Enhanced remediation of heavy metal-contaminated soils using biochar and zeolite combinations with additives: A meta-analysis.

Soil heavy metal (HM) contamination is a major concern in agricultural lands due to its potential to enter the food chain and its adverse health effects. Remediation materials such as biochar (BC) and zeolites (ZE) have been studied for their potential to mitigate risks associated with soil HM contamination. This meta-analysis evaluates changes in the availability of Cd, Cu, Pb, and Zn following the application of BC and ZE to soil, whether applied individually, in combination (BC + ZE), or with additives (BC + ZE + A). Individually, BC reduced the availability of Cd, Cu, Pb, and Zn in soil by 24.0%, 33.0%, 31.3% and 10.1%, respectively; and ZE reduced these levels by 32.4%, 18.8%, 20.3% and 38.9%. Results indicate that, on average, BC + ZE effectively decreases the availability of Cd, Cu, Pb, and Zn in soils by 32.6%, 54.3%, 35.4%, and 18.3%, respectively. The combination with additives, BC + ZE + A, reduced the Cd and Pb availability by 54.2% and 20.9%, respectively. Most studies were undertaken with Cd, representing 59% of observations, followed by Pb, Zn, and Cu, respectively, with 29%, 8%, and 5%. The small number of studies with Pb, Zn and Cu prevented the creation of subgroups involving these three HMs. Notably, the nature of the additive influences the variation in available Cd content in remediated soils. Inorganic additives combined with BC + ZE demonstrated greater effectiveness in Cd remediation, achieving reductions of available content by 86.8%, compared to those containing clay minerals or organic compounds, with reductions of 27.4% and 15.4%, respectively. These findings enhance our understanding of how BC and ZE can be utilized in soil HM remediation and their effectiveness against different metals.