DETERMINATION OF SULFATES IN SOIL?

[sagareswar]

Dear All,

How can we decide in BS 1377 Method that we should choose acid extraction ro water extraction method for the analysis of sulfates in soil?

Regards,
SAGAR

[Tom]

Sagareswar,

I am not sure this is a PHREEQC question but I can provide some thoughts as this is my research area.

The choice between acid or water extraction typically depends on a client's requirement but this may be governed by the type of sulfates present and the nature of the works which will interact with the soil. What is the reason for analysing the soil for sulfate?

The water extract will typically only leach a limited portion of the sulfates present in the soil whereas acid-extracts are often thought of as representing "total sulfate" as most soil sulfates are soluble in HCl. It is usually assumed that gypsum is the main sulfate mineral in soils. Acid-extraction is often chosen when a knowledge of the worst-case sulfate conditions of the soil is required. Acid-extraction is also used to derive terms like "oxidisable sulfides".


An extract from BS 1377-3 explains better than I can:

"Acid-soluble sulphates include all naturally occurring sulphates with rare exceptions. Calcium sulphate, the sulphate salt that is most commonly found, has a low solubility in water."

"If the sulphate present in the soil is predominantly the calcium salt, [gypsum] the total sulphate content as determined
is likely to give a misleading and pessimistic impression of the danger to concrete or cement-stabilized material arising from the presence of sulphates. In cases where the total sulphate exceeds 0.5 % it is suggested that the water soluble sulphate content of a 2 to 1 water-soil extract should be determined. If calcium sulphate is the only sulphate salt present its low solubility will ensure that the sulphate content of the aqueous extract does not exceed 1.2 g/L. Sulphate contents in excess of this figure in the soil-water extract or in the ground water as determined in this test therefore indicate the presence of other and more harmful sulphate salts."

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There is a paper by Neville (2004) which has some useful information which may help you.

Reference:

Neville (2004). The confused world of sulfate attack on concrete. Cement and Concrete Research. 34, 1275-1296.