Impact of retention phases of zinc heavy metal contaminant on the plasticity properties of bentonite in thermal improvement

In geotechnical and geo-environmental projects such as thermal stabilization, thermal remediation of contaminated soils and nuclear waste disposal, clays are always exposed to heat and heavy metal contamination. The study of the effect of heavy metal contaminants and thermal treatment on the geo-environmental engineering properties of clayey soils has long been considered by many researchers. Calcium carbonate as a major component of clay soils and as a non-plastic material reduces the plasticity properties of the soil. Calcium carbonate affects the process of heavy metal adsorption by clay particles. Accordingly, the presence or absence of calcium carbonate in the soil can have a secondary effect on the plasticity properties of clay. Generally, clayey soils retain the heavy metal contaminants by four phases. These phases include retention by cation exchange, precipitation by hydroxide carbonates (oxide and hydroxide), organic fraction and residual retention. A review of the literature studies has shown that little attention has been paid to the effect of retention phases of heavy metal contaminant on the plasticity properties of bentonite in thermal improvement from a micro-structural point of view. For this reason, this study is aimed to investigate the influence of retention phases of heavy metal contaminant on the behaviour of bentonite in thermal process. A natural bentonite soil is used in this study. The soil has been decarbonated by the use of hydrochloric acid.  To achieve the above mentioned objective, carbonated and decarbonated bentonite was prepared in a non-contaminated state and laboratory contaminated with heavy metal zinc (Zn) at concentrations of 5, 10, 20, 70 and 120 cmol/kg-soil. Contaminated and non-contaminated samples are first ground and then subjected to a temperature of 20, 110, 300, 400 and 500 °C for two hours. Then, by the use of Atterberg limit, XRD, pH and SSE experiments, micro-structural and macro-structural analysis of changes in carbonated and decarbonated bentonite plasticity properties has been investigated. According to the achieved results, in low concentrations of zinc heavy metals, calcium carbonate phase of heavy metal retention is the dominant phase in soil contaminant interaction process which prevents the change of bentonite structure. Therefore, the reason for the reduction of the Liquid limit is the reduction of the electrical charge of the clay particles, which is the result of lowering the pH. By an increase in contaminant content, all of the soil retention phases contribute to soil contaminant interaction process. Therefore, the role of calcium carbonate reduces in soil plasticity behaviour changes. As the temperature rises, Zinc metal as an accelerating agent has further reduced the Liquid limit in carbonated bentonite and decarbonated bentonite. In decarbonated bentonite, due to the absence of calcium carbonate, the clay particles adsorb more zinc. As a result, the effect of lowering the Liquid limit during increasing temperature by the heavy metal zinc in decarbonated bentonite is greater than in carbonated bentonite. Bentonite with its predominant sodium and zinc exchange cations loses its plasticity properties at temperatures of 400 and 500 °C, respectively, and the plastic limit for them is not measurable.