Effect of humic acid on the sedimentation and transport of nanoparticles silica in water-saturated porous media

Abstract

Nano silicon particles (nSiO2) is one of the most widely used industrial engineered nanomaterials (ENMs). The extensive applications of nSiO2 may pose potential risks to aquatic ecosystems and human health. Humic acid (HA) is a major component of soil and water that exists widely in the natural environment and adsorbs to the surface of nanoparticles, which affects the fate and transport of ENMs in soil. Therefore, it triggers the necessity to study the chemical reaction of HA controlling the sedimentation and transport of nSiO2. The sedimentation kinetics and transport breakthrough curves of nSiO2 with/without HA in water-saturated porous media were studied in two electrolyte (NaCl and CaCl2) solutions. The likely mechanisms were explored with both multiple technologies and numerical modeling including TEM-EDX, particle size distribution, zeta potentials, and the two-site kinetic attachment model (TSKAM). Our experimental results showed that the existence of HA generally increased the suspensivity and the transportability of nSiO2 in NaCl and CaCl2 solutions in packed sand columns at acidic pH. This result was attributed to the HA adsorption leading to the more negatively charged surface and the smaller size of nSiO2 aggregates. However, the formation of coordination complexes associated with larger cluster among nSiO2 between HA and Ca2+ contributed to the increased sedimentation of nSiO2 at alkaline pH. Subsequently, the presence of HA inhibited the transport of nSiO2 in CaCl2 solution at pH 9.0. Comparably, in NaCl at pH 9.0, HA showed the negligible effect on the nSiO2 deposition in sand. Both the attachment and detachment parameters, which were obtained from fitting the breakthrough curves of ENMs using the TSKAM, could be used to well describe the transport behavior of nSiO2 with HA under various conditions. In particular, the irreversible attachment parameters at site 2 on sand were positively related to the retention of nSiO2 with HA. The fate and transport of nSiO2 can be distinctly affected by HA depending on the ion composition, ion strength, and pH in soil. This study will provide insights for assessing the mobility of nSiO2 with HA in subsurface soil and aquatic environments.