Adsorption And Desorpton Characteristics Of Arsenate On/from Iron Oxides And Their Complexes With Humic Acids

Humic acid and iron oxides are important colloidal substances co-existenting in soils. They and their interactions significantly impact soil structure formation, differentiation of soil layers, speciation, bioavailability and biogeochenmical cycling of elements. Adsorption and desorption are the major processes affecting the mobility and transformation of arsenic in soils. Iron oxides can combine arsenate forming outer-or inner-layer complexes through specific and non-specific adsorption in the soil, which are influenced by mineral types, pH and ionic strength. In soil environment, coexisted humic acid seriously affected the adsorption of arsenic on iron oxides. In this study, the adsorption and desorption characteristics of arsenic on and from goethite, hematite, ferrihydrite and their complexs with humic acid were compared by the methods of adsorption-desorption thermodynamics and kinetics, and the influences of arsenic initial concentration, pH, ionic strength were also analyzed by batch experiments. The mechanisms of arsenic adsorption on iron oxides and their complexes with humic acids were discussed based on the adsorption kinetics in combination with XRD spectroscopy and scanning electron microscopy (SEM). The main results are as follows:Absorption-desorption characteristic of arsenic on/from iron oxides:Arsenic (arsenate) adsorption on iron oxides is increased with rising of arsenic concentration, and the rate of adsorption is fast in low concentrations and slow in high. The isothermal adsoiption thermodynamics can better fit with freundlich equation and langmiur equations. Adsorption capacity is difference among the three types of iron oxides, the order of the maximum adsorption capacity is:ferrihydrite (19.31mg/g)> goethite (8.93mg/g) The hematite (6.66mg/g), that may because of the larger specific surface area and non-crystal structure of ferrihydrite. Dynamics is good fit with the two-constant equation, Elovich equation and pseudo first order kinetics. The absorption runs rapidly at the beginning of the adsorption process and then slowing down to an equilibrium stage. The adsorption rate of ferrihydrite is fastest, which reach to90%of equilibrium after4h, goethite and hematite about12h,15h, respectively.The desorption and adsorption are significant positive correlation, that indicate the higher capacity of adsorption, the worse strength of it. The desorption rate of ferrihydrite is highest, which reach to44.42%, while goethite and hematite is15.43%and25.43%, respectively. That because the arsenate physically adsorpte in the surface of ferrihydrite. That is not solid and arsenate is easy to disperse from the surface of iron oxide. The desorption rate is fast at the beginning and slowing down. Ferrihydrite achieves the90%of equilibrium desorption in the start1h, while goethite and hematite achieve equilibrium after8h. Compared with the adsorption, desorption is faster to reach equilibrium.Acidic conditions are favorable for the adsorption. Adsorption reduces with the pH increases. When ionic strength is0.01mol/L, pH increased from2to10, ferrihydrite goethite and hematite adsorption decrease from17.35mg/g,13.58mg/g and12.64mg/g to12.83mg/g,6.47mg/g, and5.48mg/g; the decrease ranges are about26.05%,52.36%,56.66%. This is because as pH increases, the increasing OH-in solution can compete to adsorpte with arsenate.; while the positive charge of the mineral surface is in favor of the combination of arsenic in the acidic environment.High ionic strength is good for adsorption of arsenic. At pH6.5, ionic strength increases from0.01mol/L to0.5mol/L, ferrihydrite, goethite and hematite adsorption increase by0.80mg/g,1.89mg/g and1.74mg/g. the growth rate:15.25%,10.52%,32.26%. The increase of ionic strength makes the adsorbent surface electric double layer is compressed and arsenate easily diffuse into the mineral surface. That shows this adsorption is specific adsorption, iron oxide and arsenic form single-or dual-core groups, and inner complexes.The arsenic absorption-desorption characteristic on/from the complexes of iron oxides with humic acids:Humic acid and iron oxide complex reduce arsenic adsorption and adsorption rate, but did not alter the basic characteristics of arsenic adsorption on iron oxide. The order of adsorption is:ferrihydrite-humic acid (F-HA)> goethite-humic acid (G-HA)> hematite-humic acid (H-HA), the maximum adsorption is:15.17mg/g,6.85mg/g and5.17mg/g. That order is consistent with the single iron oxide, but the adsorption capacity lower by21.44%,23.29%,22.37%than corresponding single iron oxide. Humic acid inhibited the adsorption of arsenic on iron oxides resulting in adsorption decreases. That because humic acid will occupy the adsorption sites of iron oxide surface. The competitive adsorption of humic acid and arsenic inhibit arsenic adsorption. On the other hand, humic acid decreases the surface charge of iro(?) rea(?)es the surface positive charge and increases the negative charge. That is not conducive to the combination of arsenic. Humic acid can effectively reduce the desorption rate and capacity of arsenic on iron oxide.The max desorption rate of F-HA、G-HA、H-HA decrease by20.18%,6.90%and19.6%than single iron oxides.Adsorption of complexs decrease with pH increase. When ionic strength is0.01mol/L, pH increase from2to10, F-HA, G-HA and H-HA adsorption decrease from15.85mg/g,9.57mg/g and8.44mg/g to11.58mg/g,5.76mg/g and3.96mg/g. The complex downward trend slower than the single iron oxide, that because the humic acid reduces the iron oxide surface zero charge, which further inhibits arsenic adsorption in neutral and weak alkaline conditions; on the other hand, humic acid can reduce the influence of pH on the adsorption so that adsorption maintain a gentle decreasse in a certain pH range. This is associated with the buffering capacity of humic acid itself.High ionic strength can promote the adsorption of the complex. When the ionic strength increased from0.01mol/L to0.5mol/L, F-HA, G-HA and H-HA adsorption increased by2.03mg/g,3.13mg/g,2.59mg/g. Complex shows a stronger adsorption growth trend. With the increase of ionic strength, humic acid inhibition of arsenic adsorption on iron oxides can be effectively reduced, that is because humic acid contains a large number of active groups which can play its metal complexation in high concentration media.XRD analyses and SEM observations:The combination of humic acid with iron oxides did not change the crystal structure of iron oxides. It occurs only on the surface of iron oxide, which may change the surface properties of iron oxides, and some groups on the surface have been replaced by some functional groups of HA. Such changes impact the arsenic adsorption and desorption on the iron oxides.