1 Influence of the presence of Zincon the behaviour of bentonite Saswati Ray, Bismoy Roy Chowdhury, Anil Kumar Mishra & Ajay Kalamdhad Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India ABSTRACT In landfill disposal sites bentonite is used as liner material because of its high contaminant adsorption limit, high swelling capacity and low hydraulic conductivity. However, the effectiveness of liner may reduce due to the chemicals present in the leachate. Swelling and sorption capacity of bentonite may effect in the presence of various chemicals present in the leachate, which in turn reduces the thickness of the diffuse double layer (DDL). Consolidation is an important parameter of liner material, which is essential for settlement calculation. Therefore, it is necessary to study the change in the behaviour of bentonite in the presence of heavy metal. In this investigation, the effects of zinc (Zn 2+ ) of varying concentrations were studied on the behaviour of bentonite. Solutions of zinc 0 (i.e., De-ionized (DI) water), 100 and 1000 ppm concentration were prepared by dissolving salts of zinc nitrate. The results revealed that rate of consolidation and hydraulic conductivity increases with the increase in concentration because of the presence of Zn 2+ in pore water. However, liquid limit, swelling pressure and swelling potential, was decreased. Results also illustrate that at higher concentration, the impact of Zn 2+ on the behaviour of the bentonite is more significant. 1 INTRODUCTION Excessive exposure of lead can impose potential menace to the ecosystem and can cause an adverse effect on the health of adults and children. Zinc can be severely toxic to the human being if the amount exceeds the permissible limit. Various sources, for example, mining and smelting, sewage water, households, tap water system and industrial waste water containing zinc are the major sources of lead exposure to the human being and environment. This exposure can cause toxic symptoms like cholesterol balance, stomach cramp, infertility, vomiting, and nausea and can even diminish the function of immune system (Zang et al. 2012). Several researchers have attentive on the usage of low-cost adsorbent in order to reduce the processing costs. Clay minerals are economical, readily available, and a good substitute for conventional treatment of the leachate as an adsorbent (Sanchez et al. 1999). Clay minerals are used to eliminate the various type of contaminants such as removal of zinc using bentonite as an adsorbent from aqueous solution by the mechanism of adsorption and ion exchange (Mellah and Chegrouche, 1997; Kaya and Ören, 2005). Other Clay minerals like palygorskite clays, raw kaolinite is used as an adsorbent for the removal of various heavy metals from aqueous solution (Potgieter et al, 2006; Yavuz et al. 2003).Leachate contains heavy metal, which can contaminate the surrounding environment and groundwater. In order to prevent contamination by leachate, clay barrier is used in the landfill so that it cannot further migrate. Bentonite is utilised as a liner material due to its low hydraulic conductivity, contaminant adsorption capability and high swelling capacity (Dutta and Mishra, 2016). One of the utmost essential property, which helps in computing the settlement of the clay liner, is the compressibility of liner material (Mishra et al. 2010). Bentonite is an extremely compressible material. Because of the load of the waste piled in the waste disposal site, bentonite is compressed easily. When the surcharge load is applied to a saturated clay, consolidation process starts due to compression, which results in settlement, and the extent is determined after the thorough dissipation of the generated pore-water pressure (Yong and Warkentin 1975). In the past, many studies have been carried out regarding the change in the behaviour of bentonite in presence of various contaminants. Lo et al. (2004) investigated the movement of lead, zinc, and cadmium in and saturated Ottawa sand and bentonite-soil admixture and they concluded that the hydraulic conductivity of bentonite-soil admixture increases considerably when it is permeated with the metal solution. Ouhadi et al. (2006) examined the interaction of the heavy metal ion with bentonite at various pH level. They observed that at high heavy metal concentration and low pH, there is a microstructural change in bentonite. They also concluded that the rheological performance of bentonite is controlled by the osmotic phenomenon. Nakano et al. (2008) conducted the test on three locally available Japanese bentonite and one US bentonite to evaluate the hydraulic conductivity behaviour and lead retention mechanisms and they observed that carbonate plays the main role at low concentration of lead and precipitate as PbCO3. They also concluded that as a result of higher swelling capacity and montmorillonite content, hydraulic conductivity of the U.S. bentonite is lower in comparison with Japanese bentonite. Du et al. (2015) explored the consequences of different levels of lead contamination on clayey soil/calcium–bentonite backfills. They concluded that with the rise in the lead concentration pH, liquid limit and compression index (Cc) increases and the hydraulic conductivity increases to fifty-fold in comparison with clean backfills. Bentonites with different mineralogical compositions