Oxidative precipitation of arsenic(III) with manganese(II) and iron(II) in dilute acidic solution by ozone
Introduction
Arsenic is one of the unwanted constituents of most sulfide ores and concentrates which are processed in nonferrous metallurgical industries. Various types of arsenic-bearing intermediate products and process wastes have to be treated in an environmentally acceptable manner. A great deal of effort to polish technology to overcome the arsenic problem has been an increasing requisite for advances in metal production and materials processing. Conventionally, the coprecipitation of arsenic(V) with ferric hydroxide has been a practical and effective technique to remove arsenic from aqueous solutions, where ferric arsenate is precipitated at pH around 3.5 together with ferric hydroxide. This method was extensively reviewed by Harris and Krause (1993). The removal of arsenic from acidic solutions has been operated by precipitating arsenic as its sulfide which has a solubility of 28.5 mg/L in the pH range 1–4. The sulfide formation method has the advantage of fixing arsenic in a precipitate of high arsenic content but a further step is required to remove the remaining arsenic from acidic solution at an extremely low concentration level before the discard of the treated solution to the environment.
In our previous work (Nishimura and Umetsu, 1995), it was noticed that by ozone oxidation, manganese dioxide is precipitated from manganese sulfate solutions of acid concentration up to 3.0 M H2SO4 at 25 °C. Furthermore, when arsenic(III) or arsenic(V) coexists with manganese(II) in the solution, the precipitates produced by the ozone oxidation reaction at 1.0 M H2SO4 have a fairly high arsenic content of about 7 wt.%. Based on these observations, some successes have been achieved in removing arsenic(III) and arsenic(V) from manganese sulfate solution by a combination of oxidation using ozone and precipitation (Nishimura and Umetsu, 1994). Recently, more detailed data have been obtained systematically. In this paper, the oxidative precipitation reactions are presented paying attention to the factors affecting the removal of arsenic such as ozone partial pressure of the feed gas, initial arsenic concentration, pH, ozonation temperature and addition of iron(II).
Section snippets
Experimental
The equipment consists of an ozone generator of silent discharge type, a glass reaction vessel equipped with stirring impeller and sensor electrodes for monitoring pH and oxidation–reduction potential (ORP) and a pH-stat unit having a function of auto-titrator. The equipment assembly is described in detail in our previous papers Nishimura and Umetsu, 1991, Nishimura and Umetsu, 1992
Reagent grade MnSO4·4–5H2O, As2O3, As2O5, FeSO4·7H2O were used as source of manganese(II), arsenic(III),
Precipitation of As and Mn with ozone
Fig. 1 shows the variation in ORP and the concentrations of manganese and arsenic(V) of the solution with ozonation time. In the manganese(II) sulfate solution without arsenic added, the ORP shows a two-stage variation with time. The first stage consists of a steep rise right after the start of feeding of O3–O2 mixture gas, followed by a slow and steady increase ending at a stepwise shift of ORP and the second stage is a gradual increase in a higher potential range. The overall reaction in the
Conclusions
Removal of arsenic with manganese from aqueous solutions by oxidation–precipitation using ozone was investigated in the pH range of 0.4–5.0.
(1) When an O3–O2 gas mixture is supplied to solutions containing manganese(II) and arsenic(III), the oxidation of arsenic(III) to arsenic(V) takes place prior to the oxidation of manganese(II). The arsenic(V) reacts with the manganese to form a precipitate of Mn/As mole ratio around unity, believed to be MnAsO4·nH2O.
(2) The residual arsenic concentration
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