Abstract
In this paper, we present a study on the adsorption of calcium (Ca2+) onto polyacrylic acid-functionalized iron-oxide magnetic nanoparticles (PAA-MNPs) to gain an insight into the adsorption behavior of alkaline earth elements at conditions typical of produced water from hydraulic fracturing. An aqueous co-precipitation method was employed to fabricate iron oxide magnetic nanoparticles, whose surface was first coated with amine and then by PAA. To evaluate the Ca2+ adsorption capacity by PAA-MNPs, the Ca2+ adsorption isotherm was measured in batch as a function of pH and sodium chlorite (electrolyte) concentration. A surface complexation model accounting for the coulombic forces in the diffuse double layer was developed to describe the competitive adsorption of protons (H+) and Ca2+ onto the anionic carboxyl ligands of the PAA-MNPs. Measurements show that Ca2+ adsorption is significant above pH 5 and decreases with the electrolyte concentration. Upon adsorption, the nanoparticle suspension destabilizes and creates large clusters, which favor an efficient magnetic separation of the PAA-MNPs, therefore, helping their recovery and recycle. The model agrees well with the experiments and predicts that the maximum adsorption capacity can be achieved within the pH range of the produced water, although that maximum declines with the electrolyte concentration.
Highlights
Produced water generated during the production of unconventional gas from onshore activities is, on average, 600 million m3 per year [1,2]
This paper presents the results from the study of the adsorption of Ca2+, a representative alkaline earth element and a congener of Ba2+, Ra2+, and Sr2+, by iron-oxide magnetic nanoparticles (MNPs) functionalized with Polyacrylic acid (PAA)
The results indicate that at a given total cation concentration, the maximum adsorption capacity can be achieved within the pH range of the cation concentration, the maximum adsorption capacity can be achieved within the pH range of the produced water that maximum concentration
Summary
Produced water generated during the production of unconventional gas from onshore activities is, on average, 600 million m3 per year [1,2]. Nano-scale adsorbents are considered as an ideal candidate for the removal of selected ions because of their large surface area per mass and a great number of selectively active sites that can be generated on the surface Among these nano-adsorbents, the iron-oxide based magnetic nanoparticles have been extensively investigated to remove multi-valent cations, such as copper, lead, zinc, nickel [6,7,8], as they have a number of advantages, including easy control, fast separation of the spent nanoparticles from the cleaned water with the application of magnetic field gradient, and the potential for the spent nanoparticles to be regenerated and reused.
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