Introduction of N-containing moieties by ammonia plasma technique can substantially improve ciprofloxacin removal by biochar and the associated mechanisms: Spectroscopic and site energy distribution analysis

Abstract

A low-temperature plasma device was developed to introduce N-containing moieties into biochar type CS-300 to improve ciprofloxacin removal. The sorption capacity of ciprofloxacin by the treated biochars was 2.61–4.26 times that of CS-300, and the mechanisms were explained by X-ray photoelectron spectroscopy and site energy distribution analysis. The results showed that the π-π stacking mechanism dominated ciprofloxacin removal by biochars. Ammonia-plasma treatment introduced abundant amino and amide groups to CS-300. They increased the π electron density in the delocalized system in CS-300, thus enhancing ciprofloxacin removal by the π-π stacking mechanism. Plasma treatment also enhanced polar interactions between ciprofloxacin and CS-300 through hydrogen- and ionic bonding occurring at high-energy sites with energy over 10,000 J/mol, thereby increasing ciprofloxacin removal. The maximum removal efficiency of ciprofloxacin by the treated biochars reached 71.0–85.7% at pH 6, while that for CS-300 was only 31.6% and occurred at pH 4. This implied that plasma treatment not only greatly increased the maximum removal efficiency but also shifted the optimal pH from acidic to nearly-neutral condition. Our findings highlight that ammonia-plasma treatment is a promising technique to improve ciprofloxacin removal by biochars and the treated biochars have potential applications in its removal from water.