onto functionalized iron oxide magnetic nanoparticles (IOMNPs). Upon adsorption, the IOMNPs can be separated by applying a magnetic field, leaving behind softened water. IOMNP was synthesized by coprecipitation, and the amine-functionalization of its surface was obtained according to an aqueous APTES coating process. Chelating agent, polyacrylic acid (PAA), was successfully coated on amine-functionalized IOMNPs via amidation of carboxylic acid using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). PAA modification significantly enhanced the adsorption capacity of IOMNPs due to their great ability to chelate Ca2+. The effect of pH on adsorption capacity was also investigated. The adsorption capacity of Ca2+ onto PAA-IOMNPs was found to be as high as 57.2 mg/g at pH 7 from the 400 mg/L Ca2+ solution. However, in American Petroleum Institute (API) standard brine (8×104 mg/L NaCl and 2×104 mg/L CaCl2), the adsorption capacity of IOMNPs decreased to 9.8 mg/g since the high salinity screens the charges on the surface of PAA-IOMNPs and results in the formation of nanoparticle aggregates. PAA-IOMNPs can be reused after treated by acetic acid solution. A geochemical model was developed to describe the competitive adsorption of Ca2+ and H+ onto amine-functionalized IOMNPs as a function of solution pH and Ca2+ concentration. Comparison between the model and the experiments shows that the adsorption isotherms predict the behavior of the system very well. Below pH 4, adsorption of Ca2+ is negligible and becomes important above pH 7. This opens the possibility of recovering the nanoparticles after the divalent cation removal, and reusing them for the repeated water softening.