For the effluent concentration of Fetotal Fe plus
First of all, it should be noted that the aqueous concentration of As remained well above the criteria defined by USEPA drinking water standard (USEPA, 2009), and was slightly increased with progressing leaching steps. In contrast, the aqueous concentration of Zn and Pb exhibited a biphasic declining pattern, thereby sharply dropping in the initial 2–3 leaching steps and gradually reducing near or below the criteria. Meanwhile, the percentage (%) cumulative mass of leaching during eight consecutive steps was in the order of Zn ≅ Cd > Cu > Pb > As (Table S2). Note that for both As and Pb, <1% of mass that was initially present in the samples was moved into the aqueous phase. The initial concentration and declining pattern were reflected by the magnitude of ffast and kfast. For example, a relatively high aqueous concentration of Zn at the initial step followed by a dramatic PR-619 led to high values of ffast and kfast ( Fig. 1A and B). The respective values for As and Pb were very low ( Fig. 1C–F), indicating that these two elements are highly resistant to leaching over extended periods. The patterns of iron (Fe) and sulfate (SO42-) concentrations ( Fig. 1G–J) were similar to those of Cu, Zn, Cd, and Pb. Both Fe and SO42- can be produced through dissolution of iron-containing sulfide and/or sulfate minerals, such as pyrite and arsenian (i.e., As-bearing) pyrite. Sulfate anion can also be produced by oxidation of sulfide minerals. The greater concentration of Fe and SO42- in DY soil compared to that in BS soil is consistent with the XRD results which showed that pyrite and metal-sulfate minerals are abundant in DY soil. The high concentration of Fe2+ in DY soil (white circle; Fig. 1G) supports the possibility for the oxidation of Fe(II)-bearing minerals such as arsenian pyrite ( Jambor, 1994);equation(1)FeAsxS2-x+(7/2-2x)O2+H2O→Fe2++(2-x)SO42-+2H++xAsaqwhere x is the fraction of As present in pyrite. Assuming x = 0 in Eq. (1), pyrite (i.e., FeS2) oxidation occurs. In case of soils in this study, the molar mass of sulfate far exceeded that of iron, suggesting the generation of sulfate from a source other than pyrite minerals.