Adsorption and Post Adsorption Behavior of Schwertmannite with Various Oxyanions

The adsorption and post adsorption behavior of schwertmannite with various oxyanions were investigated for clean-up contaminated water with hazardous oxyanions and safe disposal of spent schwertmannite. The result of adsorption experiments showed that the maximum capacities of oxyanions adsorption onto schwertmannite are 1.023, 0.934, 0.723 and 0.313 mmol/g for arsenate, phosphate, chromate and selenate, respectively. Based on the differences in the adsorption capacities, the selectivity of oxyanion adsorption on schwertmannite decreases as the order: arsenate ≥ phosphate > chromate >> selenate. Change in the Zeta potential after adsorption by arsenate, phosphate and chromate were very different from those after adsorption by selenate and of the original schwertmannite. This difference implies that the adsorption mechanism on schwertmannite with arsenate, phosphate and chromate is different from that with selenate and sulfate. Arsenate, phosphate, and chromate ions form inner-sphere complexes with the surface of schwertmannite, while selenate and sulfate ions form outer-sphere complexes with the surface of schwertmannite. Based on a comparison with anion adsorption, strong base anions form inner-sphere complexes, which induce a strong adsorption with schwertmannite as well as it is conducive to high adsorption capacity. From the results of aging experiments, schwertmannite with sulfate and selenate changed to a more stable phase, goethite, in a short time, whereas there is no change in the XRD patterns of schwertmannite with arsenate and phosphate after 30 days. The stability of schwertmannite after the adsorption increased in the following order: sulfate ≅ selenate  chromate < phosphate ≅ arsenate. The solubility of schwertmannite with different oxyanions was calculated according to solid solution theory. The solubility of schwertmannite decreased after adsorption of oxyanions with high selectivity. It is concluded that oxyanions with high selectivity can stabilize schwertmannite by decreasing the solubility of the schwertmannite after adsorption of the oxyanions. How to cite this paper: Khamphila, K., Kodama, R., Sato, T. and Otake, T. (2017) Adsorption and Post Adsorption Behavior of Schwertmannite with Various Oxyanions. Journal of Minerals and Materials Characterization and Engineering, 5, 90-106. https://doi.org/10.4236/jmmce.2017.52008 Received: February 27, 2017 Accepted: March 28, 2017 Published: March 31, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
Water contamination is one of the serious problems worldwide. The source of the contamination is both from anthropogenic as well as from natural sources.
For example, the source of arsenic in contaminated water may be from active or abandoned mines, and also mineral processing and sediments [1] [2] [3] [4] [5].
To treat toxic elements such as arsenic, there are numerous established processes such as coagulation/co-precipitation, adsorption, and ion-exchange etc. [6] [7] [8]. Adsorption is an example of simple easily applied techniques. Adsorption has been reported in removal of hazardous cations using activated carbon, zeolites, and clays [9] [10]. However, the removal of toxic oxyanions is more difficult than that of removing cations because natural waters contain anions with similar structures, including nitrate, sulfate, and phosphate often coexisting in high concentrations. However, iron oxides and hydroxides are excellent scavengers both for hazardous cations and also hazardous oxyanions. In iron oxides and hydroxides, there are several mineral species such as goethite, hematite, and ferrihydrite etc. Schwertmannite is a meta-stable iron oxy-and hydroxy-sulfate found in acidic iron-and sulfate-rich environments such as sulfide metal mines [11] [12] [13] [14]. Schwertmannite is well known to play an important role in the removal toxic elements from the acid mine drainage [15] [16] [17] and in natural attenuation processes of hazardous elements in acid mine water [18].
As mentioned above, schwertmannite is a meta-stable phase and easily transforms to goethite in pure water, while it is also an excellent adsorbent. Both the adsorption capacity and the stability of adsorbents need to pay attention safety at the sites where adsorbents are deposited. However, it is also well known that the stability of schwertmannite is changed and stabilized after arsenate adsorption [19], but the adsorption and post-adsorption behavior of schwertmannite with other kinds of oxyanions have not been fully or systematically investigated. With this background, the adsorption properties of oxyanions including arsenate, phosphate, chromate, and selenate of schwertmannite were investigated. The stability of schwertmannite after adsorption of the above oxyanions was compared to gain a better understanding of the post-adsorption behavior of schwertmannite with the different oxyanions.

Preparation of Schwertmannite
Schwertmannite was prepared by the method previously reported by Bigham et al. [13]

Adsorption Experiments
All adsorption experiments were conducted at a constant ionic strength (I = 0.01M, NaNO 3 ) by using a 50 ml centrifuge tube adding 40 ml solution of

Oxyanion Adsorption Experiments
As shown in Figure 1, the amounts of adsorbed oxyanions increased with increasing initial concentration of anions. The pHs of the initial solution adjusted to neutral then decreased to 3.5-4.5 after adsorption ( Figure 2).
This decrease would be due to release of protons from schwertmannite during the adsorption. Further, the 2 4 SO − concentration in the reaction solutions was determined after the adsorption because 2 4 SO − originally adsorbed on schwertmannite. The 2 4 SO − concentration in the reaction solution initially 0.123 ± 0.002 mmol/g without adsorption of any other oxyanions, which is a value similar to that in previously work [21]. During the adsorption process, the   anions in the solution (Figure 3(b)). The decreasing ratios were different among the anions in the following order, arsenate ≥ phosphate > chromate  selenite.
The zeta potentials of schwertmannite before and after adsorption of the anions were measured at different pHs ( Figure 4). Schwertmannite before ad-  The schwertmannite has positive and negative potentials at pH less and more than 7. Schwertmannite with selenate changes similar to that of schwertmannite with sulfate. PZC of selenite-schwertmannite is at around 6.2. Schwertmannite with arsenate, phosphate, and chromate showed different changes from that of schwertmannite with sulfate with a PZC at around 5. Among arsenate, phosphate, and chromate, schwertmannite with chromate changes is slightly different curve at pH range between 6 and 10 with less negative potentials.

Aging Experiments
From the results of the aging experiments, the XRD patterns of samples clearly indicated the difference in the stability of schwertmannite with different oxyanions and aging times as shown in Figure 5. In schwertmannite with sulfate ions (pure schwertmannite), the peaks from goethite appeared after 7 days, while the peaks from goethite appeared after 14

Difference of Adsorption with Different Anions
Among the oxyanions, there are differences in the adsorption capacity. As men- SeO − from the thermodynamic calculation by the Geochemist's workbench (GWB) using the latest thermodynamic database released from Laurence Livermore National Laboratory in USA. From the comparison of anion adsorptions, strong base anions can achieve a strong adsorption with the schwertmannite surface, and with high adsorption capacity.
The changes in the Zeta potential after adsorption by arsenate, phosphate and chromate were very different from the adsorption by selenate and that of original schwertmannite. This difference implies that the adsorption mechanism on schwertmannite for arsenate, phosphate and chromate is different from selenate and sulfate. The curves of the Zeta potential after adsorption for oxyanions shifted to negative. The degree of the shift increased in the following order, selenate  chromate < phosphate < arsenate. This shift may be attributed to interaction between oxyanions and the schwertmannite surface. An inner-sphere complexation induced the modification of surface charge of the minerals. The inner-sphere complexation with anions makes the surface charge negative. Therefore, we may assume that arsenate, phosphate and chromate form inner-sphere complexes with the surface of schwertmannite. The selenate and sulfate form outer-sphere complexes with the surface of the schwertmannite.

Anion-Exchange with Different Anions
As described above, adsorption of oxyanions with schwertmannite such as with arsenate, phosphate and chromate is represented by the release of cion-exchange are confirmed by a plot in Figure 6, which is from the previous study for the case of sorption of As(III) and As(V) onto schwertmannite [22].
The quantity of released protons was estimated from the differences in proton concentrations which were calculated from the speciation analytical by REACT in GWB package [23]. From the slope of the regression line in Figure 6, schwertmannite released 0.61 mmol of SO 4 2after 1 mmol of As(V) adsorption.
This value is similar to the value previously reported by Fukushi et al. [19].
Overall, the anion-exchange reaction can be expressed as below: 2 4 Sch As As Sch 0.61SO 0.22H where Schindicates schwertmannite with sulfate and As-Sch is the schwertmannite with arsenate. Similarly, the anion-exchange reaction of phosphate with sulfate and chromate with sulfate become express as follows, respectively. In this study, selenate adsorption on schwertmannite was not considered to be an ion-exchange with sulfate because the capacity of selenate adsorption is limited.
Bigham, et al. [13] reported the chemical formula of schwertmannite as follows: . The mole numbers of SO 4 and mole number of each oxyanion in chemical formula were calculated by setting the mole number of Fe equal to 8 in Tables 1-3 for As-Sch, PO 4 -Sch, and Cr-Sch, respectively. As shown  In the formula above, x is the mole fraction of Sch-As, Sch-PO 4 and Sch-Cr, respectively.

Estimating the Equilibrium Constant for Anion-Exchange Reaction
The exchange reaction is described by an equilibrium constant and the distribution coefficient ordinarily expressed separately for solid and aqueous solutions [25] [26] [27]. The equilibrium constant can be used to predict the amount of an element in the solid phase when the solution chemistry is known. The anion-exchange reaction from Equations (1)-(3) can then be formulated with end-number composition, similar to the previous work by Fukushi et al. [19].    According to previous study by Fukushi et al. [19], the d K value was very similar to ex K , because in an ideal solid solution model it is assumed that the activity coefficient of this system is unity. Hence, the high value of the equilibrium constant for the anion-exchange reaction corresponds to the ratio of dissolution reaction of the mineral and mineral adsorbed by trace element. At high equilibrium constants, the adsorption capacity was highly adsorbed onto schwertmannite.

Stability of Schwertmannite with Different Oxyanions
The transformation process of schwertmannite to goethite involves dissolution and re-precipitation processes. The results from the calculation of solubility show that a low solubility suggests a high stability of the mineral [28]. In the previous studies, the stability of the mineral was described from consideration of  (20) As explained previously, K d corresponds to K ex which is the equilibrium constant for anions-exchange reaction. Further, K ex could be expressed as the ratio between the dissolution reaction of schwertmannite and schwertmannite adsorbed by different kinds of oxyanions as: sch ex As sch As The K ex for the oxyanions indicate the relative solubility of each of the schwertmannite with oxyanions, which are compared with schwertmannite with sulfate ( Table 7). The schwertmannite with arsenate has the lowest solubility and is the moststable phase among the one with other oxyanions. From Table 7, the relative solubility of schwertmannite with chromateis is smaller than that of schwertmannite with arsenate and phosphate. This implies that schwertmannite with chromate would be less stable than with arsenate and phosphate. This is consistent with the results of the aging experiments as shown in Figure 5. This is also consistent with the finding of the released Fe concentrations after the oxyanion adsorption as shown in Figure 3(b). Consequently, adsorption by oxyanions with the higher selectivity may be expected to result in meta-stable schwertmannite becoming stable.

Conclusions
In this study, the adsorption and post adsorption behavior of schwertmannite with various kinds of oxyanions were investigated to determine the potential for cleaning-up of contaminated water with hazardous oxyanions and a safe disposal of spent schwertmannite. The result of the adsorption experiments shows that the selectivity of oxyanion adsorption on schwertmannite decreases as the following order: arsenate ≥ phosphate > chromate  selenate. The adsorption mechanism on schwertmannite with arsenate, phosphate and chromate is dif- Therefore, oxyanions with high selectivity can stabilize schwertmannite by decreasing the solubility of schwertmannite after adsorption of the oxyanions.