Sulphate in soils

Method 65, p. 158. Bicarbonate extractable phosphorus extractable in soil Method 76, p. 183. Sulphur, sulphate in soil... 

Fig. 5.5. Manifold for the automated determination of sulphate in soil extracts.

FRENEY J.R. 1958. Determination of water soluble sulphate in soils. Soil Science, 86, 241-244. 

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

Patches of conductive lead sulphide can be formed on lead in the presence of sewage. This can result in the flow of a large corrosion current . Sulphate-reducing bacteria in soils can produce metal sulphides and H2S, which results in the formation of deep pits containing a black mass of lead sulphide . Other micro-organisms may also be involved in the corrosion of lead in soil . 

As indicated above, the bicarbonate ion inhibits the process, which does not occur, therefore, in many supply waters attack is most likely in waters which by nature or as a result of treatment have a low bicarbonate content and relatively high chloride, sulphate or nitrate content. The number of points of attack increases with the concentration of aggressive anions and ultimately slow general corrosion may occur. During exposure of 99-75% tin to sea-water for 4 years, a corrosion rate of 0-0023 mm/y was observed . Corrosion in soil usually produces slow general corrosion with the production of crusts of oxides and basic salts this has no industrial importance but is occasionally of interest in archaeological work. 

Singhal et al. [78, 79] have described a titrimetric method for the determination of low levels of Oxamyl residues in soils. Their investigations revealed that after hydrolysis Oxamyl gave a brown precipitate with carbon disulphide and an alkaline solution of copper(II) sulphate. This reaction suggested a procedure for the determination of Oxamyl by titration with ethylene diamine tetracetic acid of the copper remaining unreacted to the 1-(2 pyridylazo)-2-naphthol end-point indicator). The following stoichiometric reaction appeared to occur between Oxamyl and the reagents ... 

Aznarez et al. [2] have described a Spectrophotometric method using curcumin as chromopore for the determination of boron in soil. Boron is extracted from the soil into methyl isobutyl ketone with 2-methylpentane-2,4-diol. In this method 0.2-lg of finely ground soil is digested with 5ml concentrated nitric-perchloric acid (3 + 1) in a polytetrafluoroethylene lined pressure pump for 2h at 150°C. The filtrate is neutralized with 6M sodium hydroxide and diluted to 100ml with hydrochloric acid 1+l.This solution is triple extracted with 10ml of methyl isobutyl ketone to remove iron interference. This solution is then extracted with 10ml 2-methyl pentane-2,4 diol and this extract dried over anhydrous sodium sulphate. 

Pelts and Belcher [46] have described a semi-automated method for the simultaneous determination of nitrogen and phosphorus in soil. Up to 40 samples are digested with sulphuric acid using a copper sulphate-sodium sulphate catalyst at 370°C for 2.25h in a thermostatically controlled block. Nitrogen is then determined in the 0.2-2.25% range as ammonia by automatic colorimetric analyses of the indophenol blue complex at 630nm. 

Landers et al. [97] and others [98] have described a wet digestion method for the determination of total sulphur in soils. In this method the sample (1.50-500mg) is placed in a digestion flask and heated in a sand bath to dryness at 250°C with 3ml of sodium hypobromite solution. The residue is resuspended with water, neutralized with formic acid, and then hydriodic acid reduction of the sample is followed to quantitatively recover the inorganic sulphate formed by wet oxidation. 

The method [97] discussed in section 12.8.1.1 for the determination of sulphur in soils has been applied to lake sediments. Landers [97] also determined organic sulphur (carbon bonded sulphur and ester sulphate) and inorganic sulphur (sulphate and sulphide) in lake sediments. Some results... 

Kimura and Miller [29] have described a procedure for the determination of organomercury (methylmercury, ethylmercury and phenylmercury compounds) and inorganic mercury in soil. In this method the sample is digested in a steam bath with sulphuric acid (0.9M) containing hydroxy ammonium sulphate, sodium chloride and, if high concentrations of organic matter are present, potassium dichromate solution. Then, 50% hydrogen... 

BREEMEN, N. van. 1982. Soil acidification from atmospheric ammonium sulphate in forest canopy throughfall. Nature Volume 299, October 1982. 

In soils, electrons are produced by the metabolic activity of soil biota. These electrons are usually accepted by O2 dissolved in the soil solution which is then replaced by O2 from the soil air. Oxygen may, however, become deficient if all pores are filled with water as in waterlogged or compacted soils. Fe in Fe oxides may then function as an alternative electron acceptor and Fe ions will be formed according to eq. (16.3). The electrons are transferred from the decomposing biomass to the Fe oxide by microbially produced enzymes. Other potential electron acceptors in soils are nitrate, Mn and sulphate. 

Taylor, R.M. McKenzie, R.M. (1980) The influence of aluminum on iron oxides. VI. The formation of Fe(II)-Al(lll) hydroxy-chlorides, -sulphates, and -carbonates as new members of the pyroaurite group and their significance in soils. Clays Clay Min. 28 179-187... 

Calculation. The 2-ml scoop of soil was extracted via resin into 50 ml sodium sulphate extractant therefore the concentration must be multiplied by 25 to give the pg P ml in soil by resin extraction. Hislop and Cooke (1968) classified the soils with respect to mg P h air-dry soil as follows ... 

Coutinho, J. (1997) Automated method for sulphate determination in soil-plant extracts and waters. In Hood, T.M. and Benton Jones, J., Jr (eds) Soil and Plant Analysis in Sustainable Agriculture and Environment. Marcel Dekker, New York, pp. 481 94. 

Sturz, A.V., Ryan, D.A.J., Matheson, B.G, Arsenault, W.J., Kimpinski, J., Christie, B.R. Stimulating disease suppression in soils sulphate fertilizers can increase biodiversity and antibiosis ability of root zone bacteria against Streptomyces scabies. Soil Biol Biochem 2004 36 343-352. 

Bioaccessibility, and therefore oral bioavailability of soil contaminants, depends on soil type and contaminant (Davis et al., 1997 Gr0n and Anderson, 2003 Hamel et al, 1998 Ruby et al., 1999). PTMs occur in soil as a complex mixture of solid-phase chemical compounds of varying particle size and morphology, characterised by variable metal bioavailability. Mineral phases that form under acidic conditions (e.g. lead sulphate, iron-lead sulphate) will tend to be more stable in the acidic conditions of the stomach and hence less bioaccessible. By contrary, mineral phases... 

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