Adsorption and desorption characteristics of arsenic in calcareous soils as a function of time; equilibrium and thermodynamic study.

Irrigation of carbonate-rich agricultural soils with arsenic (As)-contaminated water leads to the accumulation of As in these soils. In this regard, there is an opportunity to adsorb and fix the As in soil and decrease the As transportation to the plants and subsequently the human food chain. So, the present study aimed to investigate the adsorption-desorption characteristics of As in calcareous soils and the potential of As fixation over time. First, to achieve this purpose, 53 soil samples were gathered from the study site and after the laboratory analysis, the soils were categorized into four groups based on their physicochemical properties. Then, four representative samples of these groups were selected, namely soil 1, soil 2, soil 3, and soil 4. Afterward, the As adsorption-desorption was investigated in a lab-scale batch experiment. Next, the effect of age was assessed by incubating the As-adsorbed soils for 60 days, and to study the impact of temperature, the adsorption was performed at four temperature levels (10, 20, 30, and 40 °C). Finally, the isotherm models were fitted to experimental data, and the amount of loosely and tightly held As was quantified. Results revealed that the As adsorption isotherms were L-type, in which As adsorption increased with the increase of As loading. The double-site Langmuir (DSL) estimated that a limited amount of As was adsorbed on high-energy surfaces and a large amount of As was adsorbed on low-energy surfaces. Desorption results showed that a significant amount of As desorbed immediately; however, the desorption significantly decreased with the increase of age, especially at low equilibrium concentrations. By aging the loosely held As transformed into non-labile forms so that in soils 1, 2, 3, and 4, the fraction of As adsorbed on high-energy surfaces increased from 72.5, 93.2, 63.2, and 123 mg/kg to 167, 141, 70.6, and 196 mg/kg, respectively, and the fraction of As adsorbed on low-energy surfaces decreased from 397, 256, 202, and 317 mg/kg to 182, 238, 173, and 172 mg/kg, respectively (after aging for 60 days). Aging proved to be a promising solution for decreasing As transport into the human food chain and could be employed for crops with longer irrigation cycles. ΔHad values were positive and varied from 9.26 to 13.0 kJ/mol, confirming the endothermic nature of adsorption. ΔGad values were negative and varied from - 18.8 to - 22.8 kJ/mol at all temperatures, demonstrating the spontaneous nature of adsorption.

Study of arsenic adsorption in calcareous soils: Competitive effect of phosphate, citrate, oxalate, humic acid and fulvic acid.

Arsenic (As) bio-availability in the soil is influenced by different organic and inorganic anions. In the present study, the effects of various competitive agents, including phosphate, citrate, oxalate, humic acid (HA), and fulvic acid (FA), on the adsorption of As in calcareous soils were investigated. The results revealed the presence of phosphate, citrate, and oxalate in soil has a significant impact on the arsenic retention (adsorption) in soil which increases the As bio-availability. The negative impact of the competing anions was increased at higher concentrations. The Double Site Langmuir (DSL) isotherm was best fitted to the adsorption data, which indicates that most of the As adsorbed on the low-energy surfaces (non-specific adsorption by oxides, clays, and clay-size calcite). Accordingly, in soil 1, the DSL predicted that, due to phosphate, citrate, and oxalate competition (at a concentration of 10 mM), the adsorption capacity of the high- and low-energy surfaces decreased from 86.2 to 33.5, 82.1 and 61.3 mg/kg and from 663 to 659, 335.8, and 303.5 mg/kg, respectively, Moreover, after addition of phosphate, citrate, and oxalate to the soil-As system, the Langmuir constant of high-energy surfaces decreased from 0.686 to 0.074, 0.261, and 0.301 L/mg, respectively. No regular trend was observed for the Langmuir constant of low-energy surfaces. Similarly, in soils 2, 3, and 4, the adsorption capacities of both high- and low-energy surfaces as well as the Langmuir constant of high-energy surfaces decreased by the addition of phosphate, citrate, and oxalate to the soil-As system. HA and FA did not have a significant effect on the As adsorption behavior. Phosphate, citrate, and oxalate, as interfering oxyanions, increased the As bio-availability in the calcareous soils by decreasing the As adsorption.

Arsenic geochemistry and mineralogy as a function of particle-size in naturally arsenic-enriched soils.

Naturally arsenic (As) enriched agricultural soils represent a significant global human health risk. In this study, As fractionation and mineralogy were investigated in naturally As-enriched agricultural soils and their corresponding sand, silt and clay fractions. Median As increased generally in the order (mg/kg)∶ silt (280) < bulk (314) < sand (323)

Kinetics of arsenic release from naturally contaminated soils at half saturation moisture by various extractants.

Kinetic studies on arsenic (As) release from soils are commonly performed using soils spiked or artificially contaminated with As at soil to solution ratios of 1:10 to 1:50 by mass, which make the conditions of study very different from the prevailing field situations. In this study, the kinetics of As release were investigated using different extractants including 1MK2HPO4, 0.5MNa3C6H5O7, 1MNaOH, 1MNH4F, and [Formula: see text] in five naturally As-contaminated soils (80-1680 mg total As/kg) at half saturation moisture over a long period of time (15 min-60 d). Six kinetic models were employed to fit the data. Results indicated that the trend of As release was initially rapid followed by a slow release with K2HPO4, [Formula: see text] and Na3C6H5O7 solutions, whereas with NaOH and NH4F an increasing As release was followed by a decreasing trend at longer times, and a considerable amount of the released As was re-adsorbed or precipitated. The order of the cumulative amounts of As released by extractants was [Formula: see text] . The percentage of total As released during the reaction time was less than 3% for all extractants except K2HPO4, which was between 4.0 and 8.5% in different soils. Therefore, much lower extractability of As occurred in the naturally As-contaminated soils at half saturation moisture than has previously been reported for soils spiked or artificially contaminated with As at lower soil to solution ratios (1:10 to 1:50) suggests that caution should be exercised in extending or applying the results of As release studies on artificially contaminated soils to naturally contaminated soils for remediation purposes. Finally among six kinetic models, only the power function and the simplified Elovich models described the release data satisfactorily.

Nitrate improves hackberry seedling growth under cadmium application.

The environmental toxicity of heavy metals in particular cadmium is a public concern. Cadmium is toxic for all living organisms including plants; however, plant species may show different tolerance to the presence of cadmium in their root medium. Adopting practical strategies may reduce cadmium bioavailability or increase the plant tolerance. In the present study, interaction of nitrate was investigated on cadmium treatment in hackberry (Celtis australis L.) seedlings. Different levels of nitrate (0, 50 and 100 mg/L) and cadmium (0 and 5 mg/L) were applied to seedlings via irrigation water during two consequence years. The treatments were arranged in a factorial with completely randomized design in four replications. The results of ANOVA showed that the cadmium-nitrate interaction was significant on leaf Cd concentration and root dry weight at P = 0.01, and on carotenoids and leaf dry weight at P = 0.05, while it was not significant on the rest of traits. Application of cadmium had no significant effect on new shoot growth, leaf chlorophyll and leaf fresh weight; however, it significantly reduced stomatal water conductance and photosynthesis rate, while it increased leaf transpiration rate, root and stem fresh weights, leaf Cd and proline concentrations. Application of nitrate levels, on the other hand, constantly increased the leaf nitrate concentration, new shoot growth, leaf fresh and dry weights, root fresh weight, stomatal water conductance and photosynthesis rate, whereas it reduced the necrotic points of leaves. The results indicated that the growth characteristics of hackberry seedlings were mainly influenced by nitrate but not cadmium application, and this ornamental tree is a tolerant species to high soil Cd levels.