Shankar Sharmasarkar

BTEX sorption by montmorillonite organo-clays : TMPA, ADAM, HDTMA

Organo-clay can be utilized for the containment of environmentalpollutants originating from waste sites or accidental spills. Abatch study was conducted using organo-clays produced from aWyoming montmorillonite (SWy) and three organic cations(trimethylphenylammonium (TMPA), trimethylammonium adamantane(Adam), and hexadecyltrimethylammonium (HDTMA)) to characterizeBTEX (benzene, toluene, ethylbenzene, o-, m-, p-xylene) sorption. Sorption data were fitted to two models,with Freundlich resulting in greater correlations of the datathan the Langmuir model (R2 at P ≤0.001-0.05). The Freundlich conditional index (nf),which describes the experimental sorption characteristics,decreased curvilinearly with organic-cation molecular weights,thus suggesting organo-clays with smaller cations had greaterhydrocarbon retention. Sorption of BTEX followed the order ofTMPA > Adam > HDTMA organo-clays. A similar sequence in themagnitudes of log Kd and log Komsupportedthis finding. Positive log Kom/Kow valuesfor TMPA and Adam derivatives indicated there was a greaterretention of BTEX by these organo-clays than octanol. The orderof log Kom for SWy-HDTMA, although concentration-dependent, was analogous to the log Kow order,indicating partitioning was the dominant sorption mechanism forthe HDTMA-clay. Isotherms for SWy-TMPA and SWy-Adam followed aconvex up pattern. In contract, a concave up curvature, notedfor SWy-HDTMA isotherms, was probably caused by a cosorptiveenhancement process resulting from an effective increase in organic matter content of the organo-clay due to furtherhydrocarbon sorption,in concurrence with a decrease inadsorbate activity coefficients. Values of binding affinityconstant, Kf, for SWy-TMPA were consistently higherthan SWy-Adam. The Kf values determined for totalBTEX sorption by TMPA and Adam derivatives were higher thanthose for the individual hydrocarbons. With SWy-HDTMA, the same order was observed for benzene and toluene; however, ethylbenzene and xylenes had greater Kf values thanthat for the BTEX mixture, possibly due to higher partitioningaffinity of the larger alkylbenzenes. With SWy-HDTM, thesequence of Kf values was: ethylbenzene > m-xylene > p-xylene > o-xylene > toluene >benzene. Trends for SWy-TMPA and SWy-Adam were in contrast tothat of the partitioning order, suggesting that adsorption, ratherthan partitioning, was the primary sorption mechanism for thesetwo organo-clays. With respect to the equilibriumconcentrations, the sorbed amounts for total BTEX mixture weregenerally higher than those for the individual compounds. Ascompared to benzene and toluene, the large-size alkylbenzenesshowed greater partitioning affinity due to their high hydrophobicity.

Selenite–selenate sorption in surface coal mine environment

Abstract Information is limited on the sorption characteristics of selenite (SeO 3 2− ) and selenate (SeO 4 2− ) in reclaimed coalmine environments. The objectives of this research were to evaluate the adsorption of both Se species using various isotherm models, and to quantify the relationship between sorption parameters and different soil properties. Ten coalmine soils were studied using Se treatments ranging from 0 to 2.5 mg/l. The results indicated that the percent sorption varied from 48 to 99% for SeO 3 2− , and 44 to 95% for SeO 4 2− ; however, for all soil samples, percent SeO 3 2− sorption was always greater than SeO 4 2− , indicating SeO 3 2− was the stronger adsorbate. The Langmuir model described SeO 3 2− sorption in eight samples, whereas adsorption of SeO 4 2− was described in only one sample by this model. The Freundlich model resulted in better statistical fitting of the data compared to the Langmuir model for both Se species; 10 and six samples conformed to adsorption of SeO 3 2− and SeO 4 2− , respectively. Adsorption of SeO 3 2− in nine soils and SeO 4 2− in six soils were described by the Temkin model. The best fitting of SeO 3 2− and SeO 4 2− adsorption (for all 10 soils) occurred with the Initial Mass isotherm. Higher K i values for SeO 3 2− in comparison to SeO 4 2− suggested that SeO 3 2− had a greater adsorption affinity for the coal mine soils. Difference in sorption strength of SeO 3 2− and SeO 4 2− was attributed to the structural difference of the two species based on the concepts of valence shell electron pair repulsion model and hybridization. Different adsorption indices, including Langmuir ( B l , K l B l ), Freundlich ( n f ), Temkin ( K t1 K t2 ), and Initial Mass ( K i ) model parameters, were linearly related (at P of 0.05) to various soil properties. Phosphate extractable Se and total soil Se were correlated to mainly SeO 3 2− adsorption parameters including K l B l , K t1 K t2 , and K i . Extractable Ca, Mn and Fe concentrations were correlated to B l and n f , indicating the presence of oxides or oxyhydroxides of the metals could result in greater Se retention in the mine soils. Correlations of n f with electrical conductivity, clay content, and specific surface area also suggested a possible dependence of Se adsorption on other soil properties.

Fractional partitioning for assessing solid-phase speciation and geochemical transformations of soil selenium

Fractionation techniques have been applied widely to assess the behavior of various geological trace elements. However, relatively few studies have been conducted on solid-phase speciation and geochemical assessment of selenium (Se) through fractional partitioning. Soils collected from contrasting range and mine environments of Wyoming were partitioned fractionally to investigate the significance of different Se fractions in assessing solid-phase speciation and geochemical transformations. The Se fractions extracted by the partitioning technique were aqueous (0.25 M KCI), exchangeable (1 M KH2PO4), acid-extractable (4 M HCI), sulfide and humic bound (KCIO3 + 12 M HCI), and siliceous (15.8 M HNO3 + 11.7 M HCIO4 + 28.9 M HF); each fraction was also speciated for selenite (Se+4) and selenate (Se+6). In addition, the soil samples were analyzed for total soil Se after a strong mixed-acid digestion (15.8 M HNO3 + 11.7 M HCIO4 + 28.9 M HF). High percent recovery (85–104%) was observed between total Se and sum of the partitioned fractions, which were also significantly correlated (r = 0.99). With increasing exchangeable Se, there was a corresponding increase in the aqueous fraction (r = 0.72). A similar increasing trend was found between the exchangeable and acid-extractable fractions (r = 0.51). For the aqueous fraction, Se+6 was found to be the potential species, whereas in the exchangeable fraction, Se+4 predominated. Both Se+4 and Se+6 were distributed almost evenly in the acid-extractable fraction. Speciation data for sulfide and humic bound, and siliceous fractions, however, were not conclusive. Selenium in the available and conditionally available fractions increased as the weatherable fraction decreased (for both r = −0.83). Total soil Se increased with increasing clay content (r = 0.81), and a reverse trend was observed with sand content (r = −0.69), suggesting Se is associated with the clay-sized soil fraction. Siliceous Se was linearly related to total soil Se (r = 0.91), suggesting the presence of Se in soil minerals. This fraction was again found to increase with increasing clay content (r = 0.74). Average distributions of the five Se fractions indicated the soils were dominated by the siliceous Se, with aqueous fraction comprising the least abundant Se level. Statistical analysis indicated all of the above mentioned correlations were significant at the P < 0.01 or 0.001 level.

Preliminary quantification of metal selenite solubility in aqueous solutions

Abstract Selenium solubilization can be a potential source of environmental contamination in natural systems. Currently no experimental data are available on the solubility of metal selenites in aqueous solutions. The objective of our study was to determine the solubility of Ca, Mg, Mn, and Zn selenites. A dissolution study was conducted for 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35 and 40 weeks with CaSeO 3 · H 2 O, MgSeO 3 , MnSeO 3 and ZnSeO 3 · H 2 O using different background solutions, including distilled deionized water (DI Water), 0.01 M NaCl, 0.03 M NaCl and 0.02 M Na 2 SeO 3 . For all solids, equilibrium was reached within 40 weeks. As compared to the theoretical values obtained from literature, the experimental p K 0 values for ZnSeO 3 · H 2 O were higher, whereas for MgSeO 3 the determined p K 0 values were lower; theoretical and experimental data for both CaSeO 3 · H 2 O and MnSeO 3 were comparable. A comparison of the p K 0 values also indicated that MgSeO 3 had the highest solubility in DI water, whereas in the three other solutions MnSeO 3 had the maximum solubility. The mean p K 0 values determined for CaSeO 3 · H 2 O, MgSeO 3 , MnSeO 3 and ZnSeO 3 · H 2 O in the above mentioned background solutions were 7.76, 7.56, 7.11, and 7.70, respectively, which indicated that the overall solubility order would be, Mn SeO 3 > MgSeO 3 > ZnSeO 3 · H 2 O > CaSeO 3 · H 2 O. The results of this study are fundamental for modifying existing Se thermodynamic databases and understanding selenite solid-water interactions in natural environments.

Characterization and correlation of soil and plant selenium in some range and coal mine environments of Wyoming

Abstract Selenium (Se) has been recognized as an element of environmental concern because of its natural occurrence and potential toxicity to plants, animals, and humans. Range and coal mine (reclaimed and abandoned) soils were studied using various soil extracts, including saturated paste, hot water, ammonium bicarbonate‐DTPA (AB‐DTPA), and phosphate in order to compare the Se extraction efficiency of these reagents and to characterize the soil‐plant Se correlations in these environments. Soil extraction data indicated the order of Se extractability was phosphate > AB‐DTPA > hot water > saturated paste. The saturated paste‐extractable Se was found to be typically very low. Statistically significant correlations between phosphate‐, AB‐DTPA‐, and hot water‐extractable Se were observed irrespective of the type of environment. Selenium extracted by these reagents indicated certain relationships, such as phosphate ≈ 2 AB‐DTPA, phosphate ≈ 5 hot water, and AB‐DTPA ≈ 3 hot water. Selenium content of shrub veget...

Micro-spatial variability of soil nitrate following nitrogen fertilization and drip irrigation

Micro-spatial analysis of nitrate (NO3), an environmental contaminant partially attributed to nitrogen fertilization, can be useful for estimating its distribution in soils. A study was conducted to determine the micro-spatial distribution of soil NO3 using kriging and cokriging in a drip-irrigated and nitrogen-fertilized field. One hundred soil samples were collected in a regular grid pattern from a 10 m × 20 m plot, and analyzed for soil NO3 and pH. The effect of reduced sample size on NO3 estimation was also evaluated. The pH data indicated the soils were slightly acidic to neutral with log[NO3] values ranging from 1.66 to 2.95. These parameters were inversely related; which was probably an attribute of soil nitrification process. Sample variograms and cross-variograms suggested that the spatial distribution of pH and log[NO3] could be described by linear models in the area studied, as indicated by small MSE (mean sum error), and RKV (reduced kriging variance) values close to 1. Contour maps based on kriging and cokriging estimates indicated greater homogeneity of the variables in the south-north direction than the east-west, except for zones of high NO3 and low pH in the north-central edge and north-east corner of the grid area. Cokriging of log[NO3] estimation, using pH data, improved MSE, MSSE (mean sum square error), MKV (mean kriging variance), RKV, CEE (correlation between estimated data and error), CEM (correlation between estimated and measured data) by 46, 31, 30, 22, 96, and 98%, respectively, as compared to kriging. Lower cokriging variance for any estimated log[NO3] value, as compared to the kriging analysis, indicated that cokriging provided more accurate estimates. With reduced sample observations (n) for NO3 similar conclusions were obtained; and the estimation accuracy was maintained up to n >70. Cokriging analysis with reduced n also curtailed the analytical cost, and facilitated NO3 estimation by means of pH, which was measured at a cheaper cost.

Assessment of Microirrigation for Sugarbeet Production

ABSTRACT Sugarbeet (SB) is a major value-crop in many furrow (Fu) irrigated areas of Wyoming where nitrate (NO3) contamination prevails. Microirrigation (M) may be an option for managing such pollution while sustaining SB production. With the goal of evaluating efficacy of M for SB production, in comparison to Fu practice, studies were conducted through field experiments, computer simulations, and economic assessments. The field study with SB consisted of a Fu and three M treatments (M1, M2, and M3 representing a management allowable depletion factor of 0.20, 0.35, and 0.50, respectively) and three fertilizer doses. Water use, post-harvest soil NO3 concentrations, SB yields, and water percolation were measured. Computer simulations were performed using a transport and irrigation model to predict soil NO3 concentrations and the results were compared with the experimental data. The economic feasibility of adopting M for 10 to 40 ha area was evaluated using a capital budgeting analysis. Results of field studies indicated lower water use, higher soil NO3 levels, less water percolation and greater SB yields with M than under Fu practice. The yields increased as management allowable depletion factor and fertilizer rate augmented. Computer simulations predicted that post-harvest soil NO3 concentrations were higher when SB were irrigated with M rather than Fu practice. Simulation results were comparable with the field data. The economic analysis indicated that irrigation costs decreased and SB returns increased with M practice. Results of capital budgeting showed that adoption of M in Wyoming was feasible for small-scale SB production. The profitability of the M system was dependent on the irrigation regime and the conversion area.

Understanding Selenium Mobility by Sorption and Extraction Processes in Surface Coal Mine Spoils.

Surface coal mine spoil was studied to evaluate selenium (Se) mobility and solubility using batch experiments, column-leaching techniques, and sequential and individual extraction processes. Both selenite and selenate followed linear isotherm patterns at relatively lower Se treatments, whereas over a broader solution Se concentration range the sorption mechanism followed the Langmuir model. The variation in the isotherm patterns with change in Se concentrations suggested that mechanisms for selenite-selenate sorption would be a function of the concentration of solution-phase Se. Greater isotherm slope for selenite sorption suggested higher affinity of this specie for the mine spoil, while selenate was the more liable aqueous specie. In the exchangeable phase, selenite was the predominant specie. The results also indicated that adsorbed Se may be transformed into plant-available Se with time. Cationic presence enhanced selenite immobilization, with Fe being the most effective followed by Mn and Ca. The column leading study indicated selenate had a faster mobilization rate, and there was a probable antagonistic relationship between Se species and sulfate in mine spoil solutions.