Assessing the effect of cement and nano-clay mixture on the stabilization of heavy metal contaminated soil

Stabilization/solidification (S/S) has emerged as a cost-effective method for treating a variety of wastes, particularly heavy metal (HM) contaminated soils. Among the many available fixing agents, Portland cement (PC) has been used extensively for the remediation of contaminated sites. However, there are significant environmental and technical impacts associated with PC application. Thus, the present research was conducted to address the efficacy of cement and nano-clay mixture in enhancing the S/S process. In so doing, artificially contaminated soils were first prepared by mixing kaolinite with zinc (Zn) at levels of 0 to 2%. Afterward, tow type of nano-clay (Na-Montmorillonite and Na-Cloisite), cement and cement/nano-clay (CNC) were separately added to the sample, and then, a set of macro and micro level experiments including batch equilibrium, pH, toxicity characteristic leaching procedure (TCLP), unconfined compression strength (UCS), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) analyses were carried out at various curing periods (1, 7 and 28 days) to assess the effectiveness of the additives. The results obtained show that the addition of nano-clay can increase the HM retention capability of soil; however, this may be partly lost when the treated soil are subjected to acidic TCLP solution. In addition, with increasing the HM content, due to the decrease in buffering capacity of system and the restructuring of the clay particles, the soil remediation potential at presence of nano-clay is decreased considerably. It was found that the application of sole cement may significantly enhance the HM retention capacity of soil. But in this case, the physicochemical reactions of Zn ions with cement could hinder and/or reduce the generation of hydration products phases such as calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH), resulting in the degradation of cementation structure-bonding of S/S matrix, as clearly confirmed by the formation of calcium zincate and the diminution in the cementitios compounds peak intensity in the XRD patterns of cement-treated soils. Therefore, the leaching characteristics and the mechanical properties of the S/S material with sole cement are adversely affected by increasing the amount of HM ions. As a result, a large quantity of cement (20 wt% per one percent of HM) and a long time of curing (≈ 28 days) should be employed to meet the full needs of HM immobilization in contaminated soil and give the EPA-acceptable UCS value (≥ 0.35 MPa). The TCLP and XRD test results indicate that the cement/nano-clay combination can expedite the S/S process and alleviate the deleterious influences of metal ions and acidic attack on the stabilized sample. The EDX analyses also support the increase in the development of hydration reactions and the formation of cementing materials in the presence of CNC, providing the enhancement of binding capacity that will lead to the greater strength (up to 50%) in comparison to cement application. Hence, the CNC binary system is more efficient in modifying the contaminated soil with a lower amount of binder (to about 40%) and shorter curing ages (by nearly 4 times) than that of the sole cement. Overall, it is concluded that the cement/nano-clay mixture can be utilized as an effective S/S amendment and CNC content of 15 wt% per 1% of HM can successfully remediate the contaminated soil after 7 days of curing.