As consumption of metals has been readily increased since the Industrial Revolution, a large amount of iron has been used. To meet the demand, a number of mines have been developed and operated so far. It is certain that mining industry has confer benefits on human, whereas it is not deniable that a variety of mine hazards have brought about detrimental effects on environments including human being. In particular, acid mine drainage (AMD) of low pH contains a high level of toxic heavy metals and causes water pollution in mining area. When the AMD enters surrounding water bodies, colloidal precipitates form as a result of pH-neutralization. Those colloidal particles coat the surface of rocks in the bottom of water bodies, resulting in aesthetic problems and adverse impacts on aqueous ecosystem. Especially, AMD contains a high concentration of iron ions and they are precipitated and transferred to colloidal iron (oxyhydr)oxides when the environmental conditions are changed. For this reason, the research on this phenomena is needed. Iron (oxyhydr)oxides have been used as sorbents for removal of heavy metals due to their high point of zero charge (PZC) and large specific surface area. Particularly, it is well known that they are very effective in removing arsenic, and there have been a number of studies on interactions between arsenic and iron (oxyhydr)oxides. However, such studies have been conducted separately and individually, and there have been few systematic and comprehensive studies. As a result, it is difficult to well understand how a variety of iron(oxyhydr)oxides influence the behaviour of heavy metals and arsenic. In addition, a number of technologies have been developed to purify mine wastewater. Among them, permeable reactive barrier (PRB) is extensively used. However, the PRB technique is expensive to install, requires a large space, and is applied to limited forms of pollutants. Therefore, economical and effective methods should be developed by maximizing the merits of iron (oxyhydr)oxides.
For the reason mentioned above, this study was conducted to demonstrate the effective applicability of iron (oxyhydr)oxides to remove heavy metals and arsenic from mine wastewater. First, to attain the goal, the behaviour of nano-size colloidal zero valent iron (ZVI) and iron (oxyhydr)oxides was characterized with regard to composition and concentration of background electrolyte and pH change. To find the most effective analytical method to observe their behaviour, dynamic light scattering (DLS) and real-time single-particle ICP-MS (RT-SP-ICP-MS) were comparatively studied. The results show that colloidal particles of ZVI and iron (oxyhydr)oxides were aggregated and dispersed in the pH conditions near to and far away from PZC, respectively. In terms of the effects of positive and negative ions on their behaviour, it was proven that divalent and chloride ions are more effective on their aggregation than monovalent and nitrate ions. The results on the effect of natural organic matter (NOM) indicate that the dispersion of ZVI and iron (oxyhydr)oxides was enhanced with increase of NOM content as a result of electrostatic repulsion between colloidal particles due to coating of negatively-charged NOM on their surfaces. The study suggests that the DLS is an easy and quick analytical method to characterize the surface of those particles, but is not effective in observing their aggregation/dispersion behaviour. On the contrary, it is demonstrated that the RT-SP-ICP-MS can make up the defects of DLS method, identify the particle size distribution, and measure the size and shape of particles. Consequently, the RT-SP-ICP-MS seems superior to the DLS, but they should be used together to compensate for each other.
Next, the equilibrium and kinetic studies were conducted to investigate the interaction between arsenic and ZVI/iron (oxyhydr)oxides. In the case of arsenite [As(III)], ZVI and ferrihydrite showed the most efficient sorption capacity and the maxima of arsenite sorption appeared at the pH near to PZC as a result of the highest electrostatic attraction between them at that pH. For arsenate [As(V)], most of sorbents showed lower sorption capacity than arsenite [As(III)], except for ZVI. The sorption of arsenate was decreased with increase in pH. It might be caused by the fact that arsenate is negatively charged in natural environments and the surface of iron (oxyhydr)oxides become negative as pH increases, resulting in the decrease in sorption with increase in pH due to electrostatic repulsion between them in the higher pH conditions. Based on the regression analysis of experimental data, the most suitable sorption isotherm was sought by comparing typical isotherm equations, such as Langmuir, Freundlich, Redlich-Peterson, Temkin, and BET. The results show that Langmuir equation which represents the sorption of single molecular layer was most proper to fit the experimental data. Most of arsenic sorption was observed to terminate within 4 hours and the most suitable kinetic model was investigated to be Pseudo-second-order model, and the result is consistent with the other recent researches.
With experimental results on sorption between ferrihydrite and arsenic together with the results of EXAFS analyses, the mechanism of surface interaction between them was elucidated. The EXAFS analytical results indicate that most of sorption between arsenic and ferrihydrite appeared to be binuclear bidentate complexes. Based on such results, chemical stoichiometric equations were formulated by using previous references. Among various surface complexation models (SCMs), constant capacitance model (CCM) and diffuse-double layer model (DLM) were chosen in this study to simulate arsenic sorption onto the surface of ferrihydrite. Those two selected models were operated using FITEQL program. The results show that CCM was more suitable than DLM. Also, it was confirmed that (≡FeO)2AsOH0 and (≡FeO)2AsOOH0 are the most dominant species of surface complexation for arsenite [As(III)] and arsenate [As(V)], respectively.
In order to develop an efficient method to treat AMD using ZVI and iron (oxyhydr)oxides, permeable reactive kiddle (PRK) was newly designed and tested. First, the performance of various sorbents, such as steel slags and two kinds of waste cast iron [cast iron show (CIS) and grind precipitate dust (GDP)] were compared. Next, the mixing ratio of them was optimized. The experimental results show that optimal composition of sorbent were 70% steel slag, 15% CIS, and 15% cement and the most efficient size of PRK was 1.5 to 2 times larger than the height of wastewater. Using the block-type and zigzag arrangement of PRK, the removal efficiencies of arsenic, lead, copper, and cadmium contained in artificial AMD were 99%, 58%, 23%, and 15%, respectively. This result verifies that the PRK is easily and simply installed and economically effective, compared with conventional AMD treatment methods.