Utilization of wall-breaking sludge for improving soil structure in abandoned mine land.

Promoting soil structure is considered an essential prerequisite for abandoned mine land restoration. Sewage sludge (SS) has the potential to improve soil structure. However, traditional SS application to improve soil structure requires a lot of SS, potentially exacerbating heavy metal (HM) contamination. To find an effective way to improve abandoned rare-earth mine land soil (ARLS) aggregation, we wall-broke SS and conducted an L9 (3) orthogonal design soil incubation experiment with four factors, including SS form (wall-breaking dewatered or composted SS, WBSS), water addition volume, volume ratio of WBSS to soil, and incubation time, and evaluated their effects on aggregation ability and HM availability in ARLS. The results showed that WBSS addition improved ARLS aggregation by increasing the percentage of >2 mm water-stable aggregates (by 4.53%-187.23%), mean weight diameter (by 19.18%-58.90%), and geometric mean diameter (by 23.81%-95.24%) not only in the WBSS-mixing layer but also the layer without mixed with WBSS but under the WBSS-mixing layer. These aggregate characteristic indicators positively correlated with organic matter content. Adding WBSS under a suitable condition (20% wall-breaking mixed SS or composted SS addition, 100 or 150 mL water addition, and incubation for 30 or 60 days) could produce high cementing substances (hydroxyl and carboxyl groups and mineral-associated organic carbon) but release low content of active HM. Treatment of 20% wall-breaking mixed SS addition, 150 mL water addition, and incubation for 30 days had the highest subordinate function values of aggregation ability. This treatment was more suitable for safely improving mining soil aggregation than other treatments. These results suggested that wall-breaking SS addition with suitable conditions had the potential to improve the mining soil structure.