Effectiveness of humic substances extracted from municipal sludge in remediating arsenic-contaminated soil

The humic substances (HS) extracted from sludge provide a promising, cost-effective, and environmentally friendly alternative to traditional leaching agents for the remediation of arsenic (As) contaminated soil. Therefore, this paper investigates the remediation of As-contaminated soil using HS extracted from composted sludge. The HS solution extracted using NaOH solution is SHS, and the HS solution extracted applying sodium hydroxide sodium pyrophosphate is MHS. The effectiveness of these HS agents was compared with that of other fulvic acids, considering factors such as HS concentration, pH, liquid-to-solid ratio, leaching duration. The transformation of As species in soil was also explored. The results indicated that the optimal conditions for leaching were found to be SHS and MHS concentrations of 800 mg C/L, with pH of 11, liquid-to-solid ratio of 15:1, and equilibrium time of 24 h. Under these conditions, As removal efficiencies reached 35.46% for SHS and 41.36% for MHS. MHS showed superior performance, due to the use of a mixed NaOH and Na₄P₂O₇ solution. Both SHS and MHS exhibited pseudo-second-order kinetics and conformed to the Elovich model, indicating the existence of complex heterogeneous diffusion. The leaching process influenced the behaviour of coexisting metals such as Fe and Al, which disrupt their binding with As, promoting its release. Repeated leaching significantly improved the removal efficiency of As, with MHS demonstrating the highest effectiveness. The study supports the use of HS extracted from municipal composted sludge as a cost-effective, environmentally friendly approach for As-contaminated soil remediation.