Effect of salinity gradient on the leaching of strontium, calcium, magnesium, and potassium from gypsum soil
Description
Utilizing sequential extraction, we mapped the distribution of the different cations, including Sr2+, among individual soil fractions corresponding to distinct geochemical pools. We fitted the leaching profiles using empirical kinetic models (power-law and Weibull) upon which analytical predictive models (linear + GPR) were built for Sr2+ leaching at different salinity levels employing 80/20 stratified validation, and LOLO stress testing.
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Soil was sampled in the desert in the Gharbya region in the northwest of the UAE. Soil was milled. The organic matter in soil was determined through gravimetric analyses. The mineral content was evaluated through XRF Puma S2. The sample contained 98% of gypsum. Column Experiment There were two variants of experiments: i) leaching by individual solutions with different salinity (mQ water, 0.5 mg, 5 mg, 10 mg, and 40 mg of NaCl), ii) leaching by the increasing gradient of the salinity (mQ water, 0.5 mg, 5 mg, 10 mg, 40 mg, 70 mg of NaCl). The soil was packed in the glass column Diba-Omnifit, diameter 3.5 cm, height 5 cm, with one fixed end and one adjustable end. Solution flows in a bottom-up direction, forced by the peristaltic pump at 2.6 rpm. The flow of the mobile phase in the column was 0.267 – 0.275 ml/min. The eluates were collected at the end of the column. After weighing, the eluates were diluted for the ICP-OES analyses and acidified with HCl at a pH of 2. Batch Experiment To determine the effect of salinity, a series of experiments was conducted. 1 g of dry soil was placed in the vial and immersed with 5 ml of mQ water or solutions with different concentrations of NaCl (0, 5, 40, 72.5 mg.kg-1). Every trial had two repetitions. Solutions were shaken for 3 hours at 190 rpm. Tessier’s Sequential Extraction To better understand the associations between metals bound to soil, Tessier’s sequential extraction (Leleyter et al. 2012) was applied to analyse the distribution of Sr, Ca, K, and Mg. An experimental procedure is utilized to extract i) exchangeable elements with milliQ water, T = 25°C, 1 hr (fractions 1) and 1 M ammonium acetate at a pH of 7, T = 25°C, 1 hr (fraction 2), ii) elements bound to carbonates with acetic acid buffered at a pH of 5 with ammonium acetate, T = 25°C, 5 hrs (fraction 3), iii) elements bound to Fe-Mn oxides with 1 M hydroxylammonium hydrochloride in 25% of acetic acid, pH = 2, T = 96°C, 6 hr (fraction 4), and iv) elements bound to organic matter and sulfide minerals with hydrogen peroxide, nitric acid and ammonium acetate (fraction 5). The soil:liquid mass ratio was 1:20 in each extraction. The original soils were digested in 10 mL aqua regia (HCl:HNO3 3:1) in a microwave oven. Each soil was analyzed in triplicate.
Institutions
- Khalifa University of Science and Technology