SELECTIVE REMOVAL OF ARSENIC FROM ACID MINE DRAINAGE CONTAINING FERROUS ION USING SULFIDE PRECIPITATION METHOD

摘要

Selective removal of dilute arsenic ions (As(III) or As(V)) from acid mine drainage (AMD) containingrnferrous irons was investigated using a sulfide precipitation method. 10 mg/dm3 of As(III) could bernremoved as As_2S_3 at pH ≥ 2.0 in a NaSH solution, which S/As ratio was higher than6.5 after 0.5 hrrnof reaction time. H_2S gas generation correlated with the residual As(III) concentration in AMD.rnBelow pH 2.5, ferrous ions were not found in the sludge and thus selective removal of As(III) wasrnachieved.rnOn the other hand, at pH 2.5, a NaSH solution of S/As ≥ 40 and a reaction time of 24 hr werernrequired to remove 10 mg/dm~3 of As(V) . However, at pH 3, 10 mg/dm3 of As(V) could be removedrnafter 0.5 hr of reaction time by using a NaSH solution of S/As ≥ 6 . Nevertheless, around 1/3 ofrnferrous ions were oxidized and removed as ferrihydrate together with As(V). EXAFS analysis showedrnthat As(V) was adsorbed on ferrihydrate when S/As ratio was 7, but precipitated as As_2S_3 at pHrn3,when S/As ratio was 20. These results showed that reduction of As(V) to As(III) was very slow andrnneeded enormous amount of sulfurising agent and relatively long reaction time.rnThe average primary diameter of the particles in the sludge was under 900 nm, which was increasedrnwith increasing dosage of NaSH. Particles exhibited a negative zeta potential of small value, whichrnindicates that they could coagulate to each other. Therefore, sedimentation rate of the particles fromrnsulfide precipitation method was better than the one from conventional hydroxide co-precipitationrnmethod using calcite or lime. Sedimentation of As(V) sulfide particles at pH 3 occurred at a very fastrnspeed because a small amount of ferrihydrite co-precipitated with As(V) sulfide, which acted as arncoagulant.