Arsenopyrite has been considered to be chemically unstable in the surficial environment. The new thermodynamic data suggest that arsenopyrite has similar Eh?pH stability range to pyrite, except under acid conditions (pH<4) where arsenopyrite should transform to realgar or orpiment. Arsenopyrite decomposes in oxidised waters to yield up to 1100 ppm dissolved As. These concentrations of dissolved As are several orders of magnitude lower than equilibrium solubility of arsenopyrite because of kinetic effects and development of protective oxide coatings on arsenopyrite grains.
Arsenic which is found in several different chemical forms and oxidation states, causes acute and chronic adverse health effects is a toxic trace element widely distributed in soils and aquifers from both geologic and anthropogenic sources. Arsenate (As(V)) is the thermodynamically stable form of As under aerobic condition and interacts strongly with solid matrix. However, It has been known that adsorption and oxidation reactions of arsenite (As(III)) which is more soluble and mobile than arsenate (As(V)) in soils are two important factors affecting the fate and transport of arsenic in the environment. That is, the movement of As in soils and aquifers is highly dependent on the adsorption?desorption reactions in the solid phase.
Adsorption and miscible displacement and of arsenic which could be derived from arsenopyrite rock and gravels buried in soils as landfill to treat general environmental waste treatment were investigated to identify its fate and behavior leading to groundwater To achieve this goal, we observed the types and status of arsenic in rocks, solubilities, redox potential, arsenic adsorption isotherms depending on soil solution pHs, adsorption kinetics, mobility and miscible displacement for two different soils of sandy clay loam and sandy loam with and without organic matter. From the experiments, we obtained the following results.
1. The amounts of arsenic in arsenopyrite rock fractions collected from the suburb of GyeongJu were ranged from minimum 16.04 ㎎ ㎏-1 from maximum 272.6 ㎎ ㎏-1. pH of arsenopyrite rock was 9.61 classified as a strong alkaline.
2. Water holding capacity decreased with increasing mixing ratios of arsenopyrite as well as and increasing particle size while saturated hydraulic conductivity playing a crucial role in mobility of soil substances was slightly increased with increasing mixing ratios of arsenopyrite as well as and increasing particle size.
3. From adsorption isothermal experiment about arsenic, Adsorption of As was gradually reduced with increasing pH in both kinds of soil. The results of regression analysis about Langmuir and Freundlich adsorption isotherm showed that Langmuir adsorption has a higher correlation coefficient than that of Freundlich adsorption isotherm. Sandy clay loam soil has somewhat higher ionic adsorption than that of sandy loam soil, due to higher surface area in clay and organic matter content influencing the velocity of moving fluid as well as interaction at solid and water interface.
4. The results of pH change measured from effluent showed that pH were increased by approximately max. 4, 1.1, and 1.5 units compared with pH of influent for 3, 6.64 (UPW), and 9, respectively. This represented that the effect of pH was more distinctive with increasing mixing ratios and smaller particle sizes.
5. The results of changes of Eh in the effluent showed that the maximum Eh was in the order of pH 3, pH 9, and pH 6.64, and the pore volumes to reach those maximum Ehs were 3.5, 3.2, and 3.7 for the influent pHs of 3, 6.64, and 9 in sandy clay loam, respectively.
6. The cumulative amount of As recovered from the effluent were decreased in the order of the influent pHs of 3, 6.64, and 9 in sandy clay loam and sandy loam, respectively. However, the total amount of As from sandy clay loam was significantly lower that those of sandy loam, indicating that adsorption As was much greater than that in sandy loam due to clay content.
7. We verified that the forms of arsenic ion in effluent were As (Ⅴ); HAsO42-, H2AsO4-, and H3AsO3. The changes of electrical potential and pH in soil is the most probable reason why As (Ⅴ) reverts to As (Ⅲ). In the initial stages, acidic solution as inflow water to the soil could dissolves arsenic from arsenopyrite. In the dissolution process of As from arsenopyrite gravel fractions, it turns into As (Ⅲ) form due to lower in pH. It’s assumed that redox potential of soil proceeds as anaerobic condition caused by continuous inflow water produced and As (Ⅴ) turns into arsenic (Ⅲ) by reduced redox potential. Almost every organic matter donates electron as Lewis base and it turns into As (Ⅲ) with accepting electron and hydrogen of As (Ⅴ) soil solution. As clay content increased more, surface area participating the response is widened and hydraulic conductivity is decreased. That’s why it can be stay in soil and have more time to change into the form of As (Ⅲ).
8. Adsorption kinetics of As (Ⅴ) in sandy clay loam and sandy loam under various solution pHs showed that approx. 80 % of As from solution phase was disappeared, indicating that As adsorption was instant reaction onto the surface of charged clay and organic matter. However, the instant adsorption was distinctive with decrease in pH of solution.
9. About arsenic isothermal adsorption of organic matter, more adsorption occurs in lower concentration. And it was verified that it has an inverse correlation with higher concentration. Generally, adsorption kinetic of sandy clay loam soil has more adsorption and movement than adsorption kinetic of sandy clay soil. And adsorption and movement are also increased as retention time increment is increased. In the case of organic matter, it showed relatively rapid adsorption than in soil and we ascertained two hundred eighty eight times of adsorption movement as large as equilibrium concentrations.
10. For breakthrough curves, we found that the pore volumes of water to reach the relative concentration 1 was greater in sandy clay loam than those of sandy loam, meaning that clay content influenced the transport of As in soil in addition to soil pH.
From the result mentioned above, it’s inquired that adsorption and movement of arsenic component generated from arsenopyrite decides the form of arsenic according to pH, redox potential, organic content, soil texture and water characteristic. Thus, it’s measured that we have to control condition of soil to make arsenic absorb onto soil and organic in order to avoid its inflow into ground water. It’ll prevent the water as proper drinking water from arsenic contamination and be a very useful method to apply purifying technology for polluted soil by arsenic.