Improving the phosphate adsorption performance of layered manganese oxide by ammonia plasma surface modification

Abstract

The introduction of excessive phosphate to natural water bodies often results in serious eutrophication. Adsorption is thought to be one of the best methods for phosphate recovery and removal from water. The abundance of surface functional groups and high particular surface part of layered manganese oxide (LMO) makes it an ideal agent to remove pollutants from wastewater. However, since the surface of LMO is normally negatively charged at neutral pH, it cannot efficiently clear anionic contaminants like phosphate. This study brings forward a novel development of phosphate sorbents that help remove phosphate from water. Specifically, the modification of LMO with plasma technology in the presence of ammonia (NH3) has yielded promising results. These outcomes indicated that NH3-LMO exhibited an ultrathin and transparent layered structure with numerous graphene-like wrinkles and folds. The adsorption of phosphate by NH3-LMO was very fast and followed pseudo-second-order kinetics. Due to the uniform distribution of adsorption sites on the surface of NH3-LMO, the Langmuir isotherm model could clearly support the phosphate adsorption onto NH3-LMO. The presence of chloride, sulfate, nitrate, and dissolved humic acid exerted a slight influence on the phosphate removal efficiency of NH3-LMO. Furthermore, the adsorption of phosphate by NH3-LMO was highly pH-dependent. This is mainly attributed to the transformation of pHIEP from 2.2 for LMO to 6.6 for NH3-LMO, which is positively charged at neutral pH. For the NH3-LMO reuse, the adsorption function was relatively stable after four adsorption-desorption cycles. Collectively, our research showed that NH3-LMO could be adopted as a potential adsorbent for the phosphate removal from water.