In landfill clay liner systems, temperature is maintained between 40 and 50℃ for many years because of the decomposition of wastes. Thus, the temperature effect on the diffusion and adsorption of heavy metals should be considered as a very important subject to investigate the heavy metal transfer mechanism. However, the temperature effect on the migration of heavy metals have thus far received little attention. Therefore, in this study, we conducted a combined adsorption sequential analysis for Pb, Cu, Zn and Cd at each temperature of 15, 25, 40, and 55℃ to investigate the effect of temperature on the adsorption characteristics of metals, not only for bulk samples, but also for the individual phase composing it.
We also conducted a combined diffusion sequential analysis for Zn and Cd, which have faster mobility than the others. It was carried out at each temperature of 15 and 55℃ to determine an effective diffusion coefficient and to verify partitioning characteristics of the metals for each soil constituents under the diffusion and retardation condition. Five different phases of the sequential extraction analysis are 1) exchangeable phase, 2) carbonate occluded phase, 3) Mn-oxide occluded phase, 4) organic occluded phase, and 5) Fe-oxide occluded phase. In order to extract metals from each of the occluded phase, pertinent extractants were used. The results of experiments could be used in understanding the adsorption mechanisms of metals for a natural clay soil and provide with fundamental informations to estimate the possible mobility and fate of metals under the variation of pore water environments.
According to the combined adsorption-sequential extraction analysis, it can be known that in accordance with increasing temperature from 15 to 55℃, adsorption equilibrium constants($K_ads$) and adsorption capacities($Q_0$), which represent the adsorption energy or affinity and adsorption maxima for the monolayer, respectively, tend to increase for all the phases of soil except for the adsorption in the exchangeable phase. These results mean that the adsorption reaction of metals for each individual phase is endothermic, representing that the adsorption enthalpy change is larger than zero. However, for the adsorption enthalpy change for the exchangeable phase adsorption was nearly equivalent to zero($ΔH^0>0$). However, for the adsorption enthalpy change for the exchangeable phase adsorption was nearly equivalent to zero($ΔH^0\simeq0$).
Besides, it was found that in the adsorption of lead and copper, over 50% of adsorption was occluded in carbonate phase, and in case of cadmium, over 80% of adsorption was occluded in exchangeable phase, which is an easily mobile and bio-available phase. As for zinc, at temperature below 25℃ about 60% was occluded in exchangeable phase but above 40℃ about 50% was occluded in carbonate occluded phase. The effect of temperature on each occluded phase was largely observed in Fe-oxide, Mn-oxide and organic occluded phases.
Based on the column diffusion tests, which were conducted for Cd and Zn at 15℃ and 55℃, the effective diffusion coefficient for the metals increased up to ten times with the temperature increase from 15 to 55℃. This result can be interpreted that molecular mobility of the metals increased with the temperature increase. Moreover, with the temperature increase, the migration length of Cd increased from 7mm(at 15℃) to 15mm(at 55℃), but for the case of Zn, the migration length(7mm at 15℃) was not changed with the increase in temperature. It can be interpreted that the increase of Zn adsorption quantity with the temperature increase affected the transport of the metal.
As we compare the adsorption isotherm results obtained from both diffusion and batch adsorption tests at each temperature, it can be recognized that small amount of adsorption quantity was obtained from the column diffusion tests. The difference could be explained as follows. In case of the batch adsorption tests, all the surfaces of clay particles are accessible for the heavy metal ion. However, as for the column diffusion tests, the ion solution follows specific migration paths and merely very small parts of the clay surface are in contact with the heavy metal due to the formation of peds and other micro-structural features of the soil column that contribute to the significant changes in available reactive surface.
According to the result of sequential extraction analysis following the diffusion test, Zn was mainly partitioned in the carbonate phase and this result is similar to the batch adsorption test results. As for Cd, exchangeable phase partitioning was over 60%, and hence a little difference could be seen for the variation of partitioning characteristics with the variation of temperature. However, in the case of relatively low concentration, the partitioning of the metals was occurred in the oxide phase. This result represents that the concentration loading also affects the heavy metal partitioning characteristics in natural clay soils.
Finally, as we compare the effective diffusion coefficients, an overestimated effective diffusion coefficient might be obtained if we use an overestimated retardation factor obtained from the batch adsorption test. We also know that the effective diffusion coefficient depends on the retardation factor, which could be obtained from the two methods of the batch adsorption and column diffusion tests. Therefore, in order to decide a resonable effective diffusion coefficient that can be used to design the landfill clay liner and to predict the transfer characteristics of the metals through it, we also have to consider the procedure of obtaining the retardation factors.
