Application of iron [Fe(0)]-rich substrate as a novel capping material for efficient simultaneous remediation of contaminated sediments and the overlying water body

Release of contaminants from sediments has been one of the main pollution sources causing eutrophication and malodorous black of ponds. In this study, an iron-rich substrate (IRS) was developed based on iron‑carbon micro-electrolysis and applied for simultaneous sediments and overlying water remediation. IRS obtained high ammonia and phosphate adsorption capacities (Langmuir isotherm) of 13.02 and 18.12 mg·kg⁻¹, respectively. In the 90-day long-term remediation, IRS reduced NH₄⁺-N, PO₄³⁻-P, organic-N, organic-P, TN and TP in overlying water by 48.6%, 97.9%, 34.2%, 67.1%, 53.2% and 90.4%, respectively. In sediments, IRS reduced NO₃⁻-N, NH₄⁺-N and organic-N by 98.5%, 26.5% and 6.3%, respectively. The unstable P-compounds (i.e., organic-P, Ca-bounded-P and labile-P) were effectively transferred (20.1%, 54.3% and 98.2%, respectively) into inert P-compounds (i.e., Fe-bounded-P and residual-P). Meanwhile, flux rates of nitrogen and phosphorus from sediments to overlying water were reduced from 7.02 to 4.92 mg·m⁻²·d⁻¹ (by 29.9%) and from 7.42 to 2.21 mg·m⁻²·d⁻¹ (by 70.2%), respectively. Due to micro-electrolysis, Fe²⁺/Fe³⁺/[H] were in-situ generated from IRS and NO₃⁻-N was effectively reduced. Additionally, the generation of O₂^· was promoted by Fe²⁺/[H] and strengthened the NH₄⁺-N, organic-N/P oxidation. Fe³⁺ enhanced the immobilization of PO₄³⁻ (e.g., as FePO₄·H₂O and Fe_nPO₄(OH)_3n-3). The released Fe²⁺/Fe³⁺ from IRS were finally stabilized as poorly reactive sheet silicate (PRS)-Fe and magnetite-Fe in the sediments and hardly showed side effect to sediments and water body. The developed IRS obtained advantages of high efficiency, ecologically safe and cost-effective in contaminated sediments and overlying water remediation.