A comparative study on in-situ synthesis of two iron nanoparticles in acid mine drainage using green tea and Excoecaria cochinchinensis leaves extracts

Acid mine drainage (AMD) is a type of wastewater characterized by elevated concentrations of heavy metals and metalloids, posing significant threats to ecosystems and human health. This study presents an eco-friendly method for in-situ synthesis of iron nanoparticles (FeNPs) in AMD using Excoecaria cochinchinensis leaves (EC) and green tea extracts (GT) as reducing/stabilizing agents, enabling simultaneous removal of heavy metals and metalloids. Results indicate that the two FeNPs (EC-FeNPs and GT-FeNPs) produced in AMD exhibited distinct structural and reactive properties. Firstly, the removal efficiencies of EC-FeNPs for As, Ni and Cd were 20.4 %, 15.31 %, and 11.62 %, respectively, while those of GT-FeNPs were 10.07 %, 11.7 % and 19.76 %, respectively. This indicates that EC-FeNPs were generally more effective than GT-FeNPs in removing heavy metals and metalloids from AMD. Secondly, EC-FeNPs (170.14 ± 3.22 nm) displayed larger particle sizes and more stable surface charges compared to GT-FeNPs (127.14 ± 1.94 nm), which could be attributed to differences in functional groups in the respective plant extracts. Thirdly, both EC and GT extracts acted as reductants and stabilizers in the synthesis of two FeNPs, with EC extracts providing a larger amount of phenolic compounds and carboxylic acids than GT extracts. Highlighted in this study is a novel approach for resource recovery and simultaneous removal of heavy metals and metalloids from AMD.