In situ synthesis of microbial Ca-alginate coated nZVI for Cr(VI) remediation in soil

Nano zero-valent iron (nZVI) is highly effective in removing a wide range of pollutants, including halogenated compounds, nitrates, phosphates, polycyclic aromatic hydrocarbons, and heavy metals, establishing it as a leading nanomaterial for soil and groundwater remediation. Its high injectability allows for application via grouting methods. However, a notable challenge is its susceptibility to being washed away by water during remediation, leading to significant material loss. This study proposes an innovative approach using microbially synthesized calcium alginate gel to create an in situ calcium alginate-coated nZVI composite material system (ASSA@nZVI) for nZVI immobilisation. The results demonstrate that the ASSA@nZVI system achieves uniform iron content distribution (2–2.5 mg/layer) within the sand column and effectively immobilizes nZVI. Compared with AS@nZVI and nZVI, reducing washout from approximately 45% to nearly zero. The in situ formed calcium alginate gel significantly reduces soil permeability from 1.88 × 10⁻² to 1.32 × 10⁻⁴cm/s, thereby prolonging hydraulic retention time. The ASSA@nZVI system exhibits a Cr(VI) reduction breakthrough time of up to 78 h, with a total reduction amount of 1344 μg. And aging tests show that the Cr(VI) reduction efficiency decreases by about 50% after 7 days, indicating room for further optimisation of long-term stability. Overall, this study underscores the potential of the ASSA@nZVI system to enhance the effectiveness and sustainability of nZVI-based remediation strategies.