Conventional treatment of hexavalent chromium wastewaters involves the reduction of hexavalent chromium to trivalent chromium and subsequent precipitation of trivalent chromium as hydroxide. Hexavalent chromium reduction is commonly accomplished using chemical reduction.
Hexavalent chromium is reduced to trivalent chromium at acid pH using chemical reduction agents. Sodium bisulfide, sodium metabisulfide, and ferrous sulfate are among the common reduction agents used for small plants. In larger plants, gaseous sulfur dioxide is more common and economical. Sulfur dioxide is hydrolyzed in water to yield sulfite.
Sulfite ions are oxidized by the half reaction:
Ferrous ions are oxidized by the half reaction:
Hexavalent chromium reduction proceeds with the half reaction: HCrO4-+7H++3e-^Cr3++4H2O
A combination of oxidation and reduction half reactions gives the reduction reaction for the chemical used. For the use of sulfite ions, the balanced reaction is:
This indicates that 1.5 moles sulfite ion are used to reduce 1 mole of hexavalent chromium. The reducing agent requirement can be calculated from the relevant equation
[3,105]. However, an excess of reducing agent is always needed. First, the reaction must go to completion, otherwise unreduced chromium passes through the hydroxide precipitation. To ensure that this occurs, at least 10 mg/L unreacted sulfite is required to remain after the reaction is completed. Secondly, there are reducible substances in the wastewater that consume the reducing agent. Dissolved oxygen and ferric ions are two such important substances consuming the reducing agent. Another occurrence highlighted in Reaction (17) is the consumption of hydrogen ions. This means that the reaction favors acid conditions. The reaction rate is also a function of pH. The rate of reduction reaction is so slow at pH>5.0 that it is impractical to conduct the reduction. At pH 4, completion of the reaction takes about one hour, and is even more for SO2 reduction, while at pH 2.0 the required reaction time is 5-10 minutes [3,105]. In addition to the reducing agents mentioned above, other reducers may be used. The use of scrap iron has been suggested in the literature .
Chromium reduction is a fully proven technology suited to automatic control, particularly when using SO2 addition and operating at ambient conditions. Final hexavalent chromium concentrations of 0.05 mg/L are readily obtainable. Operation of the system for continuous flow conditions is commonly realized in two reactors connected in series and mixed slowly, each having 45 minutes retention time. pH is maintained between 2.0 and 3.0 in the first tank, while the second tank pH is kept around 2.0. SO2 addition is made to maintain the ORP in the range 250-300 mV. Batch systems may also have two pH reduction stages .
Electrochemical chromium reduction is another technology used to treat hexavalent chromimum wastewaters. It uses an electric current applied to iron electrodes that dissolve to release ferrous ions to solution. Hexavalent chromium ions are reduced as ferrous ions are oxidized to ferric ions . A neutral pH is sufficient. After the reaction is completed, the solution is passed to a clarifier. Ferric hydroxide and chromium hydroxide are settled generally without a need for alkali addition. The process also removes zinc, copper, and other heavy metals. The parameters affecting the process efficiency were experimentally investigated by Kongsricharoern and Polprasert .
Another process used for chromium reduction is high pH reduction. In this reaction, FeSO4 is used for the reduction, and pH is maintained between 7.0 and 10.0. Addition of sodium sulfide was suggested to obtain additional chromium reduction and removal .
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