SPIONs as proton pump and electrostatic contributor for the simultaneous precipitation of protonated neutral red, Ag⁺ and chloride ion from aqueous solution

The development of a cheaper technique capable of simultaneous removal of organic and inorganic pollutants from water is important to alleviate cost of waste water treatment. Herein, we report the concurrent precipitation of protonated neutral red (RNH₃⁺), silver (I) (Ag+) and chloride (Cl⁻) ions from aqueous solution using greenly-synthesized biocompatible gluconic acid capped superparamagnetic iron oxide nanoparticles (SPIONs@gluconic acid). To the best of our knowledge, this is the first report on the simultaneous removal of organic and inorganic pollutants from aqueous saline solution using carboxylic acid functionalized superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs@gluconic acid were synthesized via a coprecipitation approach and characterized using transmission electron microscopy (TEM), selective area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDS), vibrating sample magnetometry (VSM) and Fourier transform infrared spectroscopy (FT-IR). The precipitation process was effected by mixing SPIONs@gluconic acid with Ag (I) complex of neutral red hydrochloride solution, followed by settling under gravity with time. A grayish black-brown precipitate was obtained which separated from a clear supernatant. The mechanism for the removal process was proposed and experimentally verified. It was found that precipitation only occurred in the presence of sufficient amount of hydrogen ion (pumped into the system by Ag⁺ and SPIONs@gluconic acid, at low pH and Ag⁺ concentration) and subsequent formation of insoluble AgCl which served as Le Chatelier's principle driven agent for the continuous precipitation of the neutral red's electrostatic–induced system. This finding indicates that SPIONs@gluconic acid may be employed successfully for a low-cost simultaneous removal of lone-pair bearing organic chloride salts and inorganic silver ions from aqueous solution at low Ag⁺ concentration. This approach may find usefulness for large scale industrial purification of combined effluents from textile and photographic industries.