Research On PH Buffering Capacity Of Geological Media On Landfill Leachate

With the rapid development of industry and quick urbanization in recent years, the emergence of more and more solid wastes result in the serious contamination of the soil and water, especially the problem brought by the landfill leachate. Some research work which is related to the mechanism of the attenuation, migration and transformation of the contaminants as well as the property of the soil contaminanted by landfill leachate had been done by some foreign and domestic environmentalists, and some achievements have been got. Some related research of the buffering capacity of soil on landfill leachate in foreign country can be achieved, but the related research was few in our country.The high concentration dissolved organic carbon can be usually found in the landfill leachate, which is mainly various organic acids. Though various organic acids exist in the landfill leachtate, the pH of the soil and groundwater contaminanted by the leachate is still neutral. This fact proved the existence of the pH buffering process. The pH buffering capacity is related to the capability to resist to the pH fluctuation when some acid or alkali matter was added into the soil. The soil's pH buffering capacity is consisted of two parts, one is the buffering capacity possessed by the soil solution, and the other is possessed by the solid substances in the soil. Generally speaking, the amount of buffering substances in the soil solution is very small, accordingly the buffering capacity is also small. More than 90% buffering capacity of the soil is attributed to the existence of the solid substance in the soil. The pH buffering system can be divided into five different system according to the different pH range. They are calcium carbonate buffering system(pH6.2-8.5), silicate buffering system(pH 5-6.2), cation exchange buffering system(pH4.2-5.0), aluminum buffering system(3.5-4.2) and ferric oxide buffering system(3.0-3.5).Correlative datum indicated that: the chemical character, the existence state as well as the concentration of the main contaminants in the pollution plume such as organic matter, ammonium nitrogen, heavy metal were dominated by the pH, as well as the attenuation of the contaminants. Therefore, a soil column and static experiment selected fine sand, silty sand, clay and soil was constructed to investigate the temporal and spatial changing tendency of the stratum medium's pH buffering capacity qualitatively and quantitatively. Simultaneously the impacts of different types and different concentrations alkalinity on the temporal and spatial changing tendency of the stratum medium's pH buffering capacity was also investigated. The results of our experiments should provide some theoretical guidance for the natural remediation and risk estimation of the landfill.The pH buffering curve fitting equation of the four stratum mediums can be got based on the data scatter points, the background pH buffering curve conform to the first order exponential decayfunction y=y₀+A₁e^-x/t, and other pH buffering curves conform to the S curve function y=A₁-A₂/(1+e^Bx-C), and then according to the formulaβ_H =ΔH/ΔpH, we can calculate the each pH buffering system's capacity (β_H) and the total buffering capacity of each stratum medium.The order of the background pH buffering capacity of the four stratum mediums which have not been contaminanted by the leachate is that: fine sand < silt sand < clay < soil, and the buffering capacity is respectively 79.9, 207.5, 456.4 mmol/(kg . pH), but the total pH buffering capacity of the soil has not been able to calculated accurately. The proportion of the secondary buffering system's capacity in the total capacity for each medium is biggest, and the proportion for fine sand, silt sand, clay and soil is respectively 37%, 52.7%, 64.4%, 16%. The order of the capacity for each buffering system of each stratum medium is that: calciurm carbonate buffering system < silicate buffering system < cation exchange buffering system < aluminum buffering system < ferric oxide buffering system.The spatial changing tendency of the pH buffering capacity of the stratum mediums which have been contaminanted by the leachate is that: the calcium carbonate buffering system's capacity for each stratum medium all gradually elevated with the distance closer to the pollution source. The calcium carbonate buffering system's capacity of the fine sand, silt sand, clay and soil increased respectively from the background value 7.89, 1.62, 18.08,0 mmol/(kg . pH) to 21.72,41.31, 68.62, 114.89 mmol /(kg·pH), and has increased 1.76 times, 24 times, 2.8 times, 115 times respectively. The silicate and cation exchange buffering system's capacity of each stratum medium did not have obvious spatial changing tendency. The capacity of the aluminum and ferric oxide buffering system all present the reducing tendency with the distance closer to the pollution source except the soil.After the alkalinity of 6 mmol/100g soil's OH^-, HCO₃^-, CO₃^2- alkalinity was added into the soil, the capacity of the total and each buffering system all increased differently with the distance closer to the pollution source, and the capacity of the calcium carbonate buffering system has increased 238 times, 198 times, 220 times respectively. The three data all exceed the 115 times greatly that was not get any alkalinity in. The silicate and cation exchange buffering system's capacity increased slightly with the distance closer to the pollution source. But the aluminum buffering system's capacity increased rapidly from the background value 149.7 mmol/(kg·pH) to the "infinity" (in this experiment cannot calculate). The capacity of the soil's secondary buffering system that was added alkalinity can not be figure out too.The temporal changing tendency of the pH buffering capacity of the stratum mediums which have been contaminanted by the leachate is that: the calcium carbonate buffering system's capacity for each stratum medium all gradually increased with the reacting time went on. The calcium carbonate buffering system's capacity of the fine sand, silt sand, clay and soil increased respectively from the background value 7.89, 1.62, 18.08, 0 mmol/(kg . pH) to 12,29.2, 55.1, 61.6 mmol /(kg·pH), has increased 0.52 time, 17.02 times, 2.05 times, 61.6 times respectively. The silicate and cation exchange buffering system's capacity of each stratum medium did not have obvious temporal changing tendency. As to the changing tendency for the capacity of the aluminum and ferric oxide buffering system with the time went on, the soil's gradually increased, the silty sand and clay's gradually decreased, and the capacity of the soil's aluminum buffering system increased in the initial time and then decreased.After the each alkalinity was added into the soil, the capacity of the calcium carbonate buffering system all increased with the concentration of added alkalinity elevated, but it decreased slightly with the time went on. The effect of different type and different concentration alkalinities on the temporal changing tendency of silicate and cation exchange buffering system is not obvious. The capacity of the aluminum buffering system increased with the concentration of added OH^- and HCO₃^- elevated, and with the time went on, the buffering capacity increased in the initial time and then decreased. However the capacity of the aluminum buffering system decreased with the concentration of added CO₃^2- elevated. The capacity of the soil's ferric oxide buffering system which was added different type and different concentration alkalinities in also can not be figure out. At the same time with the time went on, the total buffering capacity increased with the increasing alkalinity concentration of the same type except the ferric oxide buffering system. As for the same added alkalinity concentration, the total buffering capacity except the ferric oxide buffering system which was added OH" was bigger than the ones which were added HCO₃^- and CO₃^2- . After reacting with the landfill leachate, the total spatial and temporal pH buffering capacity of the fine sand increased gradually, and the silty sand and clay's decreased gradually, but the soil's increased in the initial time and decreased with the time went on. The calcium carbonate that reproduced contributed greatly to the total buffering capacity. Although the spatial and temporal changing tendency of each stratum medium's capacity were different, the order of the capacity of the four stratum medium to the leachate is that: soil > clay >> silty sand > fine sand. The total buffering capacity of the soil that was added alkalinity all increased significantly on the spatial and temporal changing tendency.