Lanthanum/Gemini surfactant-modified montmorillonite for simultaneous removal of phosphate and nitrate from aqueous solution

Abstract La3+-exchanged bentonite (Phoslock®) is a well-known adsorbent for phosphate, but it is incapable of simultaneously removing nitrate from aqueous solution. In this study, montmorillonite (Mt) was modified stepwise by lanthanum and a cationic gemini surfactant for co-adsorption of phosphate and nitrate. X-ray diffraction patterns revealed that lanthanum existed as LaCO3OH and the gemini surfactant intercalated into the interlayer space of Mt. The contents of lanthanum and gemini surfactant in obtained nanocomposite (LaOMt) were estimated using energy dispersive X-ray spectroscopy and elemental analysis, which are 0.163 and 0.435 mmol/g, respectively. Based on Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, uptake of phosphate was attributed to ligand exchange with hydroxyl and electrostatic attraction on positively charged LaCO3OH, and that of nitrate mainly resulted from the ion exchange with bromide ion and interactions with –R4N+ groups released from Mt layers. The estimated adsorption capacities of LaOMt to nitrate and phosphate were 0.84 and 0.39 mmol/g, respectively. Adsorption of phosphate was negligibly affected by nitrate, but the presence of phosphate facilitated the hydration of LaOMt and improved nitrate uptake. Compared with commercial Phoslock®, LaOMt demonstrated comparable adsorption performance in terms of phosphate removal. Considering the simultaneous uptake of nitrate and stable adsorption performance when ubiquitous anions coexisted, LaOMt is a promising adsorbent for remediation of phosphate and nitrate contaminated wastewater.