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

Abstract

Aluminium is the most abundant metal in the earth’s crust. In soil, aluminum is mainly found in the mineral form as aluminosilicates and aluminum oxides and this aluminium is in stable inactive form. In addition, Al can be found as precipitates or in very minute quantities appear in soluble forms such as conjugated organic and inorganic, and molecular ions. Aluminium mobility and as a consequence aluminium toxicity, is mainly restricted to acid environments. Depending on the soil pH these mobile forms are capable of influencing biological systems. Aluminium has low mobility under most environmental conditions. However, below a pH of 4.0 its solubility increases and aluminium is released from silicate rocks under such acidic conditions. The levels of dissolved aluminium in natural groundwater samples are generally low, probably due to its low solubility at neutral pH values. Release of acids by anthropogenic activities influence the soil acidity levels. Therefore, elevated levels of aluminium have been found in acidified soil solutions and surface waters causing harmful effects to living organisms. The present study was aimed at proving the above theoretical hypothesis and existence of a possible relationship between soil salinity and soil acidity on the concentration of mobile aluminium ions in samples obtained during the location surveys. Samples were collected from selected locations in Ratnapura, Rathupaswala, Marawila, Mabima and Muthurajawela to get different soil types and the survey results were used to test the hypothetical relationship between the presence of the stated factors, and the existence of a high concentration of mobile aluminium in the soil water samples. The analysis covered basic parameters such as soil pH, soil cation exchange capacity (CEC), soil organic matter, soil electrical conductivity and the influence of the concentration of mobile aluminium at different pH and Na+ concentration levels. The total aluminium concentration in the soil was assayed by digesting samples with strong acid. Concentration of mobile aluminium in soil samples were analyzed using the flame atomic absorption spectrophotometry. The results indicated that there is no clear relationship between mobile aluminium and total aluminium in the soil. It was also found that the concentration of mobile aluminium released increased with decrease in soil pH and that the increase was marked when the pH of soil water was less than 4.0. Highest mobile aluminium release to the soil solution was found from Mabima sites [at pH 5.00 was 0.54 (±0.06) mg kg-1 of dry soil, at pH 0.00 was 90.12 (±7.01) mg kg-1] and least in samples from Marawila sites [at pH 5.00 was 0.48 (±0.03) mg kg-1, at pH 0.00 was 4.52 (±0.36) mg kg-1]. This result confirmed that there is an effect of soil acidity on the concentration of mobile aluminium in the soil but with no direct correlation. Results also showed that the concentration of mobile aluminium released increased with increasing soil salinity and that the increase was rapid when the Na+ ion concentration was higher than 2.0 %. Highest mobile aluminium release to the soil solution was found from Muthurajawela sites [Na+ 1.0% = lower than detection limit, Na+ 5.0% = 9.87 (±0.67) mg kg-1] and least found from Marawila sites [Na+1.0% = lower than detection limit, Na+ 5.0% = 2.24 (±0.23) mg kg-1] confirming the effect of soil salinity on the concentration of mobile aluminium in the soil. The study also points towards the future opportunities for research to confirm these findings using wider samples and employing more vigorous research methodologies.