Efficacy of nano calcium silicate (NCS) treatment on tropical soils in encapsulating heavy metal ions: leaching studies validation

Abstract Soils are known to retain heavy metals through a number of processes. In this study, four different leaching methods viz. ASTM D3987, toxicity characteristics leaching procedure (TCLP), extended TCLP, and caged TCLP have been used to study the leaching behavior of Cd2+, Ni2+ and Pb2+ on red and black cotton soils. Further, these soils were amended with nano calcium silicate (NCS). Also, sequential extraction was performed on these soil mixtures to know the extent of sorption taking place in different phases of soils such as exchangeable, oxidizing, reducing, organic content and residual phases. It was found that as per ASTM D3987, all three heavy metals with red soil and black cotton soil were retained well in the short term, and over a long-term period, the metal ion was released back. When amended with NCS, both red soil and black cotton soil exhibited satisfactory retention levels over long periods which proved that NCS was responsible for encapsulation of metal ions in its matrix. To investigate the retention efficiency, extensive leaching tests were performed, and it was found that as per TCLP, extended TCLP and caged TCLP tests, the retention with NCS-amended red earth soil was 88, 90 and 94 % compared to untreated case. Sequential extraction proved that encapsulation was durable in NCS-amended soils compared to untreated ones. Hence, it is concluded that NCS-amended soil is a prospective material to attenuate heavy metals taking into consideration their long-term encapsulation effect in considered soils. This soil NCS mixture is expected to be a potent agent for in situ treatment of brown fields and an excellent substitute for existing conventional landfill liner materials.