Developing a biosorbent from <Emphasis Type="Italic">Aegle Marmelos</Emphasis> leaves for removal of methylene blue from water

Dried, mature leaves of Aegle Marmelos tree were converted to a powder, which was used as a biosorbent for dyes in water with methylene blue as a case study. The biosorbent had a surface area of 52.63 mg/g, and FTIR spectra showed the presence of –COOH, –NH₂, –R–SC=O (thioester) and R1–S(=O, =O)-N(–R2, –R3) groups on the surface. The particles were found to be porous in nature from scanning electron micrographs, and EDX measurements showed the elements C, O, Na, Mg, K, Ca and Fe on the surface. Batch adsorption experiments showed that the adsorption of the dye was preferred at near-neutral conditions. Adsorption equilibrium was achieved in ~120 min with maximum dye uptake of 19.9 mg/g. Investigation into the kinetics of adsorption indicated that second-order kinetics gave the best fit to the experimental data, and a rate coefficient of 8.0 × 10^?2 to 32.3 × 10^?2 g mg^?1 min^?1 was obtained.     

Elimination of natural organic matter by electrocoagulation using bipolar and monopolar arrangements of iron and aluminum electrodes

The aim of this research was to evaluate the efficiency of electrocoagulation (EC) for the removal of natural organic matter (NOM) by using iron (Fe) and aluminum (Al) electrodes. The effects of several operational parameters such as initial pH (3–10), time of electrolysis (5–30 min), initial concentration of organic matter (10–50 mg NOM/L), current density (0.25–1.25 mA/cm²), type of electrode material (n = 4, 2 sides × 11 cm × 10 cm, wall thickness = 2 mm, distance between each electrode = 5 mm), and type of connection of electrodes (bipolar and monopolar configurations) were explored for the removal of NOM from synthetic humic acid solution in a 2 L laboratory-scale EC cells (A _s/V = 0.110 cm^?1). The optimum conditions for the process were identified as pH = 3 and 7, electrolysis time = 20 and 10 min for Fe and Al electrodes, respectively. Using both electrodes at current density = 0.25 mA/cm² and initial concentration of organic matter = 50 mg/L, a NOM removal efficiency of almost 100% could be achieved in the bipolar mode. Based on the optimum conditions, specific reactor electrical energy consumptions were 14.90 kWh/kg Al (or 0.092 kWh/m³) and 2.88 kWh/kg Fe (or 0.11 kWh/m³). Specific electrode consumptions were obtained to be 0.0062 and 0.0382 kg/m³, and operating costs of the EC system were preliminary estimated at 0.057 and 0.119 $/m³ for Al and Fe electrodes, respectively.     

Chemical characterization and sources of PM2.5 at 12-h resolution in Guiyang,China

The increasing emission of primary and gaseous precursors of secondarily formed atmospheric particulate matter due to continuing industrial development and urbanization are leading to an increased public awareness of environmental issues and human health risks in China. As part of a pilot study, 12-h integrated fine fraction particulate matter (PM_2.5) filter samples were collected to chemically characterize and investigate the sources of ambient particulate matter in Guiyang City, Guizhou Province, southwestern China. Results showed that the 12-h integrated PM_2.5 concentrations exhibited a daytime average of 51 ± 22 µg m^?3 (mean ± standard deviation) with a range of 17–128 µg m^?3 and a nighttime average of 55 ± 32 µg m^?3 with a range of 4–186 µg m^?3. The 24-h integrated PM_2.5 concentrations varied from 15 to 157 µg m^?3, with a mean value of 53 ± 25 µg m^?3, which exceeded the 24-h PM_2.5 standard of 35 µg m^?3 set by USEPA, but was below the standard of 75 µg m^?3, set by China Ministry of Environmental Protection. Energy-dispersive X-ray fluorescence spectrometry (XRF) was applied to determine PM_2.5 chemical element concentrations. The order of concentrations of heavy metals in PM_2.5 were iron (Fe) > zinc (Zn) > manganese (Mn) > lead (Pb) > arsenic (As) > chromium (Cr). The total concentration of 18 chemical elements was 13 ± 2 µg m^?3, accounting for 25% in PM_2.5, which is comparable to other major cities in China, but much higher than cities outside of China.     

Fluoride contamination of groundwater in parts of eastern India and a preliminary experimental study of fluoride adsorption by natural haematite iron ore and synthetic magnetite

Incidence of high fluoride (F^?) in groundwater (>1.5 mg/L) in two tribal belts of eastern India, one around Chukru in the Palamau district of Jharkhand and the other around Karlakot in the Nuapada district of Odisha, has been studied. The maximum concentration of F^? in groundwater from dug wells and tube wells is 10.30 mg/L in Chukru and 4.62 mg/L in Karlakot. The groundwaters are mildly alkaline with pH ranges of 7.52–8.22 and 7.33–8.20 in Chukru and Karlakot, respectively. The F^? concentration is positively correlated with pH, electrical conductivity and SO₄ ^2? in both areas. The high F^? in groundwater resulted mainly from dissolution of biotite and fluorapatite in quartzofeldspathic gneiss. The ionic dominance pattern (in meq/L) is mostly in the order Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ among cations and HCO₃ ^? > SO₄ ^2? > < Cl^? > F^? among anions in the Karlakot groundwater. Preliminary adsorption experiments were conducted on natural haematite iron ore and synthetic magnetite to evaluate their potential for F^? removal from water. Effects of different parameters such as contact time, pH, adsorbent dose and initial F^? concentration on the adsorption capacity of these materials were investigated. Strong dependence of F^? removal on pH was observed for both the adsorbents. With natural haematite iron ore, the maximum F^? removal of 66 % occurred at an initial pH of 3.2 for a solution with F^? concentration of 3 mg/L, adsorbent dose of 7 g/L and overnight contact time. The haematite iron ore was observed to increase the pH of the F^? solution. Adsorption equilibrium was not achieved with this adsorbent even after a contact time of 45.2 h. In the case of synthetic magnetite, 84 % F^? removal was achieved after 2 min of contact time for a solution with F^? concentration of 6 mg/L, adsorbent dose of 10 g/L and initial pH of 7. The results indicate high potential of both natural haematite iron ore and synthetic magnetite as adsorbents of F^? in water.     

Iron and Pyritization in Wetland Soils of the Florida Coastal Everglades

We explored environmental factors influencing soil pyrite formation within different wetland regions of Everglades National Park. Within the Shark River Slough (SRS) region, soils had higher organic matter (62.65 ± 1.88 %) and lower bulk density (0.19 ± 0.01 g cm^?3) than soils within Taylor Slough (TS; 14.35 ± 0.82 % and 0.45 ± 0.01 g cm^?3, respectively), Panhandle (Ph; 15.82 ± 1.37 % and 0.34 ± 0.009 g cm^?3, respectively), and Florida Bay (FB; 5.63 ± 0.19 % and 0.73 ± 0.02 g cm^?3, respectively) regions. Total reactive sulfide and extractable iron (Fe) generally were greatest in soils from the SRS region, and the degree of pyritization (DOP) was higher in soils from both SRS (0.62 ± 0.02) and FB (0.52 ± 0.03) regions relative to TS and Ph regions (0.30 ± 0.02 and 0.31 ± 0.02, respectively). Each region, however, had different potential limits to pyrite formation, with SRS being Fe and sulfide limited and FB being Fe and organic matter limited. Due to the calcium-rich soils of TS and Ph regions, DOP was relatively suppressed. Annual water flow volume was positively correlated with soil DOP. Soil DOP also varied in relation to distance from water management features and soil percent organic matter. We demonstrate the potential use of soil DOP as a proxy for soil oxidation state, thereby facilitating comparisons of wetland soils under different flooding regimes, e.g., spatially or between wet years versus dry years. Despite its low total abundance, Fe plays an important role in sulfur dynamics and other biogeochemical cycles that characterize wetland soils of the Florida coastal Everglades.     

Assessment of heavy metals in sediment and in suspended particles affected by multiple anthropogenic contributions in harbours

La Goulette, Rades and Sidi Bou Said harbours are considered as the most important commercial and tourist ports in the Gulf of Tunis. They are located on the northeast coast of Tunis and receive industrial and municipal wastewaters from Tunis city. The contamination level of copper, lead, zinc, cadmium, manganese, iron, total nitrogen and total organic carbon in the surface sediments was assessed on the basis of the enrichment index factors and corresponding to sediment quality guidelines. The results revealed moderate to highly elevated concentrations near to the sites of intense industrial, shipping and/or commercial activities suggesting a direct influence of these sources. In winter and summer, concentrations varied for cadmium, 0.28–1.40 mg kg^?1; lead, 18–217 mg kg^?1; zinc, 87–459 mg kg^?1; copper, 8–121 mg kg^?1; manganese, 208–254 mg kg^?1; and for iron, 24–40 g kg^?1. Furthermore, in summer the concentration of the total organic carbon and the total nitrogen contents range between 4.3–6.5 % and 0.06–0.49 % with an average value of 5.9 and 0.15 %, respectively. Whereas, in winter, total organic carbon and the total nitrogen concentrations varied between 2.3–9.6 % and 0.03–0.22 % with an average value of 6.1 and 0.14 %, respectively. The levels of lead, copper, zinc and iron in suspended particulate matter content range between 3.1–27.5 mg kg^?1; 0.4–11.7 mg kg^?1; 1–1.5 mg kg^?1; 1.2–1.7 g kg^?1, respectively. This study revealed that heavy metals pollution is mainly localized in the commercial (Rades) and fishing (La Goulette) harbours and not in the yachting (Sidi Bou Said) harbour.     

Vertical geochemical variations and arsenic mobilization in the shallow alluvial aquifers of the Chapai-Nawabganj District, northwestern Bangladesh: implication of siderite precipitation

Core sediments from three disturbed boreholes (JOR, GHAT, and RAJ) and two undisturbed boreholes (DW1 and DW2) were collected in the study area of the Chapai-Nawabganj district of northwestern Bangladesh for geochemical analyses. In the study area, groundwater samples from fourteen As-contained private wells and five nested piezometers at both the DW1 and DW2 boreholes were also collected and analyzed. The groundwater arsenic concentrations in the uppermost aquifer (10–40 m of depth) range from 3 to 315 μg/L (mean 47.73 ± 73.41 μg/L), while the arsenic content in sediments range from 2 to 14 mg/kg (mean 4.36 ± 3.34 mg/kg). An environmental scanning electron microscope (ESEM) with an energy dispersive X-ray spectrometer was used to investigate the presence of major and trace elements in the sediments. Groundwaters in the study area are generally the Ca–HCO₃ type with high concentrations of As, but low levels of Fe, Mn, NO₃ ^? and SO ₄ ^?2 . The concentrations of As, Fe, Mn decrease with depth in the groundwater, showing vertical geochemical variations in the study area. Statistical analysis clearly shows that As is closely associated with Fe and Mn in the sediments of the JOR core (r = 0.87, p < 0.05 for Fe and r = 0.78, p < 0.05 for Mn) and GHAT core (r = 0.95, p < 0.05 for Fe and r = 0.93, p < 0.05 for Mn), while As is not correlated with Fe and Mn in groundwater. The comparatively low Fe and Mn concentrations in some groundwater and the ESEM image revealed that siderite precipitated as a secondary mineral on the surface of the sediment particles. The correlations along with results of sequential extraction experiments indicated that reductive dissolution of FeOOH and MnOOH represents a mechanism for releasing arsenic into the groundwater.     

Nitrite reduction by Fe(II) associated with kaolinite

Interactions of iron (Fe) with the nitrogen (N) cycle have emerged and contain elements of abiotic and biological reactions. One such abiotic reaction which has received little study is the reactivity of NO₂ ^? and Fe(II) associated with a major clay mineral, kaolinite. The main objective of this study was to evaluate the reactivity of NO₂ ^? with Fe(II) added to kaolinite under anoxic conditions. Stirred batch reactivity experiments were carried out with 10 g L^?1 kaolinite spiked with 25 and 100 µM Fe(II) at pH 6.45 in an anaerobic chamber. Approximately 500 µM NO₂ ^? was added to initiate the reaction with Fe(II)-loaded kaolinite. The rate of nitrite removal from solution was 2.4-fold slower in the high Fe(II) treatment when compared with the low Fe(II) treatment. A large portion of the NO₂ ^? removed from solution was confirmed to be reduced to N₂O_(g) in the Fe(II)-kaolinite slurries. However, NO₂ ^? reduction was also noticed in the presence of kaolinite-alone and to somewhat lesser extent in the presence of dithionite-citrate-bicarbonate (DCB)-treated kaolinite. Chemical extractions coupled with infrared spectroscopy suggest that Fe(III) oxide mineral impurities and structural Fe(III) in kaolinite may participate in NO₂ ^? removal from solution. Furthermore, a magnetite mineral was identified based on X-ray diffraction analysis of untreated kaolinite and DCB-treated kaolinite. Our findings reveal a novel pathway of NO₂ ^? transformation in the environment in the presence of Fe(II) associated (sorbed and impurity) with kaolinite.     

Determination of perchlorate and iodide concentrations in edible seaweeds

Perchlorate and iodide concentrations were determined in brown (Undaria pinnatifida and Laminaria japonica) and red (Porphyra sp.) edible seaweeds, which are commonly consumed by Korean people, with the use of ion chromatography, coupled with a tandem mass spectrometer. Seaweeds (i.e., good sources of iodine) are among the most important plant life in the ocean and commonly consumed as food and nutritional supplement in South Korea. All seaweed samples were purchased from different regions in South Korea. The detected concentrations of perchlorate were as follows: 19.7–620.7 μg kg^?1 dry weight (n = 11, mean concentration = 149.2 μg kg^?1 dry weight) for L. japonica and 7.3–21.7 μg kg^?1 dry weight (mean concentration = 10.6 μg kg^?1 dry weight) for U. pinnatifida. Of the 11 samples of Porphyra sp., only 1 sample showed 6.7 μg kg^?1 dry weight perchlorate. The concentrations of iodide in all seaweed samples varied from 0.44 to 6,800 mg kg^?1 dry weight. L. japonica samples (n = 11) had significantly higher iodide concentrations, with a mean of 5,261 mg kg^?1 dry weight. The bioconcentration factor values for perchlorate and iodide in the three different seaweeds varied widely and showed similar variation trends. The trend for perchlorate and iodide was Porphyra sp. < U. pinnatifida < L. japonica. The results have provided growing evidence that perchlorate frequently occurs in food products.     

The impact of sewage-contaminated river water on groundwater ammonium and arsenic concentrations at a riverbank filtration site in central Delhi,India

The groundwater abstracted at a well field near the Yamuna River in Central Delhi, India, has elevated ammonium (NH₄ ⁺) concentrations up to 35 mg/L and arsenic (As) concentrations up to 0.146 mg/L, constituting a problem with the provision of safe drinking and irrigation water. Infiltrating sewage-contaminated river water is the primary source of the NH₄ ⁺ contamination in the aquifer, leading to reducing conditions which probably trigger the release of geogenic As. These conclusions are based on the evaluation of six 8–27-m deep drillings, and 13 surface-water and 69 groundwater samples collected during seven field campaigns (2012–2013). Results indicate that losing stream conditions prevail and the river water infiltrates into the shallow floodplain aquifer (up to 16 m thickness), which consists of a 1–2-m thick layer of calcareous nodules (locally known as kankar) overlain by medium sand. Because of its higher hydraulic conductivity (3.7 × 10^?3 m/s, as opposed to 3.5 × 10^?4 m/s in the sand), the kankar layer serves as the main pathway for the infiltrating water. However, the NH₄ ⁺ plume front advances more rapidly in the sand layer because of its significantly lower cation exchange capacity. Elevated As concentrations were only observed within the NH₄ ⁺ plume indicating a causal connection with the infiltrating reducing river water.     

Enrichment pattern of leachable trace metals in roadside soils of Miri City,Eastern Malaysia

This article presents the results on distribution and enrichment pattern of acid-leachable trace metals (ALTMs) from roadside soil of Miri city, Sarawak, East Malaysia. The city is one of the fastest developing in the Malaysian region with huge petroleum resources. ALTMs Fe, Mn, Cr, Cu, Ni, Co, Pb, Zn and Cd along with organic carbon and carbonates (CaCO₃) were analyzed in 37 soil sediments collected from roadside. The enrichment of ALTMs [especially Cu (0.4–13.1 μg g^?1), Zn (9.3–70.7 μg g^?1), Pb (13.8–99.1 μg g^?1)] in the roadside soils indicate that these metals are mainly derived from sources related to traffic exhausts, forest fires and oil refineries. The comparative study and enrichment pattern of elements indicates that Mn, Cu, Zn and Pb are enriched multi-fold than the unpolluted soil and Ni, Pb, Cd in some samples compared to Sediment Quality Guidelines like Lowest Effect Level (LEL) and Effects Range Low (ERL) in the region which is mainly due to the recent industrial developments in the region.     

Evaluation of effective nutritional parameters for <Emphasis Type="Italic">Scenedesmus</Emphasis> sp. microalgae culturing in a photobioreactor for biodiesel production

Recently, microalgae are considered as lipid sources for biodiesel production. A photobioreactor was designed and fabricated for Scenedesmus sp. microalgae cultivation. The effect of several nitrogen sources, light intensity, iron ions, silicon, magnesium sulfate and ethanol concentrations on Scenedesmus sp. microalgae growth were investigated. For incubation period of 8 days, sodium nitrate and ammonium carbonate were the best nitrogen sources with biomass concentrations of 2.373 and 2.254 g L^?1, respectively. Microalgae growth was reduced using nitrogen concentrations above 0.7 g L^?1. In the first 10 days of incubation, maximum cell dry weight (0.7 g L^?1) was obtained with light intensity of 10,000 lx, whiles after that, the results were desired (1 g L^?1) using interior lighting at 7500 lx. Magnesium sulfate had a positive effect on cell growth. The biomass concentration of 1.65 g L^?1 was obtained using 0.06 g L^?1 magnesium sulfate. Maximum obtained biomass with silicon (0.7 Mm), ethanol (1.8 mL L^?1) and ferric ammonium citrate (0.02 g L^?1) was 1.7 and 1.3 and 2.16 g L^?1, respectively. Logistic model was found to be a suitable model for cell growth forecast. Fatty acid analysis showed that composition of the most dominant synthesized fatty acids, palmitic and oleic acids, was 21.16 and 33.58%, respectively. Oil produced by Scenedesmus sp. microalgae composed of 49.08% saturated and 43.53% unsaturated fatty acids has a suitable composition for a desired biodiesel.     

High-resolution profiles of iron, manganese, cobalt, cadmium, copper and zinc in the pore water of estuarine sediment

The remobilization of iron, manganese, cobalt, cadmium, copper and zinc in the pore water of estuarine sediment cores at Yingkou was assessed using diffusive equilibrium in thin films and diffusive gradients in thin films techniques. A relatively anoxic system (+33.7 to ?224.1 mV) in the sediment cores might cause the reductive release of iron, manganese and cobalt into pore water from the estuarine sediment. High-average concentrations of iron (47.85 μg ml^?1) and manganese (3.81 μg ml^?1) were observed using diffusive equilibrium in thin films on the sediment core, but the concentration of cobalt (18.02 ng ml^?1) was relatively low. A strong correlation between iron and cobalt was observed based on the vertical profiles of the metals. Manganese and iron were more readily released from the solid phase to the solution. The peak cobalt, copper and zinc concentrations were observed in the upper layer (2–4 cm) measured using diffusive gradients in thin films. However, the peak iron, manganese and cobalt concentrations were located in the deeper layer (≥7 cm). In addition, the concentration profiles measured using diffusive gradients in thin films of cobalt, copper and zinc were independent of the iron, manganese and cobalt distribution with respect to depth.     

Evaluation of trace element concentration (acid leachable) in sediments from River Pánuco and its adjacent lagoon areas, NE México

A set of forty-one surface sediment samples were collected in River Pánuco and its adjoining lagoon areas in NE Mexico to identify the enrichment pattern of trace elements. The samples were analyzed for sediment texture, carbonates, organic carbon and acid leachable trace elements (ALTEs) using autoclave method [Fe, Mn, Cr, Cu, Ni, Co, Pb, Zn, Cd, V, Be, Ba, Sr, As]. Geochemical results of Fe, Cr, Ni, Co, V and Sr in zone 1 indicate that erosion in the upland region (Sierra Madre Oriental Mountains) is very high. The above feature is supported by the supremacy of finer sediments (82.12 %), carbonates (44.67 %) and organic carbon (10.74 %), which are brought down from the drainage basin. The overall average concentration of ALTEs Mn (607 μg g^?1), Cu (28.29 μg g^?1), Ni (16.56 μg g^?1), Pb (46.11 μg g^?1), Cd (1.81 μg g^?1) and Zn (92.18 μg g^?1) indicates higher values than the lowest effect level (LEL) and effects range low (ERL) of environmental indicators. The results suggest that they are due to the increase in oil refineries, metal based industries, shipping activities and the effluent input which could enter the biological cycle and might create human health problems.     

The diagenetic geochemistry and contamination assessment of iron,cadmium, and lead in the sediments from the Shuangtaizi estuary,China

Sediment and pore water samples have been collected from the coastal tidal flat in the Shuangtaizi estuary, China, in order to investigate the geochemical behavior of iron, cadmium, and lead during diagenesis and to assess the degree of contamination. The calculated enrichment factors and geoaccumulation indices for separate elements show that anthropogenic activities have had no significant influence on the distribution of Fe and Pb in the study area, whereas the distribution of Cd has been closely influenced in this way. The high percentage of exchangeable Cd (average of 56.34%) suggests that Cd represents a potential hazard to benthic organisms in the estuary. The calculated diffusive fluxes of metals show that the most mobilized metal is Fe (9.22 mg m^?2 a^?1), followed by Cd (0.54 mg m^?2 a^?1) and Pb (0.42 mg m^?2 a^?1). Low Fe²⁺ contents in surface pore water, alongside high chromium-reducible sulfur contents, and low acid-volatile sulfur, and elemental sulfur contents at 0–25 cm depth in sediments show that Fe²⁺ is formed by the reduction of Fe oxides and is transformed first to a solid phase of iron monosulfides (FeS) and eventually to pyrite (FeS₂). The release of adsorbed Pb due to reductive dissolution of Fe/Mn oxides during early diagenesis could be a source of Pb²⁺ in pore water. From the relatively low total organic carbon contents measured in sediments (0.46–1.28%, with an average of 0.94%) and the vertical variation of Cd²⁺ in pore water, sulfide or Fe/Mn oxides (instead of organic matter) are presumed to exert a significant influence on carrying or releasing Cd by the sediments.     

Reconstruction of Recent Sedimentary Processes in a Carbonate Platform (Gulf of Batabano,Cuba) Using Environmental Radiotracers

Recent (past 100 years) sedimentary processes in the highly dynamic Gulf of Batabano (Cuba, Caribbean Sea) were investigated through the analyses of environmental radionuclides (e.g., ²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs, ^239,240Pu, and ¹⁴C) in nine sediment cores. We evaluated the mean mass accumulation rates (MARs) and the surface mixed layers (SMLs) in each sediment core. Based on these results, three sedimentary environments were identified in the study region. In the central zone, the sediments were mainly composed of carbonate transported from the southern area and showed elevated mass accumulation rates (MAR, 0.11–0.23 g cm^?2 year^?1) and relatively deep surface mixed layers (SML, 14–16 cm). The southwestern zone was characterized by lower MAR (0.05–0.08 g cm^?2 year^?1) and thinner SML (7–8 cm). In both areas, the long sediment mixing times in the SMLs (of 45–61 years) smoothed out the sedimentary records. The coastline sedimentary environments were characterized by higher MAR (0.30–0.57 g cm^?2 year^?1) and the sedimentary records displayed clear signatures of extreme climatic events such as the intensive rains in 1999 reported for La Coloma and the hurricanes Lili and Isodore in 2002. Our study shows that the application of the ²¹⁰Pb sediment dating method in dynamic costal zones is a challenging task but still may provide important information regarding sedimentation and mixing processes in the ecosystem.     

Arsenic mobilization in alluvial soils of Punjab, North–West India under flood irrigation practices

A laboratory investigation was carried out to examine the mechanism of arsenic (As) mobilization under flooded conditions (24 and 240 h) in 18 alluvial soils of Punjab, North–West India. Total dissolved As increased from a range of 3–16 μg L^?1 (mean 9 μg L^?1) to a range of 33–1,761 μg L^?1 (mean 392 μg L^?1) with the increase in flooding period from 24 to 240 h. The amount of As mobilization varied depending upon redox potential (pe) created by flooding conditions. After 24 h of flooded conditions, pe of soil water suspension ranged from ?1.75 to 0.77 (mean ?0.24). Increasing the flooding period to 240 h, pe of soil water suspension decreased in the range of ?4.49 to ?2.74 (mean ?3.29). Pourbaix diagram identified arsenate (HAsO₄ ^2?) as predominant species in most of the alluvial soil–water suspensions under oxidized conditions, after 24 h of equilibration period, which ultimately transformed to arsenite (H₃AsO₃ ⁰) after 240 h of anaerobic condition due to more reduced status. The solid phase identified was orpiment (As₂S₃). Identification of iron and manganese species in alluvial soil water suspension by Pourbaix diagram indicated decline in both soluble Fe²⁺ and SO₄ ^2? concentration due to the formation of iron sulfide mineral phase after 240 h under anaerobic conditions. In these soils, decline in soluble Fe was also due to the precipitation of vivianite [Fe₃(PO₄)₂·8H₂O]. Elevated arsenic content and low pe value were measured in aquifers located in paddy growing fields comparative to aquifers of other sites. Large degree of variability in As concentrations was recorded in aquifers located at same sites. Thus, it is better to analyze each aquifer for their As content rather than to depends on the prediction on As content of neighbouring wells. The present investigation elucidates that flood irrigation practices in Punjab for growing paddy crop could induce the geochemical conditions favorable to mobilize arsenic from surface soils which could eventually elevate its content in the underlying shallow aquifers. Water abstracted from these aquifers by hand pumps or tube wells for drinking purposes could create hazards for local population due to loading with arsenic concentration above the safe limits. Thus, to avoid further contamination of shallow aquifers with arsenic, it is advisable to shift the flooded rice cultivation to other upland crops having lesser water requirement.     

Sorption and desorption of mercury(II) in saline and alkaline soils of Bahía Blanca, Argentina

The objective of this work was to study sorption–desorption and/or precipitation–dissolution processes of Hg(II) compounds considering an eventual contact of soils with Hg-bearing wastes. In addition, this study contributes new data about Hg(II) chemistry in alkaline systems. Saline and alkaline soils with low organic matter (<1 %) and high clay content (60–70 %) were obtained near a chlor-alkali plant. Batch techniques were used to perform the experiments using 0.1 M NaNO₃ solutions. Total Hg(II) concentrations ranged from 6.2 × 10^?8 to 6.3 × 10^?3 M. Sorption of Hg(II) was evaluated at two concentration ranges: (a) 6.2 × 10^?8 to 1.1 × 10^?4 M, and (b) 6.4 × 10^?4 to 6.3 × 10^?3 M. At low Hg(II) concentrations, adsorption occurred with a maximum sorption capacity ranging from 4 to 5 mmol/kg. At high Hg(II) concentrations, sorption–precipitation reactions occurred and maximum sorption capacity ranged from 17 to 31 mmol/kg. The distribution of Hg(II) hydrolysis products showed that Hg(OH)₂ was the predominant species under soil conditions. According to sorption experiments, X-ray diffraction and chemical speciation modelling, the presence of Hg(OH)₂ in the interlayer of the interstratified clay minerals can be proposed. Hg(OH)₂ was partially desorbed by repeated equilibrations in 0.1 M NaNO₃ solution. Desorption ranged from 0.1 to 0.9 mmol/kg for soils treated with 5.8 × 10^?5 M Hg(II), whereas 2.1–3.8 mmol/kg was desorbed from soils treated with 6.3 × 10^?3 M Hg(II). Formation of soluble Hg(II) complexes was limited by low organic matter content, whereas neutral Hg(OH)₂ was retained by adsorption on clay mineral surfaces.