Junxia Li

Iodine mobilization in groundwater system at Datong basin, China: Evidence from hydrochemistry and fluorescence characteristics

Characterizing the speciation of iodine in groundwater is essential for understanding its hydrogeochemical behavior in aquifer systems. To quantify the variations in iodine speciation and assess factors controlling the distribution and transformation of iodine, 82 groundwater samples and 1 rain water were collected from the Datong basin, northern China in this study. Factor analysis (FA) and excitation emission matrix with parallel factor analysis (EEM-PARAFAC) were used to clarify the potential relationships among iodine species and other hydrochemical parameters. The iodine concentrations of groundwater range from 6.23 to 1380 μg L(-1) with 47% of samples exceeding its drinking water level of 150 μg L(-1) as recommended by the Chinese government. 57% of samples have ratios of iodate to total iodine greater than 60%, while iodide as the major species in 22% of the samples. Significant amounts of organic iodine with concentrations higher than 100 μg L(-1) were observed in 9 groundwater samples. Redox conditions of groundwater system strongly affect iodine concentration and speciation of inorganic iodine in groundwater, and extremely reducing condition restricts the iodine release from sediments into groundwater. The results of FA show that iodine mobilization in groundwater is related to the nature of dissolved organic matter. EEM-PARAFAC model demonstrates the dominance of terrestrial DOM sources and the presence of microbial activities in groundwater system of the Datong basin. It is proposed that degradation of organic matter and reductive dissolution of iron oxyhydroxides are major hydrogeochemical processes responsible for the mobilization of iodine release and the genesis of organic iodine.

Cl/Br ratios and chlorine isotope evidences for groundwater salinization and its impact on groundwater arsenic, fluoride and iodine enrichment in the Datong basin, China.

In order to identify the salinization processes and its impact on arsenic, fluoride and iodine enrichment in groundwater, hydrogeochemical and environmental isotope studies have been conducted on groundwater from the Datong basin, China. The total dissolved solid (TDS) concentrations in groundwater ranged from 451 to 8250 mg/L, and 41% of all samples were identified as moderately saline groundwater with TDS of 3000-10,000 mg/L. The results of groundwater Cl concentrations, Cl/Br molar ratio and Cl isotope composition suggest that three processes including water-rock interaction, surface saline soil flushing, and evapotranspiration result in the groundwater salinization in the study area. The relatively higher Cl/Br molar ratio in groundwater from multiple screening wells indicates the contribution of halite dissolution from saline soil flushed by vertical infiltration to the groundwater salinization. However, the results of groundwater Cl/Br molar ratio model indicate that the effect of saline soil flushing practice is limited to account for the observed salinity variation in groundwater. The plots of groundwater Cl vs. Cl/Br molar ratio, and Cl vs δ(37)Cl perform the dominant effects of evapotranspiration on groundwater salinization. Inverse geochemical modeling results show that evapotranspiration may cause approximately 66% loss of shallow groundwater to account for the observed hydrochemical pattern. Due to the redox condition fluctuation induced by irrigation activities and evapotranspiration, groundwater salinization processes have negative effects on groundwater arsenic enrichment. For groundwater iodine and fluoride enrichment, evapotranspiration partly accounts for their elevation in slightly saline water. However, too strong evapotranspiration would restrict groundwater fluoride concentration due to the limitation of fluorite solubility.

Hydrogeochemistry of co-occurring geogenic arsenic, fluoride and iodine in groundwater at Datong Basin, northern China.

Abnormal levels of co-occurring arsenic (As), fluorine (F) and iodine (I) in groundwater at Datong Basin, northern China are geochemically unique. Hydrochemical, (18)O and (2)H characteristics of groundwater were analyzed to elucidate their mobilization processes. Aqueous As, F and I ranged from 5.6 to 2680 μg/L, 0.40 to 3.32 mg/L and 10.1 to 186 μg/L, respectively. High As, F and I groundwater was characterized by moderately alkaline, high HCO3(-), Fe(II), HS(-) and DOC concentrations with H3AsO3, F(-) and I(-) as the dominant species. The plots of δ(18)O values and Cl/Br ratios versus Cl(-) concentration demonstrate build-up of more oxidizing conditions and precipitation of carbonate minerals induced by vertical recharge and intensive evaporation facilitate As retention to Fe (hydr) oxides, but enhance F and I mobilization from host minerals. Under reducing conditions, As and I can be simultaneously released via reductive dissolution of Fe (hydr) oxides and reduction of As(V) and I(V) while F migration may be retarded due to effects of dissolution-precipitation equilibria between carbonate minerals and fluorite. With the prevalence of sulfate-reducing condition and lowering of HCO3(-) concentration, As and I may be sequestered by Fe(II) sulfides and F is retained to fluorite and on clay mineral surfaces.

In situ treatment of arsenic contaminated groundwater by aquifer iron coating: Experimental Study

In situ arsenic removal from groundwater by an aquifer iron coating method has great potential to be a cost effective and simple groundwater remediation technology, especially in rural and remote areas where groundwater is used as the main water source for drinking. The in situ arsenic removal technology was first optimized by simulating arsenic removal in various quartz sand columns under anoxic conditions. The effectiveness was then evaluated in an actual high-arsenic groundwater environment. The arsenic removal mechanism by the coated iron oxide/hydroxide was investigated under different conditions using scanning electron microscopy (SEM)/X-ray absorption spectroscopy, electron probe microanalysis, and Fourier transformation infrared spectroscopy. Aquifer iron coating method was developed via a 4-step alternating injection of oxidant, iron salt and oxygen-free water. A continuous injection of 5.0 mmol/L FeSO4 and 2.5 mmol/L NaClO for 96 h can form a uniform goethite coating on the surface of quartz sand without causing clogging. At a flow rate of 7.2 mL/min of the injection reagents, arsenic (as Na2HAsO4) and tracer fluorescein sodium to pass through the iron-coated quartz sand column were approximately at 126 and 7 column pore volumes, respectively. The retardation factor of arsenic was 23.0, and the adsorption capacity was 0.11 mol As per mol Fe. In situ arsenic removal from groundwater in an aquifer was achieved by simultaneous injections of As(V) and Fe(II) reagents. Arsenic fixation resulted from a process of adsorption/co-precipitation with fine goethite particles by way of bidentate binuclear complexes. Therefore, the study results indicate that the high arsenic removal efficiency of the in situ aquifer iron coating technology likely resulted from the expanded specific surface area of the small goethite particles, which enhanced arsenic sorption capability and/or from co-precipitation of arsenic on the surface of goethite particles.

Monitoring and modeling the effects of groundwater flow on arsenic transport in Datong Basin

Although arsenic-contaminated groundwater in the Datong Basin has been studied for more than 10 years, little has been known about the complex patterns of solute transport in the aquifer systems. Field monitoring and transient 3D unsaturated groundwater flow modeling studies were carried out on the riparian zone of the Sanggan River at the Datong Basin, northern China, to better understand the effects of groundwater flow on As mobilization and transport. The results indicate that irrigation is the primary factor in determining the groundwater flow paths. Irrigation can not only increase groundwater level and reduce horizontal groundwater velocity and thereby accelerate vertical and horizontal groundwater exchange among sand, silt and clay formations, but also change the HS− concentration, redox conditions of the shallow groundwater. Results of net groundwater flux estimation suggest that vertical infiltration is likely the primary control of As transport in the vadose zone, while horizontal water exchange is dominant in controlling As migration within the sand aquifers. Recharge water, including irrigation return water and flushed saltwater, travels downward from the ground surface to the aquifer and then nearly horizontally across the sand aquifer. The maximum value of As enriched in the riparian zone is roughly estimated to be 1 706.2 mg·d−1 for a horizontal water exchange of 8.98 m3·d−1 close to the river and an As concentration of 190 μg·L−1

In-situ arsenic remediation by aquifer iron coating: Field trial in the Datong basin, China.

An aquifer Fe-coating technology was evaluated for in-situ As remediation. The groundwater in the aimed aquifer has low dissolved Fe(II) concentration and high As(III) concentration, which has a low affinity toward Fe-oxides/hydroxides. To overcome these challenges, dissolved Fe(II) (5.0 mM) and NaClO (2.6 mM) were injected into the studied aquifer to promote the formation of Fe oxides/hydroxides and to oxidize As(III) into As(V), thus removing aqueous As via adsorption and/or co-precipitation. During field experiment, As concentration in groundwater from the pumping well significantly decreased. Fe and As speciation calculations indicate that incorporation of negatively charged As(V) into goethite was the probable mechanism for As removal. Both chemical sequential extraction results and spectroscopic data also support that alternating injection of Fe(II) and NaClO can achieve aquifer Fe coating and immobilize As via adsorption onto Fe oxides/hydroxides. Geochemical modelling further confirms that although competition for sorption sites between As and other dissolved species is expected in the natural groundwater system, high surface area of the Fe oxides/hydroxides can provide sufficient sites for As retention. The ability to effectively decrease As concentration of in-situ aquifer Fe-coating technology indicates that this approach should have extensive applicability to similar high As groundwater occurred worldwide.

The influence of irrigation-induced water table fluctuation on iron redistribution and arsenic immobilization within the unsaturation zone

Given the long-term potential risk of arsenic (As)-contaminated agricultural soil to public health, the redistribution of iron (Fe) and immobilization of As within the unsaturation zone during irrigation and consequent water table fluctuations were studied via a column experiment and corresponding geochemical modeling. Experimental results show that As and Fe accumulated significantly at the top of the column during irrigation. A tremendous increase in As and Fe accumulation rates exists after water table recovery. It was deduced that Fe(II) and As(III) were oxidized directly by O2 at the period of low water table. But the production of hydroxyl radical (OH) was promoted at the period of high water table due to the oxidation of adsorbed Fe(II). The generated OH further accelerate the oxidation of Fe(II) and As(III). Moreover, the combination of As and Fe is more stronger at the top of the column due to the transformation of combined states of As from surface complexation into surface precipitation with the growth of Fe(III) minerals. This study details the processes and mechanisms of As and Fe immobilization within the unsaturation zone during different irrigation periods and accordingly provides some insights to mitigate As accumulation in topsoil.

Chlorobenzenes Contamination in Soils/Sediments at a Site of Decommissioned Plant in Central China

China is one of the most important producers and suppliers of the chlorobenzenes (CBs), accounting for more than 50% of the world production. Although CBs contamination at the sites of chemical production factories is well-known, the knowledge on spatial distribution and contaminated degree of CBs are still limited. To better understand the occurrence of CBs, a total of 161 soil/sediment samples were collected from 36 sampling points within a 20 m deep from a site of decommissioned chlorobenzenes-producing factory in Wuhan City, Hubei Province, central China. The results showed that monochlorobenzene (MCB) is the predominant contaminants with the ΣCBs levels ranging from 0.05 to 8 600 mg/kg dry weight (dw) with a mean value of 384 mg/kg dw. The vertical distribution of CBs indicated that CBs may have clearly transported to deeper sections in the subsurface. Statistical analyses showed that there are no good correlations between the ΣCBs concentration with soil organic matter (SOM) content and the moisture content.

Variations of uranium concentrations in a multi-aquifer system under the impact of surface water-groundwater interaction

Understanding uranium (U) mobility is vital to minimizing its concentrations in potential drinking water sources. In this study, we report spatial-seasonal variations in U speciation and concentrations in a multi-aquifer system under the impact of Sanggan River in Datong basin, northern China. Hydrochemical and H, O, Sr isotopic data, thermodynamic calculations, and geochemical modeling are used to investigate the mechanisms of surface water-groundwater mixing-induced mobilization and natural attenuation of U. In the study site, groundwater U concentrations are up to 30.2 μg/L, and exhibit strong spatial-seasonal variations that are related to pH and Eh values, as well as dissolved Ca2+, HCO3-, and Fe(III) concentrations. For the alkaline aquifers of this site (pH 7.02-8.44), U mobilization is due to the formation and desorption of Ca2UO2(CO3)30 and CaUO2(CO3)32- caused by groundwater Ca2+ elevation via mineral weathering and Na-Ca exchange, incorporated U(VI) release from calcite, and U(IV) oxidation by Fe(OH)3. U immobilization is linked to the adsorption of CaUO2(CO3)32- and UO2(CO3)34- shifted from Ca2UO2(CO3)30 because of HCO3- elevation and Ca2+ depletion, U(VI) co-precipitation with calcite, and U(VI) reduction by adsorbed Fe2+ and FeS. Those results are of great significance for the groundwater resource management of this and similar other surface water-groundwater interaction zones.

Organic and inorganic colloids impacting total iodine behavior in groundwater from the Datong Basin, China

The geochemical behaviors of colloids in aquifers played an important role in determining the fate of iodine in groundwater system. To decipher the impact of colloids on iodine mobilization in aquifers, three successive pore-sized colloids filtration (0.45μm, 30kDa and 5kDa) were conducted on in-situ during groundwater sampling. The results showed that the distribution ratios (f) of total iodine (IT) and iron in the dissolved solution (i.e., 5kDa ultrafiltered) were from 0.78 to 0.99 and from 0.56 to 0.94, respectively. Natural organic matter (NOM) in the colloidal fractions obtained the f values ranging from 0.14 to 1.00. The decreased Eh values from recharge area to discharge area indicated redox potential of groundwater system changed from oxidizing to reducing along groundwater flowpath, and interestingly, the corresponding iodine fractions in groundwater were decreasing in dissolved solution and increasing in colloidal fractions. Inverse correlation between Fe and DOC and positive correlation between iodine and DOC suggested the occurrence of reductive dissolution of iron oxyhydroxides and degradation of organic iodine in groundwater system. Iodine distribution increased in dissolved solution and decreased in colloids with pH increase. Moreover, as pH increased, f (Fe) and f (DOC) decreased in dissolved solution and increased in colloids. Relatively weak correlation of f (IT) with f (Fe) and strong relationship between f (IT) and f (DOC) in the large grain size fractions suggested the Fe-OM complexes controlled iodine distribution in groundwater, which depends on the presence of Fe bridges. Negligible association of iodine with Fe and NOM in the small grain size fractions might be attributed to the effects of abundant OH- content in groundwater.