Biochar and sodium carboxymethyl cellulose (CMC) improve the soil matrix for treating highway rainwater runoff

In view of the rainwater pollution problems in highway runoff on the Northwest Expressway, especially heavy metal pollution and poor soil water retention, this study focused on the sandy soils along the highway sides in Qilian Mountain National Park, Gansu Province. It investigated the removal of pollutants as well as water and fertilizer retention effects in simulated highway rainwater runoff using four amendments: carboxymethyl cellulose (CMC), corn straw biochar, corn cob biochar, and rice husk biochar, both individually and in combination. Through permeability tests, evaporation tests, and static adsorption experiments, the optimal soil treatment for runoff rainwater was determined. Experiments showed that mixing soil with quartz sand in a 4:6 ratio and adding 5% rice husk biochar and 0.1% CMC can significantly enhance the adsorption of various pollutants and improve soil moisture retention. We packed the initially selected improved soil matrix into columns, initiated operation, and set the influent flow rate to 9 mL/min. During stable operation, the average retention rates of nutrients-ammonia nitrogen (NH₄⁺-N), total phosphorus, and chemical oxygen demand-from highway runoff were 81.75%, 66.72%, and 77.97%, while the average removal rates of copper (II) (Cu²⁺) and chromium (VI) (Cr⁶⁺) were 91.94% and 84.18%, respectively. After operation, the residual forms of copper (Cu) and chromium (Cr) in the soil matrix accounted for 56.96% and 52.28%. The transformation of Cu and Cr into stable residual forms with low migration risk effectively reduced their environmental impact.