Transport behavior of nanoplastics in activated carbon column

Abstract

Nanoplastics can be produced directly from some artificial products, such as cosmetics, or indirectly from the breakup of large pieces of plastic waste. They have a small particle size, large specific surface area, and stable structure and can concentrate toxic compounds in water. The discharge of nanoplastics into the water environment through urban piping systems or surface runoff may lead to the contamination of surface water resources, which poses a great threat to the safety of drinking water. As a common adsorbent, granular activated carbon (GAC) is widely used in the advanced treatment of drinking water. However, most of the studies focused on the transport ability of nanoplastics in quartz sand, and there is a lack of research on the migration behavior of nanoplastics in activated carbon media. In this study, the stability and pore characteristics of GAC were studied, and its regeneration efficiency was investigated. The transport curves of PSNPs, which have a particle size of 98 ± 9 nm and specific surface area of about 67 m²/g, were compared under different ionic strengths, ionic species, flow rates, pH, and humic acid (HA) concentrations. And DLVO theory was used to analyze the transport behavior of nanoplastics in activated carbon column. All experiments were performed at room temperature to make the results generalizable. The results showed that GAC had stable pore structure and excellent adsorption capacity. The surface area and pore volume of GAC are 759 m²/g and 0.357 cm³/g, respectively. And the regeneration rate of GAC can reach 90% and 83.3% after the first two regeneration cycles. On the other hand, at high ionic strength and low pH, the repulsive barrier between PSNPs and activated carbon gradually disappeared; then, more PSNPs were deposited in the activated carbon media, and the concentration of PSNPs in the effluent water was lower. Both the flow rate and HA promoted the transport of PSNPs, but the breakthrough curves of PSNPs did not change significantly when the HA concentration was further increased. At the same ion concentration, PSNPs tend to deposit on the surface of activated carbon in the background solution of Ca²⁺ compared with Na⁺. This study reveals the migration mechanism of PSNPs in the activated carbon filter column, which is of great importance to ensure the safety of drinking water and human health.

Keywords: DLVO theory; Drinking water; Granular activated carbon (GAC); Nanoplastics; Transport behavior.