The experiments about entrainment performance, as well as mass-transfer efficiency, of multidowncomer (MD) sieve tray were investigated in a 1200-mm-diameter tower with air/water system. The results indicate that the entrainment rate of MD sieve tray is lower and its behavior with increased liquid rate contradicts that of the conventional sieve tray. According to the nonmonotone variation phenomenon of entrainment at various liquid loads given by Lu and Kister, we developed the Double-Transition Point Theory to interpret the entrainment features under different contact regimes and the entrainment mechanism of MD sieve tray. By comparing the correspondence match of entrainment, pressure drop, and efficiency curves, we discussed the operating upper limit and mass-transfer efficiency. Therefore, based on the formulas of Fell’s and Hunt’s correlations, we developed the model for MD sieve tray where a semitheoretical derivation on entrained droplets in the froth regime was made.

In present study, a novel method was developed to synthesize siderite under high temperature and high pressure (SID-HTP). SID-HTP was characterized by N2 adsorption-desorption isotherms (BET), XRD, SEM, and FTIR and utilized to remove arsenic(V) (As(V)) from aqueous solution. Results showed that, under oxic condition, pH had ignorable effect on As(V) adsorption. However, adsorption capacity increased with increasing pH from 2 to 7 and remained relatively constant at higher pH until 10 under anoxic condition. Higher adsorption was obtained in the presence of oxygen, showing oxygen-enhanced As(V) adsorption on SID-HTP. In both cases, adsorption equilibrium was achieved within 12 h and adsorption process was better described by pseudo-second-order kinetic model. The equilibrium data fitted well with Langmuir isotherm model for As(V) adsorption. The maximum adsorption capacity increased with increasing temperature, which was up to 42 mg g−1 at 55 °C in the presence of oxygen. Thermodynamic study revealed that the adsorption was a spontaneous and endothermic process. The mechanism of oxygen-enhanced adsorption was mainly ascribed to the –OH on the surface of FeOOH (goethite and lepidocrocite) in the SID-HTP. It suggested that SID-HTP would be a potentially attractive adsorbent for As(V) removal.

Sediments were taken from the Hetao basin to investigate effects of different lithologies and grain size distribution on As behaviors during oscillation of redox conditions. Total digestion and sequential extraction procedures showed that Fe and Mn oxide minerals played important roles for As accumulation. The co-variation of As, CIA (Chemical Index of Alteration), ICV (Index of Compositional Variability) and Al2O3/SiO2 indicated that As were enriched in strongly chemical weathering sediments weathered from mature mudrocks. Two sediments samples with the same As contents (21.9 μg/g) but different lithologies (fine sand versus clay) were chosen for redox oscillation experiments. As released from clayey (C-1) suspension was lower than those in C-2 (grounded sand) and C-3 (sand). Under reducing conditions, the maximum As concentration in C-1 was 9.99 μg/L, however, it was 149 μg/L in C-2 and 120 μg/L in C-3. It inferred that the lithology and mineralogy play an important role in As fixation and mobilization. The grounded sands released more As in reducing conditions and sequestrated more As in oxic conditions relative to the pristine sands.

•The build-up of groundwater salinity was related to natural processes.•Non-direct evaporation and mineral/evaporite dissolution contribute to groundwater salinity.•Co-occurrence of high As and high salinity groundwater were found.•High salinity groundwater (high SO4) may enhance As hazard in reducing environment.The quality of groundwater used for human consumption and irrigation in the Hetao Basin of Inner Mongolia, China is affected by elevated salinity as well as high arsenic (As) concentrations. However, the origin of high salinity and its potential impact on As mobility in the Basin remain unclear. This study explores both issues using stable isotopic compositions and Cl/Br ratios of groundwater as well as the major ions of both groundwater and leachable salts in aquifer sediments. Limited variations in δ18O and δ2H (− 11.13 to − 8.10, − 82.23 to − 65.67) with the wide range of Total Dissolved Solid (TDS, 351–6734 mg/L) suggest less contribution of direct evaporation to major salinity in groundwater. Deuterium excess shows that non-direct evaporation (capillary evaporation, transpiration) and mineral/evaporite dissolution contribute to > 60% salinity in groundwater with TDS > 1000 mg/L. Non-direct evaporation, like capillary evaporation and transpiration, is proposed as important processes contributing to groundwater salinity based on Cl/Br ratio and halite dissolution line. The chemical weathering of Ca, Mg minerals and evaporites (Na2SO4 and CaSO4) input salts into groundwater as well. This is evidenced by the fact that lacustrine environment and the arid climate prevails in Pleistocene period. Dissolution of sulfate salts not only promotes groundwater salinity but affects As mobilization. Due to the dissolution of sulfate salts and non-direct evaporation, groundwater SO42 − prevails and its reduction may enhance As enrichment. The higher As concentrations (300–553 μg/L) are found at the stronger SO42 − reduction stage, indicating that reduction of Fe oxide minerals possibly results from HS− produced by SO42 − reduction. This would have a profound impact on As mobilization since sulfate is abundant in groundwater and sediments. The evolution of groundwater As and salinity in the future should be further studied in order to ensure sustainable utilization of water resource in this water scarce area.Download high-res image (147KB)Download full-size image

High groundwater arsenic (As) and salinity have been detected in aquifers of the Hetao Basin in Mongolia which have caused serious public health concerns. The objective of this study was to characterize the distributions of the soluble components in sediment in different lithologies and depths and to assess the relationship between soluble As in sediments and dissolved As in groundwater.One hundred and one sediment samples and 13 groundwater samples were collected from four boreholes at varied depths. In addition to total chemicals and mineralogical phases of sediments, the soluble components (including major ions and As, Fe, and Mn) in sediments and dissolved chemicals in groundwater were analyzed.Clay or silty clay had relatively higher EC values (189–805 μS cm−1) than aquifer sands (approximately 92–261 μS cm−1). The major soluble components were Na+, Ca2+, HCO3−, and SO42−, which were more variable in clay samples than fine sand samples. Soluble As concentrations ranged between 2 and 950 μS cm−1, and high contents generally occurred in clay sediments with high contents of soluble Fe and Mn. A comparison of chemicals between soluble components in sediments and dissolved species in groundwaters at matched depths showed that chemicals were preferentially partitioned into sediments at the mountain front and deep aquifers (>60 m), while partitioned into groundwater in the shallow aquifers (<60 m) of the flat plain. Arsenic was preferentially partitioned into groundwater in aquifers with relatively low dissolved SO42−.Groundwater components were mostly sourced from corresponding sediments. In clay sediments, As was desorbed from the surface sites along with other soluble components. Under reducing conditions, reduction of Fe oxides with high surface sites for As adsorption led to a weak association of As with other phases (such as carbonates), and therefore resulted in high dissolved As concentrations and low As partition between sediments and groundwater in deep aquifers.

•Only inorganic As was found in saliva samples.•Both organic and inorganic As were observed in urine, hair and nail samples.•Saliva was a potential biomarker for chronic As exposure.•The major pathway of As intake was from grains, fruits and vegetables.•Arsenic intake from crops had potential risk to residents' health.Seventy saliva samples, seventy urine samples, seventy nail samples, seventy hair samples, eight drinking water samples and ninety-three crop samples were collected from four villages of the Hetao Basin in Inner Mongolia to determine arsenic (As) exposure biomarkers and evaluate relationship between As uptake and human health risk. Trivalent As (As(III)), pentavalent As (As(V)), dimethylarsinic acid (DMA), arsenobetaine (AsB) and monomethylarsonic acid (MMA) were found in all urine samples. Only As(III) and As(V) were detected in saliva samples. In nail and hair samples, DMA, MMA, As(III) and As(V) were observed. Based on total As contents in crops and drinking water, the local residents' daily intake of total arsenic (TDIAs), the hazard quotient (HQ), and the cancer risk (R) were assessed. Male, older and cases of skin lesion participants generally had higher As contents in saliva, urine, nail and hair samples in relative to others. Salivary, urinary, nail and hair As were not significantly affected by body mass index (BMI) and smoking. Good correlations were observed between TDIAs and salivary, urinary, nail and hair As, showing that saliva, urine, nail and hair samples can be used as biomarkers of As exposure. Individually, levels of arsenicosis were positively correlated with TDIAs. The relationship between TDIAs and prevalence of arsenicosis concluded that, although As levels in crops and drinking water did not exceed national standards, they still pose a potential threat to human health. It was suggested that the maximum permissible levels of crop As and drinking water As should be re-evaluated for protecting human health.Download high-res image (130KB)Download full-size image

An anaerobic nitrate-reducing Fe(II)-oxidizing bacterium, Pseudogulbenkiania sp. strain 2002, was used to investigate As immobilization by biogenic Fe oxyhydroxides under different initial molar ratios of Fe/As in solutions. Results showed that Fe(II) was effectively oxidized, mainly forming lepidocrocite, which immobilized more As(III) than As(V) without changing the redox state of As. When the initial Fe/As ratios were kept constant, higher initial Fe(II) concentrations immobilized more As with higher Asimmobilized/Feprecipitated in biogenic lepidocrocite. EXAFS analysis showed that variations of initial Fe(II) concentrations did not change the As–Fe complexes (bidentate binuclear complexes (2C)) with a fixed As(III) or As(V) initial concentration of 13.3 μM. On the other hand, variations in initial As concentrations but fixed Fe(II) initial concentration induced the co-occurrence of bidentate binuclear and bidentate mononuclear complexes (2E) and bidentate binuclear and monodentate mononuclear complexes (1V) for As(III) and As(V)-treated series, respectively. The coexistence of 2C and 2E complexes (or 2C and 1V complexes) could contribute to higher As removal in experimental series with higher initial Fe(II) concentrations at the same initial Fe/As ratio. Simultaneous removal of soluble As and nitrate by anaerobic nitrate-reducing Fe(II)-oxidizing bacteria provides a feasible approach for in situ remediation of As-nitrate cocontaminated groundwater.

The role of sulfur cycling in arsenic behavior under reducing conditions is not well-understood in previous investigations. This study provides observations of sulfur and oxygen isotope fractionation in sulfate and evaluation of sulfur cycling-related biogeochemical processes controlling dissolved arsenic groundwater concentrations using multiple isotope approaches. As a typical basin hosting high arsenic groundwater, the western Hetao basin was selected as the study area. Results showed that, along the groundwater flow paths, groundwater δ34SSO4, δ18OSO4, and δ13CDOC increased with increases in arsenic, dissolved iron, hydrogen sulfide and ammonium concentrations, while δ13CDIC decreased with decreasing Eh and sulfate/chloride. Bacterial sulfate reduction (BSR) was responsible for many of these observed changes. The δ34SSO4 indicated that dissolved sulfate was mainly sourced from oxidative weathering of sulfides in upgradient alluvial fans. The high oxygen–sulfur isotope fractionation ratio (0.60) may result from both slow sulfate reduction rates and bacterial disproportionation of sulfur intermediates (BDSI). Data indicate that both the sulfide produced by BSR and the overall BDSI reduce arsenic-bearing iron(III) oxyhydroxides, leading to the release of arsenic into groundwater. These results suggest that sulfur-related biogeochemical processes are important in mobilizing arsenic in aquifer systems.

Effects of pH, As species, and Fe/Mn minerals on the fractions of adsorbed As in aquifer sediments were evaluated. Kinetic data showed that As adsorption was controlled by diffusion through the external film. Isothermal data of both As(III) and As(V) fitted the Langmuir isotherm well, revealing a monolayer adsorption process. Sequential extraction demonstrated that water-soluble As and non-specifically sorbed As were the major fractions of adsorbed As. Assessing the relationship between the Freundlich KF and the increases in the amounts of As fractions showed that the pH played a key role in weakly adsorbed As, especially water-soluble As. Although inorganic As species converted each other during the adsorption processes, more non-specifically sorbed As was adsorbed in As(V)-treated sediment than in As(III)-treated sediment, showing that the electrostatic selectivity controlled the non-specific adsorption. Additionally, specifically sorbed As and As associated with the amorphous phases were predominated by Fe/Mn minerals, especially Fe(III) (hydr)oxides. These results suggested that pH, As species, and Fe/Mn minerals would regulate the As fractions in aquifer sediments, and therefore control As cycling in aquifer systems.

An exponential function shortcut calculation (EFSC) method is proposed for the estimation of the number of theoretical plates, NT, in distillation units. We set up the new concept of the thermal state equation, β-line equation, and define the cut ratio of the q line, δ, as an independent variable to interpret the stage separating capacity. The EFSC model is established on the basic data of NT calculated by the previously presented exponential function rigorous calculation (EFRC) method. The data sources of the EFSC method are much greater than those of the Gilliland correlation. The curves of the EFSC model go through the points (X = 0, Y = 1) and (X = 1, Y = 0), whose physical significance fully coincides with the characteristics of R ∼ NT. Validation of the EFSC method indicates that its accuracy is close to that of the plate-to-plate calculations and the EFRC method and higher than that of the Gilliland correlation.

Indigenous microbes play crucial roles in arsenic mobilization in high arsenic groundwater systems. Databases concerning the presence and the activity of microbial communities are very useful in evaluating the potential of microbe-mediated arsenic mobilization in shallow aquifers hosting high arsenic groundwater. This study characterized microbial communities in groundwaters at different depths with different arsenic concentrations by DGGE and one sediment by 16S rRNA gene clone library, and evaluated arsenic mobilization in microcosm batches with the presence of indigenous bacteria. DGGE fingerprints revealed that the community structure changed substantially with depth at the same location. It indicated that a relatively higher bacterial diversity was present in the groundwater sample with lower arsenic concentration. Sequence analysis of 16S rRNA gene demonstrated that the sediment bacteria mainly belonged to Pseudomonas, Dietzia and Rhodococcus, which have been widely found in aquifer systems. Additionally, NO3−-reducing bacteria Pseudomonas sp. was the largest group, followed by Fe(III)-reducing, SO42−-reducing and As(V)-reducing bacteria in the sediment sample. These anaerobic bacteria used the specific oxyanions as electron acceptor and played a significant role in reductive dissolution of Fe oxide minerals, reduction of As(V), and release of arsenic from sediments into groundwater. Microcosm experiments, using intact aquifer sediments, showed that arsenic release and Fe(III) reduction were microbially mediated in the presence of indigenous bacteria. High arsenic concentration was also observed in the batch without amendment of organic carbon, demonstrating that the natural organic matter in sediments was the potential electron donor for microbially mediated arsenic release from these aquifer sediments.

Synthesized siderite was used to remove As(III) and As(V) from water solutions under anoxic conditions and oxic conditions. Results showed that As adsorption on synthetic siderite under anoxic conditions was around 10 mg/g calculated with Langmuir isotherm. However, the calculated As adsorption on synthetic siderite under oxic conditions ranged between 115 and 121 mg/g, which was around 11 times higher than that under anoxic conditions. It was found that 75% siderite was transformed into goethite during oxic adsorption. However, synthetic goethite had lower As adsorption capacity than siderite under oxic conditions, although its adsorption capacity was a little higher than siderite under anoxic conditions. It suggested that the coexistence of goethite and siderite bimineral during mineral transformation probably contributed to the robust adsorption capacity of siderite under oxic conditions. Results of extended X-ray absorption fine structure (EXAF) spectroscopy indicated both As(III) and As(V) formed inner-sphere complexes on the surface of As-treated solid regardless of substrates, including the bidentate binuclear corner-sharing (2C) complexes and the monodentate mononuclear corner-sharing (1V) complexes. Monodenate (1V) and bidentate (2C) complexes would be related to high As adsorption capacity of siderite under oxic conditions. It showed that more Fe atoms were coordinated with As atom in the monodentate complexes and the bidentate complexes of As(V)/As(III)-treated siderite under oxic conditions, in comparison with As(V)/As(III)-treated siderite under anoxic conditions and As(V)/As(III)-treated goethite. Calcinations of natural siderite resulting in the coexistence of goethite and siderite greatly increased As adsorption on the solid, which confirmed that the coexistence of bimineral during mineral transformation from siderite to goethite greatly enhanced As adsorption capacity of siderite adsorbent. The observation can be applied for modification of natural siderite for As removal from high As waters.

Patchy distribution of high As groundwater has normally been observed in As-affected areas. Spatial and temporal evolutions help in better understanding mechanisms of As mobilization and in developing effective strategies for ensuring drinking water safety. Four multilevel samplers were installed approximately along the groundwater flow path to investigate spatial and temporal variations in groundwater As in the Hetao basin, Inner Mongolia. Both water chemistry and groundwater level were monitored for about two years. Groundwater As concentration generally showed increasing trends, and Eh values showed decreasing trends along the flow path, indicating that As was mobilized via reductive dissolution of Fe oxides. However, in evaporation discharge area, shallow groundwater As was generally lower than those upstream and downstream. In addition to evaporation, siderite and pyrite precipitations controlled groundwater As concentrations. The negative correlations between As concentration and SIpyrite (or SIsiderite) implied that siderite and pyrite precipitations would scavenge groundwater As and lower As concentration. Temporal variation showed different trends in different locations. It may reflect replenishment of high/low As groundwater for the increase/decrease in As concentrations, resulting from water level fluctuation. The increase trends in groundwater As concentrations at depth around 15 m in the discharge areas would result from the increase in the recharge of underlying groundwater (20 m) with high As concentration due to enhanced evaporation in the seasons with high water levels. The investigation suggested that monitoring of groundwater As should be routinely carried out to ensure the drinking water safety in the As-affected areas.

The study has investigated the feasibility of using synthetic siderite for F− removal from aqueous solution. Batch experiments were performed to test effects of adsorbent dosage, contact time, initial F− concentration, temperature, solution pH, and coexisting anions on F− removal. Results show that the kinetic rate of F− adsorption was high in the first 2 h, and thereafter significantly decreased. The kinetic data was better fitted to the pseudo-second order kinetic model than the pseudo-first order kinetic model. In comparison with Langmuir isotherm, both Freundlich and Redlich–Peterson isotherms better described the adsorption process, which indicates that the multilayer adsorption should be involved in the process of F− removal. Thermodynamic study manifests that F− adsorption on synthetic siderite was spontaneous and exothermic in nature. The synthetic siderite had high adsorption capacity for F− removal, which was up to 1.775 mg/g in the batch with an adsorbent dosage of 5 g/L and an initial F− concentration of 20 mg/L at 25 °C. The adsorption was relatively independent on solution pH between 4 and 9. The presence of Cl− and NO3− had less impact on F− adsorption, while PO43− significantly affected F− removal from aqueous solution. Results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) suggest that the high adsorption capacity possibly arose from both coprecipitation of ferric hydroxide with F− and adsorption of F− on the fresh goethite.Synthetic siderite was employed as adsorbent for fluoride removal from aqueous solution by batch method. The mineral composition and morphological analysis of the pristine and used adsorbents was determined by X-ray diffraction analysis and scanning electron microscopy respectively to discuss the mechanism of the adsorption.

•Modified granular natural siderite (MGNS) was fabricated and fully characterized.•MGNS had higher adsorption rate and capacity for As(III) relative to natural siderite.•MGNS was a pH-insensitive, highly selective adsorbent for As(III) removal.•Arsenic was effectively removed from real high-As groundwater by MGNS.•A fast oxidation from As(III) to As(V) would occur after As(III) adsorption on MGNS.Although natural siderite has been investigated to remove both As(III) and As(V), it has relatively low adsorption rate and capacity. It is crucial to enhance its adsorption characteristics for As removal prior to being used in practical application. Modified granular natural siderite (MGNS) was fabricated through addition of organic binder, extrusion granulation and calcination, and evaluated for adsorption characteristics by means of batch and column tests. Results showed that MGNS had higher adsorption rate and capacity for As(III) in comparison with natural siderite. Arsenic(III) adsorption achieved equilibrium at 24 h, with adsorption capacity of 9.43 mg/g estimated from Langmuir isotherm at 25 °C. Column tests showed that there was less difference in total As loads in MGNS-packed filters for As(III)-spiked deionized water, As(III)-spiked tap water, and real-world high-As groundwater. The coexistence of anions had no significant effect on As adsorption in both batch and column experiments. Results of XRD, SEM and BET analysis indicated that MGNS, as an Fe(II)/(III) hybrid system, had a much larger specific surface area relative to the pristine natural siderite due to massive spherical aggregates attaching to the siderite matrix. XANES spectra showed that As(V) was the major species in the adsorbent after As(III) adsorption. Its proportion in total As slightly increased with the increase in contact time. Adsorption and heterogeneous oxidation of As(III) were believed to be the main mechanisms of As(III) removal by MGNS. This study suggested that MGNS is a potential adsorbent for effectively removing As from As-contaminated groundwater in filter application.

High arsenic water has been a global focus of both scientists and water supply managers because of its serious adverse impact on human health and wide distribution in the world. Processes of redox, sorption, precipitation, and dissolution release arsenic in both natural systems and in environments intensely modified by human activities. In natural systems, groundwater arsenic is controlled by lithologic geochemistry, sedimentation conditions, hydrogeologic setting and groundwater chemistry. However, in the intensely human-affected systems (such as mining and tilling areas), arsenic mobilization is dependent on the composition of the primary materials, treatment methods, storage design, and local climate. Well-designed experimental systems aid in characterizing sorption, precipitation, and redox processes associated with arsenic dynamics during water-rock interaction. Continued investigations of field sites will further refine understanding of the processes favoring arsenic mobility in the range of natural and man-made systems. The combination of field and experimental studies will lead to better understanding of arsenic cycling in all systems and sustainable management of water resources in arsenic-affected areas.

Twenty-nine wells were selected for groundwater sampling in the town of Shahai, in the Hetao basin, Inner Mongolia. Four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2.5–20 m. Results show that groundwater As exhibits a large spatial variation, ranging between 0.96 and 720 μg/L, with 71% of samples exceeding the WHO drinking water guideline value (10 μg/L). Fluoride concentrations range between 0.30 and 2.57 mg/L. There is no significant correlation between As and F− concentrations. Greater As concentrations were found with increasing well depth. However, F− concentrations do not show a consistent trend with depth. Groundwater with relatively low Eh has high As concentrations, indicating that the reducing environment is the major factor controlling As mobilization. Low As concentrations (<10 μg/L) are found in groundwater at depths less than 10 m. High groundwater As concentration is associated with aquifers that have thick overlying clay layers. The clay layers, mainly occurring at depths <10 m, have low permeability and high organic C content. These strata restrict diffusion of atmospheric O2 into the aquifers, and lead to reducing conditions that favor As release. Sediment composition is an additional factor in determining dissolved As concentrations. In aquifers composed of yellowish-brown fine sands at depths around 10 m, groundwater generally has low As concentrations which is attributed to the high As adsorption capacity of the yellow–brown Fe oxyhydroxide coatings. Fluoride concentration is positively correlated with pH and negatively correlated with Ca2+ concentration. All groundwater samples are over-saturated with respect to calcite and under-saturated with respect to fluorite. Dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F− adsorption–desorption are likely controlling the concentration of F− in groundwater.Highlights► Groundwater As and F− concentration show a large spatial variation. ► Within depths <30 m, As concentrations increase with increasing depth. ► Fluoride concentrations do not show a consistent trend with depth. ► Desorption, evaporation and precipitation are controlling F− concentration. ► Reducing environment is the major factor controlling As mobilization.

Due to the importance of colloids in regulating element transport and mobility in aquifers, As distribution in the colloidal fraction needs to be identified in high As groundwaters. Groundwater samples were filtered in the field through a progressively decreasing pore size (0.45 μm, 100, 30, 10, 5 kDa) using a filtration technique under a N2 atmosphere. Major and trace elements and organic C (OC) were measured in (ultra)filtrates. The studied groundwater samples have typical physio-chemical characteristics of the basin waters. Declines in concentrations of alkali (Na, K), alkaline-earth (Mg, Ca, Sr, Ba) elements, Mo, Si and Se during ultrafiltration are smaller relative to other elements. Arsenic, Cu, Cr, U and V are generally about 30% lower in 5 kDa ultrafiltrates in comparison with 0.45 μm filtrates. Around 50% of Fe, OC and Al are bound to colloids with grain size between 5 kDa and 0.45 μm. Two types of colloids, including large-size Fe colloids and small-size organic colloids, have been identified. Results indicate that As would be more likely to be associated with small-size organic colloids than Fe colloids. SEM images and EDS analysis and synchrotron XRF analyses confirm the association of As with NOM with molecular weights of 5–10 kDa. The better correlation between As(V) and OC in the 5–10 kDa fraction indicates that the small-size organic colloids have a greater affinity for As(V) than As(III). Arsenic associated with organic complexes may not be immobilized by adsorption, and, therefore, easily transported in the aquifer. Thus, the presence of As-containing colloidal complexes in high As groundwaters must be considered in the modeling of As transport in the aquifers.Research highlights► Both Fe colloids and organic colloids are present in high As groundwaters. ► Arsenic is more likely to be associated with small-size organic colloids. ► Iron colloids do not significantly affect As distribution and transport.

Although high As groundwater has been observed in shallow groundwater of the Hetao basin, little is known about As distribution in deep groundwater. Quantitative investigations into relationships among chemical properties and among samples in different areas were carried out. Ninety groundwater samples were collected from deep aquifers of the northwest of the basin. Twenty-two physicochemical parameters were obtained for each sample. Statistical methods, including principal component analysis (PCA) and hierarchical cluster analysis (HCA), were used to analyze those data. Results show that As species were highly correlated with Fe species, NH4-N and pH. Furthermore, result of PCA indicates that high As groundwater was controlled by geological, reducing and oxic factors. The samples are classified into three clusters in HCA, which corresponded to the alluvial fans, the distal zone and the flat plain. Moreover, the combination of PCA with HCA shows the different dominant factors in different areas. In the alluvial fans, groundwater is influenced by oxic factors, and low As concentrations are observed. In the distal zone, groundwater is under suboxic conditions, which is dominated by reducing and geological factors. In the flat plain, groundwater is characterized by reducing conditions and high As concentrations, which is dominated by the reducing factor. This investigations indicate that deep groundwater in the alluvial fans mostly contains low As concentrations but high NO3 and U concentrations, and needs to be carefully checked prior to being used for drinking water sources.

This study investigated the potential of the Fe(II)-oxidizing bacteria in removing arsenic in aqueous environment. The bacteria were isolated from the batch of tap water and rusty iron wires, and were acclimated to culture media amended with arsenic concentrations, gradually increasing from 100 μg L−1 to 100 mg L−1. Acclimated bacteria with enhanced arsenic tolerance were used to remove arsenic from the aqueous solution. These bacteria belonged to Pseudomonas species according to 16S rRNA gene sequences. Extracellular enzymes produced by these bacteria played important roles in microbial Fe(II) oxidization and Fe oxide precipitation. Moreover, these bacteria survived and propagated in high arsenic condition (100 mg L−1 As). However, after As(III/V) acclimation, morphological characteristics of the bacteria showed some changes, e.g., shrinking of long bacillus. XRD (X-ray diffraction) patterns indicated that Fe oxide precipitations by Fe(II)-oxidizing bacteria in Fe-rich culture medium were poorly-crystallized ferrihydrites. Adsorption on the biogenic ferrihydrites greatly contributed to high arsenic removal efficiency of Fe(II)-oxidizing bacteria.

•Groundwater As is predominantly regulated by active As forms in sediments.•The scale of geochemical factors is vital in better understanding relative contribution to groundwater As concentrations.•Groundwater flushing decreases As concentration at the multi-basin scale.•Redox conditions are the key factor controlling groundwater As at the scale of contaminated sites.High As groundwater has been widely found in inland basins of P.R. China, which has posed a serious health risk to the local residents. Although these inland basins experience the same arid to semiarid climate and are filled with Quaternary sediments, As concentrations show big variations. Three inlands basins have been investigated to characterize hydraulic conductivities and geochemistry of the groundwater and sediments, and evaluate their controls on dissolved As concentrations. Dissolved As concentrations ranged between <0.1 and 105 μg/L (average 27.8 μg/L), between <0.1 and 338 μg/L (average 94.0 μg/L), and between 0.33 and 857 μg/L (average 130 μg/L) in the Yinchuan basin (YC), the Songnen basin (SN), and the Hetao basin (HT), respectively. In the YC, although Fe and Mn concentrations are the highest, groundwater has the lowest As concentration. Ionically bound As fraction and strongly adsorbed As fraction of sediments are the highest in the HT and the lowest in the YC basin. Groundwater As is predominantly regulated by these mobilizable As forms in sediments. However, the predominant factors controlling groundwater As are dependent on the scale of the study area. At the average multi-basin scale, groundwater flushing evidently decreases groundwater As concentrations. At the individual scale of the sampling site, due to the similar groundwater flow rate, redox conditions are the key factor controlling groundwater As, with more As partitioned into groundwater of lower Eh values. Variations in these factors controlling groundwater As suggest that safe wells can be expected in the basins with high groundwater flow rates and at sites with higher groundwater Eh values.Download high-res image (215KB)Download full-size image

•Spatial distribution of groundwater As was investigated in Xinjiang.•High As groundwater was found in the north, south and east of Xinjiang.•Arsenic concentrations generally increased with the increase in well depths.•Desorption and/or reductive dissolution led to As enrichment in groundwater.•Model-based prediction of groundwater As would deviate from real data.Although potential contamination of groundwater As is expected to occur in Xinjiang, P.R. China, few data are available for the regional distribution of groundwater As. In this study, the spatial distribution of groundwater As was investigated in the Chepaizi (CPZ-N) and Shihezi (SHZ-N) areas of northern Xinjiang, the Balikun-Yiwu Basin (BY-E) in eastern Xinjiang, and the Tarim (TRM-S) and Yanqi (YQ-S) basins in southern Xinjiang. Arsenic concentrations greater than 10 μg/L were found in 12% of analyzed groundwaters. All groundwater samples collected in CPZ-N had As concentrations greater than 10 μg/L (25–185 μg/L), 30% in SHZ-N (<0.25–49 μg/L), 2.7% in BY-E, and 6.1% in TRM-S and YQ-S. No high As groundwater (As >10 μg/L) was found in the eastern and southern TRM-S and YQ-S. Distribution of groundwater As showed a tremendous spatial variability, which greatly varied over a short distance horizontally. Arsenic concentration generally increased with increasing sampling depth. The spatial distribution of groundwater As would be regulated by As source and hydrogeochemical processes. Higher pH and/or lower ORP values were generally observed in high As groundwater (>10 μg/L) in comparison with low As groundwater (<10 μg/L). Arsenic mobility in SHZ-N and CPZ-N may result from As desorption under relatively high pH conditions, and more tentatively from reductive dissolution of Fe(III) oxides in BY-E and TRM-S. However, detailed mechanisms of As mobilization in these regions need further investigation.

•High As groundwaters occur in inland basins and river deltas in China.•Twenty provinces have found high As groundwaters among 34 provinces.•Aquifers are characterized by reducing conditions and low groundwater flow rates.•High As groundwater has high Fe, Mn concentrations and low NO3-, SO42- concentrations.•Both redox processes and desorption processes result in As mobilization.China is a typical high-As region, where 20 provinces have high As groundwaters among 34 provinces. These groundwaters usually occur in both arid–semiarid inland basins and river deltas. In the inland basins, mainly distributed in the northwest of China, shallow groundwaters usually have high As concentrations in alluvial lacustrine or lacustrine sediment aquifers, while high As groundwater mainly occurs in fluvial–marine sedimentary aquifers in the river deltas, which have been affected by transgression. In both the inland basins and the river deltas, high As groundwaters, mainly occurring in reducing conditions, are characterized by high Fe and Mn concentrations, high pH and HCO3- concentration, and relatively low NO3- and SO42- concentrations. Although As contents are well correlated to Fe/Mn contents in the aquifer sediments, groundwater As concentrations are generally independent of sediment As contents. Redox processes, microbe-related reduction, and desorption processes are the major geochemical processes for As enrichment in groundwaters. In reducing conditions, both reductive dissolution of Fe oxides and reductive desorption of As are believed to result in As mobilization, which would be catalyzed by indigenous microbes. Although decomposition of the low-molecular weight organic matter during microbe metabolization would also release the colloid-bound As into groundwater, the cycling of colloidal As still needs to be further investigated during redox processes. Besides, high pH and high HCO3- lead to As desorption from adsorption sites in the aquifer systems. However, the contribution of competitive desorption to high As concentrations is still unknown and remains to be discovered, relative to reductive dissolution of Fe oxides, especially in the inland basins.

Temporal variations in groundwater As concentration are crucial for understanding the mechanisms of As cycling and developing effective strategies for sustainable usage of low As groundwater in As-affected areas. Little is known about temporal variations in As concentration of shallow groundwater from the Hetao basin. Groundwater samples were taken once each year in July/August from 2006 to 2010. Another sampling campaign was carried out in November 2006. Groundwater tables were monitored as well. Results showed that water levels were higher during December–April than during May–September in the front of the alluvial fans where irrigation relied mainly on groundwater. In contrast, the highest water levels were observed in May–June and in November in the flat plain region where the diverted Yellow River water was utilized for irrigation. Concentrations of Na+, Cl−, and HCO3− were relatively constant over 4 years, although most wells showed slight decreasing trends in concentrations of Ca2+ and Mg2+. Although concentrations of As in most wells were lower in 2006 than in 2007, there were no significant changes in As concentration between 2006 and 2010 at p = 0.05. Shallow groundwaters sampled in November from the flat plain region with surface water irrigation showed generally higher As concentrations than in July. This was caused by more reducing conditions due to less oxygen dispersing into the aquifers when irrigation water flooded the soil and perhaps the un-saturated zone, as evidenced by the lower redox potential of shallow groundwater in November. Results of μ-XRF showed that As contents were well correlated with Fe contents in the sediment samples from the shallow aquifers. The most plausible explanation for the decoupling between temporal variation in As concentration and in Fe concentration was expected to be the reductive desorption of As(V), due to the analogical variation trends between As(III) and total As.Highlights► Groundwater level temporally fluctuated during 4 years. ► Slight increasing trends in As concentration were observed over 4 years. ► Iron concentrations generally kept relatively constant. ► Increasing As concentrations would be associated with increasing water levels. ► Decoupling of temporal variation in As with Fe should be reductive desorption of As.

•In situ experiments were used to delineate As mobilization from Fe(III) oxides.•Arsenic desorption occurred from ferrihydrite, goethite and hematite.•Arsenic was released from reductive dissolution of ferrihydrite.•Transformation of ferrihydrite to secondary Fe minerals favored As mobilization.•Goethite and hematite would be the major oxides controlling groundwater As content.Although reductive dissolution of Fe(III) oxides has been well accepted for As mobilization in alluvial aquifers, the key factors controlling this process are poorly understood. Arsenic(V)-adsorbing ferrihydrite, goethite and hematite were used to examine in-situ mobilization and transformation of adsorbed As(V) and Fe(III) oxides. In the Hetao basin, seven wells with wide ranges of groundwater As were selected to host As(V)-Fe(III) oxides sand. During 80 d experiments, As was firstly desorbed and then released via reductive dissolution of iron oxide from ferrihydrite, while only desorption was observed from goethite/hematite sand. Desorbed As was predominantly controlled by groundwater HCO3− and DOC, while reductive dissolution-related As release was mainly regulated by ORP values, DOC and Fe(II) concentrations. Mineral transformation from ferrihydrite to lepidocrocite and goethite/or mackinawite would also contribute to As release. Arsenic species was transformed from As(V) to As(III) on ferrihydrite, but remained unchanged on goethite and hematite. Arsenic partition between As-Fe(III) oxide sand and real groundwater ranged between 0.012 and 0.102 L/g. Kd-sand between As-goethite sand/As-hematite sand and groundwater fell within the ranges observed between sediments and groundwater. This study suggests that As desorption, reductive dissolution and mineral transformation of ferrihydrite would be the major processes controlling As mobility.

Four multilevel samplers were installed in both As-affected areas and low-As areas for long-term monitoring of chemical variations of shallow groundwater (< 30 m in depth). Results showed slight increasing trends in As concentrations in groundwaters with high As concentration (> 50 μg/L), and in groundwaters at 15 m depth in low-As areas. Variations in As concentrations were in line with those of Fe(II) in low-As areas (< 50 μg/L), while incompatible variations were generally observed in groundwaters from As-affected areas. This indicated that authigenic siderite and pyrite immobilized As in groundwaters with high As concentration (>200 μg/L), being released from aquifer sediments via reductive dissolution of Fe oxides.

Drinking water in the Hetao basin relies significantly on deep groundwater. Little is known about the occurrence of As and its distribution in deep groundwaters of the Hetao Basin. This study observed that this deep groundwater is affected by the presence of As in a nearby mountainous area. Arsenic concentrations gradually increase from the alluvial fans to the flat plain of the basin. Four hydrogeochemical zones were distinguished based on hydrogeological conditions, together with the distribution of redox-sensitive elements and their relationship with As concentrations. Although As mobilization is fundamentally controlled by the reductive dissolution of Fe-bearing minerals, reduction of SO42-also affects groundwater As concentrations. If the supply of SO42- is sufficient, further reduction of SO42-may lead to a decrease of As levels through co-precipitation with pyrite. This information is helpful for choosing localities for safe drinking water wells in mountain-edge alluvial fans.

•Increasing Ce anomaly along a flow path would be related to changing redox conditions.•REE geochemistry is associated with precipitation and dissolution of Fe/Mn oxides.•Europium anomaly would result from weathering of plagioclase in aquifer sediments.•REEs are useful as tracers of groundwater flow conditions and geochemical processes.Rare earth element (REE) geochemistry is a useful tool in delineating hydrogeochemical processes and tracing solute transport, which can be used to reveal groundwater chemical evolution in the complexed groundwater systems of the North China Plain (NCP). Groundwaters and sediments were collected approximately along a flow path in shallow and deep aquifers of the NCP to investigate REE geochemistry as a function of distance from the recharge zone. Groundwater REE concentrations are relatively low, with ranges from 81.2 to 163.6 ng/L in shallow groundwaters, and from 65.2 to 133.7 ng/L in deep groundwaters. Speciation calculation suggests that dissolved REEs mainly occur as dicarbonato (Ln(CO3)2−) and carbonato (LnCO3+) complexes. Although along the flow path groundwater REE concentrations do not vary substantially, relatively lower HREEs are observed in central plain (Zone II) compared to recharge area (Zone I) and discharge plain (Zone III). Shale-normalized REE patterns are characterized by different degrees of enrichment in the HREEs, as indicated by the variation in average (Er/Nd)NASC value. The similar REE compositions and shale-normalized REE patterns of shallow and deep groundwaters demonstrate that interactions of groundwaters between shallow and deep aquifers possibly occur, which is likely due to the long-term groundwater over-exploration. Cerium anomalies (Ce/Ce∗ = CeNASC/(LaNASC × PrNASC)0.5) generally increase from Zone I, through Zone II, to Zone III, with trends from 0.79 to 3.58, and from 1.22 to 2.43 in shallow groundwaters and deep groundwaters, respectively. This is consistent with the variations in oxidation–reduction potential and redox sensitive components (i.e., dissolved Fe, Mn, NO3− and As concentrations) along the flow path. Positive Ce anomaly and redox indicators suggest that redox conditions progressively evolve from oxic to moderate anaerobic in the direction of groundwater flow. In the recharge zone (Zone I), groundwater low positive Ce anomalies are likely due to partially oxidative scavenging of Ce(III) to Ce(IV), and HREE enrichment would result from preferential scavenging of the LREEs relative to the HREEs during Fe/Mn oxides/oxyhydroxides precipitations, which is well supported by the low concentrations of dissolved Fe and Mn. In the down-gradient (Zone II and Zone III), reductive dissolution of Fe/Mn oxides/oxyhydroxides increases positive Ce anomalies along the flow path. The positive correlations between (Er/Nd)NASC values and dissolved Fe/Mn concentrations suggest that reductive dissolution of Fe/Mn oxides/oxyhydroxides, as well as readsorption, are the geochemical controls on groundwater REE fractionation patterns. Groundwaters mostly have positive Eu anomalies (Eu/Eu∗ = EuNASC/(SmNASC × GdNASC)0.5), which would be the result of chemical weathering of feldspars (e.g., plagioclase) detected in aquifer sediments by XRD technique.