Characterization of fluoride-tolerant halophilic Bacillus flexus NM25 (HQ875778) isolated from fluoride-affected soil in Birbhum District, West Bengal, India

A new Gram-positive, nonpigmented, rod-shaped fluoride-tolerant bacterial strain, NM25, was isolated from waterlogged muddy field soil collected from the fluoride endemic area of Rampurhat II block (average fluoride in water, 4.7 mg/l, and in soil, 1.5 mg/kg) in Birbhum District, West Bengal, India. The study was undertaken to characterize the fluoride-tolerant bacterial isolate, to determine its role in bioaccumulation of fluoride, and to analyze the water and soil quality of the bacterial environment. The isolate was positive for catalase, lipase, urease, protease, oxidase, and H₂S production, but negative for indole production, nitrate reduction, and Vogues–Proskauer test. The organisms were sensitive to recommended doses of ofloxacin, kanamycin, rifampicin, levofloxacin, vancomycin, gatifloxacin, gentamicin, doxycycline, streptomycin, and nalidixic acid but resistant to ampicillin. Based on the phenotypic characteristics, 16S rRNA gene sequence, and phylogenetic analysis, the bacterial isolate NM25 was identified as Bacillus flexus. The G+C content of the 16S rDNA was 53.14 mol%. This strain tolerated up to 20 % (w/v) NaCl in nutrient agar medium and was grown at the pH range 4–12. It reduced fluoride concentration up to 67.45 % and tolerated more than 1,500 ppm of fluoride in brain–heart infusion agar medium.     

Predicting potentially plant-available lead in contaminated residential sites

Lead (Pb)-based paints pose a serious health problem to people living in residential settings constructed prior to 1978. Children are at a greater risk to Pb exposure resulting from hand-to-mouth activity in Pb-contaminated residential soils. For soil Pb, the most environmentally friendly, potentially cheap, and visually unobtrusive in situ technology is phytoremediation. However, the limiting factor in a successful phytoremediation strategy is the availability of Pb for plant uptake. The purpose of this study was to establish a relationship between soil properties and the plant-available/exchangeable Pb fraction in the selected Pb-based paint-contaminated residential sites. We selected 20 such sites from two different locations (San Antonio, Texas and Baltimore, Maryland) with varying soil properties and total soil Pb concentrations ranging between 256 and 4,182 mg kg^?1. Despite higher Pb levels in these soils that exceeds US EPA permissible limit of 400 mg kg^???1, it is known that the plant-available Pb pools are significantly lower because of their sorption to soil components such as organic matter, Fe?CMn oxides, and clays, and their precipitation in the form of carbonates, hydroxides, and phosphates. Principal component analysis and hierarchical clustering showed that the potentially plant-available Pb fraction is controlled by soil pH in the case of acidic Baltimore soils, while soil organic matter plays a major role in alkaline San Antonio soils. Statistical models developed suggest that Pb is likely to be more available for plant uptake in Baltimore soils and a chelant-assisted phytoextraction strategy will be potentially necessary for San Antonio soils in mobilizing Pb from complexed pool to the plant-available pool. A thorough knowledge of site-specific factors is therefore essential in developing a suitable and successful phytoremediation model.     

Uncertainty based optimal monitoring network design for a chlorinated hydrocarbon contaminated site

An application of a newly developed optimal monitoring network for the delineation of contaminants in groundwater is demonstrated in this study. Designing a monitoring network in an optimal manner helps to delineate the contaminant plume with a minimum number of monitoring wells at optimal locations at a contaminated site. The basic principle used in this study is that the wells are installed where the measurement uncertainties are minimum at the potential monitoring locations. The development of the optimal monitoring network is based on the utilization of contaminant concentration data from an existing initial arbitrary monitoring network. The concentrations at the locations that were not sampled in the study area are estimated using geostatistical tools. The uncertainty in estimating the contaminant concentrations at such locations is used as design criteria for the optimal monitoring network. The uncertainty in the study area was quantified by using the concentration estimation variances at all the potential monitoring locations. The objective function for the monitoring network design minimizes the spatial concentration estimation variances at all potential monitoring well locations where a monitoring well is not to be installed as per the design criteria. In the proposed methodology, the optimal monitoring network is designed for the current management period and the contaminant concentration data estimated at the potential observation locations are then used as the input to the network design model. The optimal monitoring network is designed for the consideration of two different cases by assuming different initial arbitrary existing data. Three different scenarios depending on the limit of the maximum number of monitoring wells that can be allowed at any period are considered for each case. In order to estimate the efficiency of the developed optimal monitoring networks, mass estimation errors are compared for all the three different scenarios of the two different cases. The developed methodology is useful in coming up with an optimal number of monitoring wells within the budgetary limitations. The methodology also addresses the issue of redundancy, as it refines the existing monitoring network without losing much information of the network. The concept of uncertainty-based network design model is useful in various stages of a potentially contaminated site management such as delineation of contaminant plume and long-term monitoring of the remediation process.     

The role of disinfectants and sanitizers during COVID-19 pandemic: advantages and deleterious effects on humans and the environment

Environmental Science and Pollution Research - Disinfectants and sanitizers are essential preventive agents against the coronavirus disease 2019 (COVID-19) pandemic; however, the pandemic crisis...     

Kinetics of metribuzin release from bentonite-polymer composites in water

A series of bentonite polymer-composites (BPCs) loaded with metribuzin were studied for their controlled release in aqueous medium. The release of active ingredient from BPCs was significantly lower as compared to commercial metribuzin formulation. The results revealed that the cumulative metribuzin release was highest (81%) from the BPCs containing 8% clay (commercial bentonite) and 2% metribuzin which correspond to the lowest (14 days) half-life values i.e., time required for 50% release of active ingredient (t_1/2). The metribuzin release from the BPCs decreased with increased concentration of clays in polymer matrix and the release was further decreased with BPCs prepared with pure nano-bentonite. BPCs containing 12% clay and 2% metribuzin showed maximum t_1/2 values i.e., 25 and 51 days for commercial bentonite and pure nano-bentonite as clay sources, respectively. The differential behaviour in the metribuzin release rates from BPCs was ascribed due to variations in crosslinking of metribuzin in the composites. As metribuzin release was found to be slower in BPCs compared to commercial formulation, it could be used for control of weeds tailored to different crops.     

Evaluating a drinking-water waste by-product as a novel sorbent for arsenic

Arsenic (As) carcinogenicity to humans and other living organisms has promulgated extensive research on As treatment technologies with varying levels of success; generally, the most efficient methods come with a significantly higher cost burden and they usually perform better in removing As(V) than As(III) from solution. In the reported study, a novel sorbent, a waste by-product of the drinking-water treatment process, namely, drinking-water treatment residuals (WTRs) were evaluated for their ability to adsorb both As(V) and As(III). Drinking-WTRs can be obtained free-of-charge from drinking-water treatment plants, and they have been successfully used to reduce soluble phosphorus (P) concentrations in poorly P-sorbing soils. Phosphate and arsenate molecules have the same tetrahedral geometry, and they chemically behave in a similar manner. We hypothesized that the WTRs would be effective sorbents for both As(V) and As(III) species. Two WTRs (one Fe- and one Al-based) were used in batch experiments to optimize the maximum As(V) and As(III) sorption capacities, utilizing the effects of solid:solution ratios and reaction kinetics. Results showed that both WTRs exhibited high affinities for soluble As(V) and As(III), exhibiting Freundlich type adsorption with no obvious plateau after 2-d of reaction (15000 mg kg-1). The Al-WTR was highly effective in removing both As(V) and As(III), although As(III) removal was much slower. The Fe-WTR showed greater affinity for As(III) than for As(V) and reached As(III) sorption capacity levels similar to those obtained with the Al-WTR-As(V) system (15000 mg kg-1). Arsenic sorption kinetics were biphasic, similar to what has been observed with P sorption by the WTRs. Minimal (<3%) desorption of sorbed As(III) and As(V) was observed, using phosphate as the desorbing ligand. Dissolved Fe2+ concentrations measured during As(III) sorption were significantly correlated (r2=0.74, p<0.005) with the amount of As(III) sorbed by the Fe-WTR. Lack of correlation between Fe2+ in solution and sorbed As(V) (r2=0.2) suggests reductive dissolution of the Fe-WTR mediating As(III) sorption. Results show promising potential for the WTRs in irreversibly retaining As(V) and As(III) that should be further tested in field settings.     

Assessment of groundwater contamination from fertilizers in the Delhi area based on 0, N03 and K composition

Increasing application of nitrogen fertilizers in the irrigated lands of the studied area is likely to create a blanket non-point source of nitrate. Groundwater contamination from fertilizers, in this context, has been reported as derived from N0₃⁻, K⁺ and ¹⁸0 composition of groundwater. The data suggest both point and non-point sources of groundwater pollution. Thirty-three percent of the groundwater samples showed nitrate contents exceeding the general acceptable limit of 20 p.p.m. and 15% of the samples crossed the maximum permissible limit of 45 p.p.m. High nitrate levels are associated with high δ¹⁸O values, clearly indicating that significant quantities of evaporated (isotopically enriched) irrigation water infiltrate along with fertilizer nitrate to the groundwater system. Different δ¹⁸O---N0₃⁻ trends suggest isotopically distinct, non-point source origins which vary spatially and temporally, due to different degrees of evaporation/recharge and amounts of fertilizer applied. A scatter diagram of N0₃⁻ vs K⁺ suggests a common source of these ions when the concentration is less than 40 p.p.m. The investigation indicates that a combination of isotope (¹⁸0) and hydrochemical data can clearly characterize the impact of fertilizer on groundwater. Application of high nitrate, high potassium groundwater irrigation can minimize the requirement for inorganic fertilizers and bring down the cost of cultivation considerably, through appropriate management of fertilizer and water and modifications in agronomic practices and strategies on crops grown. Such practices will help protect groundwater from further degradation.     

Bioremediation of petroleum hydrocarbons in contaminated soils: comparison of biosolids addition, carbon supplementation, and monitored natural attenuation

Two methods of biostimulation were compared in a laboratory incubation study with monitored natural attenuation (MNA) for total petroleum hydrocarbon (TPH) degradation in diesel-contaminated Tarpley clay soil with low carbon content. One method utilized rapid-release inorganic fertilizers rich in N and P, and the other used sterilized, slow-release biosolids, which added C in addition to N and P. After 8 weeks of incubation, both biostimulation methods degraded approximately 96% of TPH compared to MNA, which degraded 93.8%. However, in the first week of incubation, biosolids-amended soils showed a linear two orders of magnitude increase in microbial population compared to MNA, whereas, in the fertilizer-amended soils, only a one order of magnitude increase was noted. In the following weeks, microbial population in the fertilizer-amended soils dropped appreciably, suggesting a toxic effect owing to fertilizer-induced acidity and/or NH(3) overdosing. Results suggest that biosolids addition is a more effective soil amendment method for biostimulation than the commonly practiced inorganic fertilizer application, because of the abilities of biosolids to supplement carbon. No statistically significant difference was observed between the biostimulation methods and MNA, suggesting that MNA can be a viable remediation strategy in certain soils with high native microbial population.     

Effect of soil properties on arsenic fractionation and bioaccessibility in cattle and sheep dipping vat sites

Historical use of high arsenic (As) concentrations in cattle/sheep dipping vat sites to treat ticks has resulted in severe contamination of soil and groundwater with this Group-A human carcinogen. In the absence of a universally applicable soil As bioaccessibility model, baseline risk assessment studies have traditionally used the extremely conservative estimate of 100% soil As bioaccessibility. Several in-vitro, as well as, in-vivo animal studies suggest that As bioaccessibility in soil can be lower than that in water. Arsenic in soils exists in several geochemical forms with varying degree of dissolution in the human digestive system, and thus, with highly varying As bioaccessibility. Earlier batch incubation studies with As-spiked soils have shown that As bioaccessibility is a function of soil physicochemical properties. We selected 12 dipping vat soils collected from USA and Australia to test the hypothesis that soil properties exert a significant effect on As bioaccessibility in As-contaminated sites. The 12 soils varied widely in terms of soil physico-chemical properties. They were subject to an As sequential fractionation scheme and two in-vitro tests (IVGS and IVGIA) to simulate soil As bioavailability in the human gastrointestinal system. Sequential As fractionation results showed that the majority of the As measured in the dipping vat soils resided either in the Fe/Al hydroxide fraction, or the Ca/Mg fractions, or in the residual fraction. Water-extractable As fraction of the 12 soils was typically <10% of the total, reaching values up to 23%, indicating minimal leaching potential, and hence, lower risk of As-contamination from exposure to groundwater, typically used as drinking water in many parts of the world. Partial individual correlations and subsequent multiple regression analyses suggested that the most significant soil factors influencing As bioaccessibility were total Ca+Mg, total P, clay content and EC. Collectively, these soil properties were able to explain 85 and 86% of the variability associated with the prediction of bioaccessible As, using IVGS and IVGIA in-vitro tests, respectively. This study showed that specific soil properties influenced the magnitude of soil As bioaccessibility, which was typically much lower than total soil-As concentrations, challenging the traditional risk assessment guideline, which assumes that soil As is 100% bioaccessible. Our study showed that total soil As concentration is unlikely to provide an accurate estimate of human health risk from exposure to dipping vat site soils.