The occurrence, transport mechanisms, and removal of silver nanoparticles in the water environment

Abstract

There is a heightened concern from communities on silver nanoparticles (AgNPs) disposal from industrial products into the water environment because of their toxicity to aquatic habitats. Aeration and coagulation methods to remove AgNPs have limitations such as complicated procedure involvement and the use of toxic materials. Hence, adsorption has the potential to be a simple and greener method to remove AgNPs. The aim of this study was to evaluate the occurrence, transport mechanisms, and removal of AgNPs in water environment. Water samples were collected from two sewage treatment plants (STPs), Melana River, and Skudai River. Effects of AgNPs on bacteria isolated from the rivers and STPs were also investigated. Transport mechanism of AgNPs in river was evaluated using constructed river model as a pilot study. AgNPs removal from the water via two different activated carbons derived from oil palm shell (ACfOPS) and coconut shell (ACfCS) were examined. In this respect, adsorption mechanism of AgNPs onto the proposed activated carbons was evaluated using physio-chemical approaches. In terms of kinetic behaviors, evaluation of nine kinetic adsorption models using six statistical indicators were scrutinized. Specific analytical methods namely UV-Vis spectrophotometer, FTIR, FESEM and SEM-EDX, and molecular analysis have been employed to gather the objectives. This study found that the concentration of AgNPs in the rivers and STPs ranges from 0.1 to 10.2 mg/L and 0.1 to 20.0 mg/L, respectively. AgNPs have antibacterial capability against all examined bacteria depending on their size and bacteria types. In the transport mechanism inspection, this study found that about 95% of the entering AgNPs into the river system was transmitted via the effluent. ACfOPS and ACfCS removed up to 99% of AgNPs from the water. The interaction mechanism between AgNPs and the activated carbon surface was mainly due to electrostatic force interaction via binding Ag+ with O- presented in the activated carbon to form AgO. Experimental adsorption data can be best described using the mixed 1,2-order kinetic model. Furthermore, this study also found that the proposed model outperformed existing kinetic models that share the same number of parameters. Findings from this study are useful for the monitoring of AgNPs in water environment and providing a method to remove AgNPs.