Removal Of Chromium(Ⅵ) And Arsenite(Ⅲ) In Polluted Waters Through Adsorption Onto Biosynthetic Schwertmannite

Chromium and arsenite are two toxic heavy metals which have been considered as the priority controlled pollutant in many countries because of their higher mobilization and carcinogenic. In recent years, increasing awareness of water pollution with Cr(VI) and As(III) and their long-term bio-toxicity has drove us to pay more efforts towards pollution abatement. Cr and As have several industrial applications such as in tannery, metal finishing operations and pesticide factory where chromium occurs most frequently as Cr(VI) and As as As(III) in waters, respectively. Among many treatment methods for eliminating Cr(VI) and As(III), adsorption onto solid adsorbents is an attractive approach due to its cost-efficiency, simplicity, easily operation, and higher removal efficiency.Schwertmannite with molecular formula Fe₈O₈(OH)_8-2x(SO₄)_x, a secondary mineral often found in SO₄^2-rich acidic mining drainges, was found to attenuate Cr(VI) and As(III) in the acidic dranges because of its large specific surface area and channel structure.Schwertmannite was synthesied through the oxidation of FeSO₄ faciliated by acidithiooxdans ferroxidans LX₅ in this study. Biosynthetic schwertmannite was employed to adsorb Cr(VI) and As(III) ions in aqueous solutions. Desorption of Cr(VI) absorbed schwertmannite indicated that 0.005mol/L of NaOH was an effective agent in the regeneration of adsorbent. Furthermore, the adsorption behaviors of Cr(VI) and As(III) on schwertmannite have been mainly studied under different conditions in term of different initial concentration, pH, temperature and co-existing heavy metals and anions. Results showed that the removal of Cr(VI) and As(III) was strongly dependent on both pH and their concentrations. The maximum adsorption capacity was 55.0mg/g obtained at pH6.0～7.0 for Cr(VI)and 67.7mg/g at pH8.5～9.0 for As(III), respectively. Removal efficiency of Cr(VI) was constant over 90% in the range of Cr(VI) concentration in solution from 5.0 mg/L to 100mg/L. Likewise, above 98% of removal efficiency for As(III) was also obtained when As(III) in solution was less than 1.0mg/L. In addition, thermodynamic parameters for Cr(VI) and As(Ⅲ) adsorption have been determined at 20℃～52℃for Cr(Ⅵand 8℃～48℃for As(Ⅲ). Negative△G°showed the spontaneity of the adsorption processes while positive△H°and△S°values meant that the reaction system was endothermic. The addition of chloride or nitrate hardly affected the Cr(Ⅵ) and As(Ⅲ) adsorption, whereas the co-exist sulfate and phosphate greatly interfered with the adsorption within a certain concentration range. The interferential ability followed PO₄^3-＞SO₄^2-＞NO₃^-=Cl^-. Besides, schwertmannite had a stronger anti-interfering capacity for co-existing anions than activated carbon in Cr(Ⅵ) solution with pH≥5.0, in which the adsorption capacity of schwertmannite for Cr(Ⅵ) was 3.5 times higher than activated carbon. It was noted that the adsorption capacity of schwertmannite for As(Ⅲ) was higher than activated carbon in a wide pH range from 2.0 to 10.0. It was concluded that the Cr(Ⅵ)-rich wastewater could be purified effectively if the mixture of schwertmannite and activated carbon were used under different pH condition. The co-presence of Cu²⁺, Zn²⁺ and Cd²⁺ in Cr(Ⅵ)or As(Ⅲ)containing solution had neglectable impact on Cr(Ⅵ) and As(Ⅲ) adsorption to schwertmarmite, indicating that schwertmannite had a more selective absorption for Cr(Ⅵ) and As(Ⅲ). Adsorption isotherm of Cr(Ⅵ) could be fitted by Langmuir isotherm instead of Freundlich isotherm with a better correlation coefficient, while Freundlich equation was more suitable for describing the adsorption process of As(Ⅲ).It was presumed through using the technology of EXAFS, FT-IR as well as some mathematic models that the adsorption of Cr(Ⅵ) and As(Ⅲ) onto schwertmannite involved in the ligand exchange reaction replacing SO₄^2- and Fe-O with Cr(Ⅵ) and As(Ⅲ), complexation reaction with active radicals, electrostatic attraction and some other chemical reaction Among adsorption mechanisms, ions change reaction might play an important role in the adsorption of Cr(Ⅵ) and As(Ⅲ) from DKR equation. To sum up, schwertmannite was an effective adsorbent which exhibited an higher treatment efficiency and stability for removal of Cr(Ⅵ) and As(Ⅲ) anions from aqueous system.