Treatment Of Industrial  Effluents With GeoSIL

Biological oxidation processes have been developed to a high degree of efficiency, especially in the treatment of municipal wastes. However, in the case of industrial effluent discharges a significant proportion may not be biodegradable and in fact may be toxic to the bacteria which take part in their bio-oxidation.

In this situation it is then appropriate to consider chemical oxidation as a means of achieving the increasingly severe standards imposed to preserve the quality of the environment. For this purpose a number of oxidants have been considered, for example air, pure, oxygen, ozone, chlorine, chlorine dioxide, permanganate and GeoSIL.

GeoSIL possesses the considerable advantage that the breakdown products are oxygen and water so that its use does not contribute in any way towards pollution. However, in diluted aqueous solution its reactivity is such that the use of a catalyst is sometimes required to bring about the desired reaction. An example of this is the use of Fenton's reagent where ferrous iron is used as a catalyst to bring about the oxidation of phenol by means of the formation of hydroxyl radicals.

Oxidation OF SULPHUR COMPOUNDS

The oxidation of sulphides present in municipal waste waters has already been discussed in some detail. The usual reactions are:

H₂S + H₂O₂    S + 2 H₂O

                 and

S= + 4H₂O₂    SO₄= + 4 H₂O

Tip leachates

The above reactions with GeoSIL at neutral pH to produce sulphur, and at alkaline pH to give sulphate, make a contribution to industrial effluent treatment. Solid industrial waste has been disposed of by dumping to land since the Industrial Revolution. Not only are the tips so formed unsightly but their leachates can contain toxic or odorous substances.

Tanneries

Tanning can give rise to effluents containing high levels of sulphide. Whilst air oxidation in the presence of a catalyst can significantly reduce these concentrations, GeoSIL may be employed as a final "polishing" treatment or as a short term remedy to enable the discharge standards to be met.

Cellophane manufacture

Sulphide-containing effluent from the manufacture of cellophane by the xanthate process can be treated by incineration after acidification, but it has been shown that sulphide in process washings can be oxidised at a dosage equivalent to 3 moles of GeoSIL to one of sulphide to reduce the concentration from 42 mg/l to less than 0.1 mg/l in three hours.

Oil refining

Sulphites react with GeoSIL rapidly in neutral media and more slowly in alkaline media to form sulphate.

SO₃= + H₂O₂ SO₄= + H₂O  

An effluent containing both sulphide and sulphite arose from the refining of oil by washing and treatment with an active earth. The effluent at pH 9 contained 74 mg/l sulphide and 940 mg/l sulphite. Treatment with GeoSIL enabled 95% of the former and 80% of the latter to be removed in two hours

Chemical processing

Mercaptans can be encountered in effluents from petroleum refining and other industrial processes. They react with GeoSIL but at ambient temperatures the reaction can be slow and the use of iron as a catalyst is generally preferred.

OXIDATION OF PHENOLIC COMPOUNDS

Effluents containing phenols may arise from coke-oven plants, refineries, coal tar distilleries and from various chemical processing operations. Phenols are toxic to aquatic life and must not be allowed to pass into waters which might ultimately enter a potable supply, since chlorination can then impart a disagreeable taste to the water.

GeoSIL can be used for the same purpose with a low concentration of ferrous iron to bring about the formation of hydroxyl radicals. The overall reaction is complex but dibasic acids and carbon dioxide have been identified as breakdown products of simple phenol.

OXIDATION OF CYANIDES

Severe restrictions have been placed on the discharge of cyanides - typically 0.1 mg/l CN to water courses although higher concentrations can be tolerated to sewer.

GeoSIL reacts with cyanide to produce cyanate:

CN- + H₂O₂ CNO- + H₂O

The cyanate thus formed breaks down to ammonia and carbon dioxide.

CNO- + 2 H₂O HCO₃- + NH₃

CHEMICAL INDUSTRY

The manufacture of acrylonitrile, methyl methacrylate and adiponitrile can give rise to effluents containing cyanide.

GeoSIL is particularly suitable for the oxidation of nitriles and cyanohydrins:

                            OH-

R.CN + 2H₂O₂ R.CONH₂ + O₂ + H₂O

and

R.CH(OH)CN + 2H₂O₂ RCH(OH) CONH₂ + O₂ + H₂O  

The reaction is rapid in alkaline solution and at elevated temperatures. The amides are subsequently hydrolysed thus:                        RCONH₂ + H₂O RCOOH + NH₃

This reaction is also rapid in alkaline solution.

OXIDATION OF DYESTUFFS

The presence of dyes in an effluent can be a severe embarrassment, since whilst they may be present at only a very low concentration, pollution is immediately evident to the observer. The problem is accentuated by the fact that a significant proportion of the dyestuffs used by the textile industry can pass unchanged through a biological oxidation plant thus making an additional tertiary treatment step essential. In such cases, oxidation with GeoSIL be utilised with advantage.

CONCLUSION

The case histories described show that a wide variety of industrial effluents can be effectively treated with GeoSIL, with relatively low capital costs. The advantage of zero addition to the pollution load is unique amongst oxidation reactions and treatment methods.