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Assessment of the leachability and mechanical stability of mud from a zinc-plating plant and waste zeolite binding with portland cement

In the recent years, rapid industrialization has led to an increasing in the quantity of hazardous wastes. These waste materials must be properly disposed or reuse in new composite materials. This includes methods of waste treatment and stabilization/solidification (S/S) process with cementitious or pozzolanic binders. S/S process reduce the hazardous potential of waste materials by transforming wastes into less soluble form and improve the handling and physical characteristics of wastes. Cement-based S/S process is based on the reactions of the Portland cement and constituents from the waste material (heavy metals). This study has examined the immobilization of mud from a zinc-plating plant and waste zeolite materials with Portland cement type CEM I. Determination of leachability was assessed by NEN 7345 leaching test. The study of leaching behaviors of metals is an important way to obtain valuable information about the chemical speciation of contaminants in the S/S waste matrix and their potential environmental risks. The prediction of leaching behavior in the environment can be accomplished by means of predictive mathematical models such as mechanistic and empirical leach models. They can help identify the leaching mechanism and can provide methods for correlating leach information. The mechanical stability of immobilized waste materials was tested by measuring compressive and flexural strength. Evaluation of stabilization/solidification process was performed by determining the diffusion coefficients (De) and leachability indices (LI). According to the Canadian Agency for the Environment for the leachability index above 9 process is successful, and the immobilized material is suitable for "controlled utilization" (rehabilitation of quarries, lagoons and road construction). For leachability index between 8 and 9, the immobilized material can be disposed of in sanitary landfills. For leaching index less than 8, material is not suitable for disposal. The samples with the addition of electroplating waste showed average mobility of zinc ions with leachability index values above 9 for all samples. With addition of waste zeolite in cement-mud system, zinc has become even less mobile with even greater leachability index. The mechanisms controlling the release of Zn from the sample were determined using a diffusion model developed by de Groot and van der Sloot. According to this model, if the slope of the linear regression is 0.5, Zn release is slow and diffusion is the controlling mechanism. If the slope is close to 1, the controlling mechanism is dissolution and if the slope is close to 0, the mechanism is wash-off. Results show that controlling mechanism for leaching of zinc is wash-off (except for sample with the addition of 5 wt. % of waste). For samples with lower addition of waste zeolite, leaching mechanism is diffusion, whereas at higher additions, the leaching mechanism is wash-off. From the results of measuring the compressive strength of mortars with different additions of mud and zeolite it can be concluded that the samples with the additions showed lower compressive strength than the sample without additives. According to standard EN 197-1, early compressive strength for CEM I should be ≥ 20 MPa (after two days of hydration), and normalized strength (after 28 days of hydration) should be ≥ 42.5 MPa. With regard to the standard limit, Samples 1 and 2 meet these conditions. The addition of a mixture of mud and zeolite leads to an even greater reduction in compressive strength. Standard prescribed limit satisfied only a Sample 4. With regard to the standard limit for this type of cement, mud shares are eligible to 5 wt. % and a mixture of mud and zeolite up to 10 wt. %. FTIR spectra indicate the negative effect of zinc on the Portland cement hydration. Lower intensity peaks were observed at the same time with movement the peak towards smaller and larger values of wave numbers.

portland cement ; mud from a zinc-plating plant ; waste zeolite ; mechanical stability ; leaching of zinc

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