Study of the Effects of Soil Acidity and Salinity on Aluminium Mobility in Selected Soil Samples in

In the earth's crust, aluminium is the most common metal. Aluminum is primarily present as aluminosilicates and aluminum oxides in the mineral form in soil and this aluminum is in a stable inactive form. In addition, Al can be present in soluble forms such as conjugated organic and inorganic, and molecular ions as precipitates or in very minute amounts. The mobility of aluminium and, as a result, the toxicity of aluminium is limited primarily to acid environments. These mobile types are capable of affecting biological processes depending on the soil pH. Under most environmental conditions, aluminium has poor mobility. However, its solubility increases below a pH of 4.0 and aluminium is released under such acidic conditions from silicate rocks. In natural groundwater samples, the levels of dissolved aluminium are usually poor, possibly due to its low solubility at neutral pH values. The release of acids from anthropogenic activities influences the level of soil acidity. In acidified soil solutions and surface waters, elevated levels of aluminium have also been observed, causing adverse effects on living organisms. The purpose of the present study was to demonstrate the above-mentioned theoretical hypothesis and the presence of a potential relationship between soil salinity and soil acidity in the concentration of mobile aluminium ions in the samples collected during the site surveys. In order to obtain different soil types, samples were collected from selected locations in Ratnapura, Rathupaswala, Marawila, Mabima and Muthurajawela, and the results of the survey were used to test the hypothetical relationship between the presence of the specified factors and the existence of a high mobile aluminium concentration in soil water samples. The study covered basic parameters such as soil pH, soil cation exchange ability (CEC), organic matter of the soil, electrical conductivity of the soil and the effect of mobile aluminium concentrations at different levels of pH and Na+. By digesting samples with strong acid, the total aluminium content in the soil was assayed. The concentration of mobile aluminium in soil samples was analyzed using spectrophotometry for atomic flame absorption. There is no direct relationship between mobile aluminium and total aluminium in the soil, the results showed. It was also found that the released mobile aluminum concentration increased as the soil pH decreased and that the increase was marked when the soil water pH was less than 4.0.0. The maximum mobile release of aluminium to the soil solution was observed at Mabima sites [at pH 5.00, 0.54 (±0.06) mg kg-1 of dry soil, at pH 0.00, 90.12 (±7.01) mg kg-1] and at least 0.48 (±0.03) mg kg-1 at pH 5.00, at pH 0.00, 4.52 (±0.36) mg kg-1]. This finding demonstrated the influence of soil acidity on the mobile aluminium concentration in the soil, but without a clear correlation. Results also showed that the released mobile aluminium concentration increased with increasing soil salinity and that when the Na+ ion concentration was higher than 2.0 percent, the increase was rapid. The maximum mobile aluminum release to the soil solution was observed from Muthurajawela sites [Na+ 1.0 percent = lower than the detection limit, Na+ 5.0 percent = 9.87 (±0.67) mg kg-1] and the least detected from Marawila sites [Na+1.0 percent = lower than the detection limit, Na+ 5.0 percent = 2.24 (±0.23) mg kg-1], confirming the effect of soil salinity on soil mobile aluminum concentrations. The report also points to potential research opportunities to validate these results using larger samples and the use of more vigorous research methodologies.

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