Effects of Composted Straw, Biochar, and Polyacrylamide Addition on Soil Permeability and Dynamic Leaching Characteristics of Pollutants in Loessial Soil in Urban Greenbelts According to Indoor Simulation Experiments

Urban greenbelt soil is currently severely degraded and unable to meet the needs of sponge city construction. Therefore, this study involved adding modified materials, such as decomposed straw, straw biochar, and PAM (polyacrylamide), to greenbelt soil (collected from the Xixian New Area, a pilot city for sponge city construction in China). This study was conducted to explore the effects of adding modified materials on soil physical properties and pollutant adsorption capacity through indoor simulation experiments and dynamic leaching experiments (in the dynamic leaching experiments, the medium thickness was 40 cm, and a water outlet was set every 10 cm to collect the filtrate). In this study, three experimental treatments were set up: (1) soil–sand–decomposed straw + PAM (SSJ), (2) soil–sand–biochar + PAM (SSB), and (3) soil–sand–decomposed straw–biochar + PAM (SSBJ). In the three treatments, the addition amounts of soil, sand, and PAM (0.01 g·mL−1) were constant at 560 kg·m−3, 624 kg·m−3, and 76 L·m−3, respectively. The addition amounts of decomposed straw in the SSJ and SSBJ treatments were 100 kg·m−3 and 50 kg·m−3, respectively. The amounts of added biochar in the SSJ and SSBJ treatments were 32 kg·m−3 and 16 kg·m−3, respectively. The saturated hydraulic conductivity and saturated water content of the different treatments increased by 92.90–107.10% and 19.07–32.17%, respectively, compared with the background values. As the depth increased, the leaching concentrations of N and COD (chemical oxygen demand) at 40 cm in the different treatments increased by 282.66–1374.02% and 435.10–455.84%, respectively, compared with those at 10 cm. However, the leaching concentrations of Cu, Zn, Cd, and P changed little with increasing depth. As the flow load increased, the leaching concentration of the pollutant pattern was not obvious. After the leaching of pollutants stabilized, at 40 cm, the leaching concentrations of N, P, and COD for the SSJ, SSBJ, and SSB treatments were 5.46–56.30 mg·L−1, 0.14–2.06 mg·L−1, and 1034.23–1531.40 mg·L−1, respectively. The retention rates of Cu, Zn, and Cd showed a small trend over time, and the retention rates were all above 86%. Overall, the SSB treatment had a strong ability to intercept N, P, and COD, whereas the SSBJ treatment had a strong ability to intercept Cu, Zn, and Cd. These research results can provide a reference for the improvement of greenbelts in sponge city construction.