Beneficial utilization of sewage sludge ash residues for the stabilization/solidification of As-, Cr-, and Cu-contaminated marine sediments

Abstract

The management of incinerated sewage sludge ash (ISSA) residue after phosphorus recovery and dredged marine sediment contaminated with heavy metal(loid)s such as As(V), Cr(VI), and Cu(II) in coastal cities is a major concern due to the limited land space and stringent environmental regulations for solid wastes. This study proposes a novel recycling routine for the generated ISSA residue based on its high iron content. Briefly, the high iron ISSA residue was co-pyrolyzed with a biomass waste (peanut shell) at 1050 °C under N2 atmosphere, yielding a composite material containing zero valent iron (ZVI). Due to the versatile applications of (ZVI) in environmental remediation, including the adsorption removal of heavy metal(loid)s, it could be an alternative low-cost candidate for the solidification/stabilization (S/S) treatment of contaminated marine sediments. The composite material was reused as a partial substitute of cement (at substitution rates of 20%) for the S/S treatment of contaminated marine sediments. The results showed that all the treatment groups could effectively reduce the leaching of As and Cu from contaminated sediments to a comparable level according to the toxicity characteristic leaching procedure (TCLP). CK (cement alone), SBC and BC groups showed a poor immobilization effect toward Cr(VI). Nevertheless, FBC, FSBC, and RBC significantly reduced the leaching of Cr(VI). Replacement of cement by these composite materials significantly reduced the mechanical strength of the S/S products compared to CK. Particularly, BC group demonstrated the lowest mechanical strength, which was 49% lower than that of CK. Even though the lowest mechanical strength was greater than 3 MPa, S/S products met the requirement for recycling as filling materials under local law. The results demonstrated the feasibility of producing ZVI containing biochar composites from ISSA residue and peanut shell, which could be further reused to substitute cement and enhance immobilization efficacy, particularly toward Cr(VI) for contaminated sediment.