Effect of montmorillonite and humic acid on cotransport of g-C3N4 and lead: The role of triazine ring in lead adsorption and deposition in soil components

In the past few years, Graphitic carbon nitride (g-C3N4) has been produced on a large scale and has been widely used because of its excellent optical properties and physicochemical stability. It is easy to ignore the risk of g-C3N4 transport in the environment, including soil and groundwater, due to the non-toxicity of metals. Based on this situation, we investigated the effects of pumping flow rate, ionic strength, and the presence of lead (Pb) on the transport behavior of g-C3N4 in quartz sand, montmorillonite, and humic acid-coated sand through column experiments. The retention of g-C3N4 in the media and the adsorption mechanism of Pb onto g-C3N4 were analyzed by FTIR and XPS. The results showed that fast flow rate and low ionic strength were favorable conditions for the transport of g-C3N4. The cotransport experiment results showed that Pb reduced the outflow of g-C3N4 by more than 50%. Contrarily, 6.95%, 0.43%, and 11.01% Pb were hindered in quartz sand, montmorillonite, and humic acid-coated sand due to the presence of g-C3N4. Compared with the uncontaminated media, the recoveries of g-C3N4 in Pb contaminated quartz sand, montmorillonite, and humic acid-coated sand were reduced by 0.34%, 5.86%, and 15.34%, respectively. Concomitantly, which were attributed to the special triazine ring structure and the abundant bonding modes with soil components and Pb, g-C3N4 particles were easier to deposit in montmorillonite and humic acid-coated sand, and could be used as the carrier to release 6.70% and 5.64% Pb from contaminated quartz sand and humic acid-coated sand.