In-situ and ex-situ remediation of potentially toxic elements by humic acid extracted from different feedstocks: Experimental observations on a contaminated soil subjected to long-term irrigation with sewage effluents

The complicated reactions among elemental pollutants and humic substances (HS) in soil matrix are not fully understood. Herein, natural humic acid (HA) was evaluated for its foreseeable in-situ and ex-situ remediation effects on a Typic Torripsamment soil subjected to long-term irrigation with sewage effluents. Humic acid was extracted from Vertic Torrifluvents soil (HA(S)), farmyard manure (HA(FYM)) and wheat straw (HA(WS)). Physicochemical analyses illustrated the higher surface reactivity of HA(WS) compared to other types. Ecological risk assessment indices showed hazardous soil contamination due to the cumulative impact of potentially toxic elements (PTEs). The extractability of PTEs showed variations in HA-amended treatments. Mobile and/or available fractions of some PTEs (As, Cd, Mn, Ni, Pb and Zn) showed higher extractability compared to the unamended treatment suggesting the potentiality of HA to serve as a washing agent. Other PTEs (Cr, Fe and V) showed less extractability suggesting the immobilization effect of HA. The ex-situ kinetic removal of aqueous PTEs increased sharply, peaked at similar to 240 min, and reached equilibrium at 48 h. HA(WS) showed higher in-situ and ex-situ remediation efficiency compared to other types given the uniqueness of its surface reactivity.Derjaguin-Landau-Verwey-Overbeek (DLVO) theory well-interpreted the behavior of HA transport in sand column, and the actual mobility fitted well with Advection-dispersion-reaction (ADR) model. Attachment of HA onto sand beads occurred via secondary energy minimum, and the mass balance calculations showed recovery values of about 56.7, 48.9 and 44.7% for HA(S), HA(FYM) and HA(WS), respectively. In essence, data of this research provide insights for understanding combined washing-stabilization mechanisms involved in PTEs remediation by HA amendments. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study evaluated the effects of different sources of humic acid on the remediation of polluted soil. The results showed that humic acid extracted from wheat straw had the best performance in both in-situ and ex-situ remediation, with higher surface reactivity. The study also revealed the impact of humic acid on the extractability of PTEs.