Preoxidation

5.1. Chemicals

The effective removal of arsenic from water usually requires the complete oxidation of As(III) to achieve the required drinking water standard. There are various means of oxidation available, but in drinking water treatment, additional aspects must be considered, such as the limited list of chemicals, residual oxidants and oxidation by-products or the oxidation of other inorganic and organic water constituents. Thus, the national drinking water and treatment regulations will therefore be important in the selection of the most feasible oxidant.

The need for a special oxidation step is caused by the very slow kinetics of oxidation by dissolved oxygen [2]. Oxygen would be a preferred oxidant as some problems with other chemicals can be avoided. There are also technical and operational problems whenever chemicals are added continuously in small-scale water treatment plants, where operators are not available all the time. Thus, a pre-oxidation step adds operational complexity.

Effective oxidants include free chlorine, hypochlorite, ozone, permanganate and hydrogen peroxide/Fe2+ (Fenton's reagent), but not chloramines [20] and peroxosulfate. Chlorine is widely used for this purpose, but may lead to chlorination by-products, namely trihalomethanes (THM's), from reactions with natural organic matter (NOM). In Germany, the application of chlorine is allowed only for disinfection purposes, not for general oxidation.

Ozone, widely used in surface water treatment for oxidation and disinfection, is very effective, but not feasible for As(III) oxidation. It must be produced on-site from air or oxygen and is a strong oxidant, which would also partly oxidize the NOM, an unwanted reaction in cases of geogenic arsenic in groundwater.

The most feasible chemical oxidants found to date are potassium permanganate and Fenton's reagent (H202/Fe2+), if the removal of the As(V) is to be accomplished by precipitation/coagulation and rapid filtration. Permanganate as a weak oxidant oxidizes As(III), ferrous and manganese ions specifically and rapidly. The Mn(IV)-hydroxide formed must be filtered afterwards, together with the precipitated As(V). The presence of Fe(II) and Mn(II) leads to a higher consumption of KMnO,*. A dose of approximately 1 - 2 mol KMn04 per mol As(III) is usually needed. It was found that the DOC concentration has no significant influence on the dose necessary. Thus, problems regarding the formation of biodegradable organic carbon should not be expected [17].

The singular application of H2O2 may be successful, if the raw water contains already sufficient ferrous ions to induce OH-radical formation. The mass ratio of H2O2 to Fe2+ should be varied to find the optimum for a maximum oxidation. The dosages required are usually in the range of 0.3 - 2 mg L"'. The ferric ions formed will precipitate As(V). The residual H2O2 may not exceed 0.1 mg L"1 (German standard), which can be achieved by careful dosage adjustment. High concentrations of bicarbonate reduce the oxidation of As(III) by scavenging of the OH-radicals. The use of H2O2 may also lead to a partial oxidation of NOM.

In comparing these chemical oxidation processes, preference may be given to KMnC>4 in the case where there is a subsequent precipitation/filtration step and to chlorine or hypochlorite, if regulations permit.

5.2. Biological oxidation

The biological oxidation of As(III) in fixed-bed reactors represents a simple treatment process which does not require additional chemicals. Investigation of the feasibility of this process as a pre-treatment for arsenic removal showed a sustainable and mainly biological oxidation using common filter material such as sand or pumice. After adaptation periods of a few days to a few weeks, fixed-bed filters operated at filter rates of 25 m h"1 and fed with As(III) concentrations of over 100 |ig L"1 showed high oxidation rates and reliably oxidized As(III) down to effluent concentrations of 2 ng L"1 [36], Such a biological filter is generally insensitive to changes in the operational conditions. However, backwash events will reduce the oxidation rate for a short period of time. This technique is particularly suited for the treatment of ground waters where arsenic removal is the only treatment goal. These investigations suggest that biological oxidation also occurs in other filter media, e.g. in iron removal plants. The possibility of biological arsenic oxidation in GFH / activated alumina fixed-bed reactors may represent an interesting focus of research aiming at the oxidation and removal of As(III) in one reactor only [36].

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