Investigating the role of ferrous iron in the arsenic(V)-iron(II,III) coprecipitation process system.

摘要

In this thesis, the effects of iron(II) on arsenic(V) removal from acidic sulphate solutions in lime neutralization systems were investigated. The role of Fe(II) was analyzed via different types of experiments. Firstly, 2-stage continuous coprecipitation (CCPTN) circuit experiments were run, involving variable Fe(II)/Fe(III) fractions whilst maintaining an Fe(tot)/As(V) molar ratio of 4, and the resultant products were subjected to stability testing. It was found that CCPTN results were reproducible; that increasing the Fe(II) content resulted in somewhat lower initial arsenic(V) removal, but still better results than those obtained from equimolar Fe(III)-As(V) solutions in the absence of ferrous pointing to the latter's beneficial effect on arsenic(V) precipitation and retention. Coprecipitates aged at constant pH 8, drifting pH and at various temperatures reached pseudoequilibrium after several months. Notably, long term stability testing of the coprecipitates showed that up to an Fe(II)/Fe(III) ratio of 1 at 20°C, As(V) release was maintained below 1 mg/L after 463 days with "drifting pH" stabilized at 5 increasing only to 1.9 mg/L upon pH adjustment to 8.;Secondly, the behaviour of Fe(II) was studied in batch reactor set-ups as part of hydrolysis and oxidation experiments with and without As(V). It was shown in the absence of As(V), Fe(II) precipitated out of solution completely between pH 7.5 and 8.5. Subsequent oxidation of the ferrous hydroxide slurry was found to proceed via a series of transformations starting from ferrous hydroxide to green rust, to magnetite and finally goethite. The oxidation kinetics were governed by oxygen mass transfer. In the presence of As(V) both Fe(II) and As(V) precipitated from solution starting at pH 3 with the latter ultimately dropping below 1 mg/L between pH 6.5 to 9 via the proposed precipitation of a ferrous arsenate compound (symplesite). Subsequent oxidation of the Fe(II)-As(V) slurry at constant pH 8 led to destabilization of the ferrous arsenate phase and concomitant partial release of As(V). The bulk control of As(V) in the latter case switched from ferrous arsenate to ferric arsenate or arsenate adsorption on freshly formed iron(III) hydroxide.