Arsenic removal by goethite and jarosite in acidic conditions and its environmental implications

Schwertmannite (Fe(8)O(8)(OH)(5.5)(SO(4))(1.25)), jarosite (KFe(3)(SO(4))(2)(OH)(6)) and goethite (FeOOH) control natural attenuation of arsenic in acid mine drainage (AMD) impacted areas. Batch experiments were conducted to examine the sorption capacity of synthetic goethite and synthetic jarosite at highly acidic pH (1.5-2.5), at two ionic strengths (0.02-0.15 mol dm(-3), NaCl) and at sulphate concentrations in the range of 5 x 10(-3) to 2.8 x 10(-1) mol dm(-3). In the absence of competitive effects of other anions, K-jarosite presents better removal efficiency than goethite for As(V). The maximum sorption capacity is estimated to be 1.2 x 10(-4) and 7.0 x 10(-6)mol m(-2) for jarosite and goethite, respectively, under similar experimental conditions. The variation of arsenic sorbed on goethite as a function of the equilibrium arsenic concentration in solution fits a non-competitive Langmuir isotherm. In the case of K-jarosite, sorption data could not fit a Langmuir or Freundlich isotherm since sulphate-arsenate anion exchange is probably the sorption mechanism. Ionic strength and pH have little effect on the sorption capacity of goethite and jarosite in the small range of pH studied. The presence of sulphate, which is the main anion in AMD natural systems, has a negative effect on arsenic removal since sulphate competes with arsenate for surface sorption sites. Moreover, mobilization of arsenic in the transformation of schwertmannite to jarosite or goethite at pH 2-3 is proposed since the sorption capacities of goethite and K-jarosite are considerably lower than those reported for schwertmannite.