Mercury distribution in a typical shallow lake in northern China and its re-emission from sediment.

Mercury (Hg) re-emission from sediment is an important process in the biogeochemistry cycles of Hg in the aquatic ecosystem. The contribution of Hg released from sediment to water remains unclear for some shallow lakes. Lake Nansi is a typical shallow lake in northern China that is divided into upper and lower lakes by a dam. The Hg species in the water profile and sediment were measured from two sampling sites in the lake. Nansi Lake was not markedly contaminated by Hg. The Hg profile in the sediment indicated that the demand for energy and the policy management in the catchment influenced the Hg accumulation in its sediment. On the basis of Fick's first law and the sedimentation rate, the diffusion flux of dissolved Hg from sediment to overlying water and the accumulation flux were estimated. According to one-year scale estimation, approximately 10%-13% the Hg in the sediments can be re-released into the overlying water. The Hg diffused from the sediment accounted for 7.9%-16% of the Hg in the overlying water. These results of this study improve the understanding of the sources of pollution in water and enable researchers to focus on the contribution of sediment to the pollution of water in shallow lakes.

Distribution characteristics of heavy metals in surface soils from the western area of Nansi Lake, China.

Surface soil samples collected from the western area of Nansi Lake, China, were analyzed for selected heavy metals including As, Cd, Pb, and Zn, to determine their spatial distributions and environmental effects. The average concentrations of As, Cd, Pb, and Zn in soil were 13.21 mg/kg, 0.20 mg/kg, 23.94 mg/kg, and 79.95 mg/kg, respectively. The concentration of As, Cd, and Zn was approximately 1.44-, 2.33-, and 1.25-fold higher than its background values in study area, respectively. Meanwhile, the concentrations of heavy metals progressively decreased from east to west within the study area, in a step-function distribution. The differences in the heavy metal distribution characteristics might be caused by the lake water irrigation and agricultural activities such as fertilizer and pesticide use. There were significant positive correlations between the values of OrgC, Al2O3, and Fe2O3 and concentrations of heavy metals. According to the Geo-accumulation index (Igeo) and the potential ecological risk index (PERI), Cd posed higher potential ecological risk in surface soil when compared with As, Pb, and Zn. These results could provide the scientific basis on which to evaluate the distribution of heavy metals under natural and anthropogenic influences in the surface soil near Nansi Lake, China.

Selective and simultaneous high resolution 2-D imaging of AsIII, CrIII and SbIII and dissolved oxygen by developing a new DGT technique comprising a hybrid sensor.

A new diffusive gradients in thin films technique (HR-MPTS DGT) with mercapto-functionalized attapulgite in a binding gel was developed for simultaneous two-dimensional (2-D) chemical imaging of AsIII, CrIII and SbIII selectively at the submillimeter scale, combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. The HR-MPTS DGT exhibited selective accumulation of AsIII, SbIII and CrIII (> 97%), yet negligible accumulation of AsV, SbV and CrVI (< 2%). Accumulation of AsIII, CrIII and SbIII on the binding gel had a linear relationship (R2 > 0.99) with the corresponding standardized laser ablation signals, proving the feasibility of LA-ICP-MS analysis. Analysis for AsIII, CrIII and SbIII was provided with favorable analytical precision (relative standard deviation <10%). With the purpose of evaluating the dynamics of AsIII, CrIII, SbIII and O2 in the rooting zone, a hybrid sensor, which comprises the HR-MPTS gel overlying an O2 planar optode, was deployed in rhizosphere sediments. Results showed that the consumption of both AsIII and SbIII due to the oxidation extended ~4.48 mm into the sediments, which was consistent with the extension length of the oxidized sediment layers around the roots created by O2 leakage.

New application of lanthanum-modified bentonite (Phoslock®) for immobilization of arsenic in sediments.

Lanthanum-modified bentonite (LMB, Phoslock®) is a well-known capping agent for phosphorus immobilization in sediments. Herein, LMB was used to immobilize As in sediments. Batch capacity experiments for arsenate and arsenite adsorption were carried out to obtain adsorption isotherms and kinetics using the Langmuir and Freundlich model calculations. High-resolution (HR) diffusive gradients in thin films (DGT) were applied to monitor the changes of weakly bound As fraction near sediment-water interface (SWI). The interaction of As(III) and As(V) with LMB was influenced by pH and initial mineral composition. As(V) was more obviously adsorbed than As(III) at pH 4 to 9, with mean adsorption of 3.89 mg g-1 and 0.04 mg g-1, respectively, while at pH > 9 As(III) was preferentially adsorbed. After LMB amendment for 2 months, the maximum As removal efficiency in the pore and overlying water reached 84.5% and 99.3%, respectively. The capping agent remained stable in the top sediments, while the maximum DGT labile As content decreased to 0.89 and 0.51 µg L-1 in dosage-and time-treatments. The As concentration inflection point moved down to a deeper layer. As species changed from labile exchangeable-As to Fe-oxide-bound and residual As. The proportion of mobile As finally decreased to 10.5% of the total As in the upper 20-mm layer sediment. The increase of Kd (the distribution coefficient at SWI) and k1 (adsorption rate constant) and the decrease of Tc (response time of (de)sorption) in the DGT-induced fluxes model (DIFS) indicated the time-dependent impediment of As release from the sediment due to LMB immobilization.

O2 distribution and dynamics in the rhizosphere of Phragmites australis, and implications for nutrient removal in sediments.

Root-triggered microscale variations in O2 distribution in the rhizosphere of young Phragmites australis are important for nutrient removal in sediments. In this study, the micro-scale O2 dynamics and the small-scale changes of soluble reactive phosphorus (SRP) and ammonium (NH4+) in the rhizosphere of P. australis were investigated using planar optodes and high-resolution dialysis (HR-Peeper), respectively. Results suggested that root O2 leakage has a highly variable distribution depending on the stage of root growth, the site of O2 leakage gradually shift from the entire emerging main roots to the main root tip and subsequently shifted the emerging lateral roots. The O2 concentration increased in the rhizosphere with increasing light intensity and O2 levels in the overlying water. Continuous O2 release from the lateral roots causes the formation of iron plaque on the surface of lateral roots, which reduce the mobility of P by adsorption of iron plaque in the rhizosphere. The oscillation of oxic-anoxic root zones improves nitrogen removal through the processes of anammox, heterotrophic denitrification and nitrification. This work from the micro-scale demonstrates that the O2 concentration is the spatio-temporal variations in the rhizosphere, and it presents an important role for nutrient removal in sediments.

A new DGT technique comprising a hybrid sensor for the simultaneous high resolution 2-D imaging of sulfides, metallic cations, oxyanions and dissolved oxygen.

A new diffusive gradients in thin films technique (HR-ZCA DGT) was developed for simultaneous two-dimensional (2-D) chemical imaging of sulfides, metallic cations and oxyanions (S, Cd, Co, Fe, Cu, Mn, Ni, Pb, Zn, As, Cr, Mo, Sb, Se, V, P and W) at the submillimeter scale, combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. A novel binding gel was prepared using a double precipitation method with AgI and zirconium oxide (Zr-oxide) deposited sequentially on a preformed Chelex-100 resin gel. A good linear relationship was observed (R2>0.99) between mass accumulation of the 17 assessed elements on the binding gel and the corresponding standardized laser ablation signals (signals of elements divided by signals of internal standard 13C), proving the feasibility of LA-ICP-MS analysis. Good analytical precision (RSD<12 %) was achieved for all 17 elements. A hybrid sensor comprising the novel DGT binding gel overlying an O2 planar optrode was then tested in sediments to evaluate the dynamics of O2 and multiple elements. Results showed that the mobility of As, P and W were controlled by precipitation/dissolution processes with Fe/Mn oxides. V, Co, Ni, Zn, Mo, Cd and Sb were released at the sediment surface with the oxidation of iron sulfides.

A new DGT technique comprised in a hybrid sensor for the simultaneous measurement of ammonium, nitrate, phosphorus and dissolved oxygen.

A new diffusive gradients in thin films technique (ZrO-AT DGT) with zirconium oxide, A-62 MP and T-42H resins containing in a single binding gel was developed for simultaneous measurement of nitrate (NO3-N), ammonium (NH4-N) and phosphate (PO4-P). The DGT uptake was found to be independent of pH variation from 3.2-8.7. Ionic strengths below 5, 10 and 750 mmol·L-1 NaCl did not affect DGT uptake of NH4-N, NO3-N and PO4-P, respectively. This new DGT was deployed in natural freshwater environments, with in situ measurements of the three nutrients found to be accurate. It ensured that rinsing the exposed surface of the DGT device at 3-day intervals can prevent biofouling. Additionally, a hybrid sensor comprising the novel DGT binding layer overlying an O2 planar optrode was tested in sediments to evaluate the dynamics of O2 and the three nutrients. Results showed that PO4-P and NO3-N fluxes decreased while fluxes of NH4-N increased under aerobic conditions. Nearly simultaneous variation in O2 and NO3-N was observed at the sediment-water interface (SWI) and transformation of NO3-N and PO4-P was found to be sensitively influenced by O2 dynamics.

Enhanced DGT capability for measurements of multiple types of analytes using synergistic effects among different binding agents.

There is a requirement for simultaneous measurements of diverse analytes to reveal their biogeochemical couplings in the environment. In this study, we first realized simultaneous measurements of three types of analytes, including sulfide [S(-II)], metallic cations, and oxyanions by diffusive gradients in thin films (DGT) using a new mixed ZrO-CA binding gel. The ZrO-CA gel exhibited faster absorption rates for oxyanions than previously reported ZrO-Chelex gel, and faster absorption rates for cations when the gel was saturated with S(-II). It implies that there were synergistic effects among the three binding agents which should facilitate the DGT uptake. The technique's performance was validated under laboratory conditions in mixtures of dissolved compounds. It was shown to be independent of the solution's pH (5-9) and ionic strength (from 1 to 3 mM to 750 mM). The capacities of the ZrO-CA DGTs without or with [S(-II)] saturation to measure As(III), As(V), Cd(II), Fe(II), and P(V) individually were significantly greater than those of other DGTs except for Zr-oxide based DGTs. The measurements in sediment revealed similar distributions of three types of analytes in the vertical profile and confirmed the feasibility and advantage of the ZrO-CA DGT. This study provides a new perspective to enhance the DGT capability in measurement of multiple analytes through utilizing the synergistic effects among different binding agents.

Seasonal antimony pollution caused by high mobility of antimony in sediments: In situ evidence and mechanical interpretation.

Antimony (Sb) mobilization in sediments and its impact on water quality remained to be studied. In this study, high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) technique were used to measure soluble Sb and labile Sb in sediment-overlying water profiles for a full year in a eutrophic region of Lake Taihu. Results showed that the highest mean concentrations of soluble Sb in overlying water (11.27 and 6.99 µg/L) appeared in December 2016 and January 2017, due to oxidation of Sb(III) to Sb(V) by Mn and Fe oxides, all of which exceeded the surface or drinking water limits set by China, United States and European Union. From April to November 2016, the concentrations of soluble Sb remained low with small monthly fluctuations and mean values ranging from 1.79 to 2.93 µg/L. This was attributed to the predominance of insoluble Sb(III) in sediments under anoxic conditions. The concentration of soluble Sb was slightly higher in summer than in autumn, due to the complexation of Sb(III) with DOM, as shown under anaerobic incubation. The mobility of inorganic Sb in sediments was mostly determined by the transition between Sb(III) and Sb(V), with Sb pollution in bottom water during winter being of concern.

Simultaneous measurement of fifteen rare earth elements using diffusive gradients in thin films.

A new method for simultaneous measurement of fifteen rare earth elements (REEs) [La (Ⅲ), Ce (Ⅲ), Pr (Ⅲ), Nd (Ⅲ), Sm (Ⅲ), Eu (Ⅲ), Gd (Ⅲ), Tb (Ⅲ), Dy (Ⅲ), Ho (Ⅲ), Er (Ⅲ), Tm (Ⅲ), Yb (Ⅲ), Lu (Ⅲ), and Y (Ⅲ)] was established in this study using the diffusive gradients in thin films (DGT) technique with Chelex®100 binding gel. Five different types of ion exchange resins (Chelex®100, D418, D001-cc, 001 × 7, and HSTY®-SS) were selected for the initial investigation of their adsorption performance for REEs. The Chelex®100 binding gel had the greatest uptake efficiencies of >95% for the fifteen REE ions, which was used for all subsequent experiments. The binding gel exhibited rapid binding dynamics to REEs in mixed solution of the fifteen REE ions. Elution efficiencies ranging from 86.5% to 93.8% for these REEs were obtained based on extraction using 2.0 M HCl. The Chelex®100 DGT uptake of the fifteen REE ions increased linearly with the deployment time and found to be independent of pH (3-9) and ionic strength (3 mM-100 mM). The capacities of Chelex®100 DGT for measurement of the mixed elements were determined at a range of 5.39-6.75 µg cm-2. Application of the DGT for soil analysis showed that Chelex®100 DGT was a useful tool in simultaneous measurement of the fifteen REE ions, even in a soil with high concentrations of REEs. This study demonstrated the advantage of Chelex®100 DGT in simultaneous measurement of the fifteen REE ions due to high uptake efficiencies and a wide tolerance to environmental interference.

Spatial trends and pollution assessment for mercury in the surface soils of the Nansi Lake catchment, China.

Surface soil samples collected from Nansi Lake catchment were analyzed for mercury (Hg) to determine its spatial trends and environmental impacts. Results showed that the average soil Hg contents were 0.043 mg kg-1. A positive correlation was shown between TOC and soil Hg contents. The main type of soil with higher TOC contents and lower pH values showed higher soil Hg contents. Soil TOC contents and CV values were both higher in the eastern catchment. The eastern part of the catchment, where the industry is developed, had relatively high soil Hg contents and a banding distribution of high Hg contents was corresponded with the southwest-northeast economic belt. Urban soils had higher Hg contents than rural soils. The urbanization pattern that soil Hg contents presented a decreasing trend from city center to suburb was shown clearly especially in the three cities. Soil Hg contents in Jining City showed a good consistency with the urban land expansion. The spatial trends of soil Hg contents in the catchment indicated that the type and the intensity of human activities have a strong influence on the distribution of Hg in soils. Calculated risk indices showed that the western part of the catchment presented moderately polluted condition and the eastern part of the catchment showed moderate to strong pollution level. The area with high ecological risk appeared mainly along the economic belt.

Successful control of internal phosphorus loading after sediment dredging for 6years: A field assessment using high-resolution sampling techniques.

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6years, due to their geographical proximity (a line distance of approximately 9km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353mg/m2/d to 0.174, 1.58 and 0.048mg/m2/d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom.

Mechanisms driving phosphorus release during algal blooms based on hourly changes in iron and phosphorus concentrations in sediments.

Algal growth causes a drastic change in aquatic conditions over a diel cycle, which may induce sensitive feedback systems in sediments, causing P release. In this study, a microcosm experiment was performed using a suction sampler (Rhizon) to observe changes in soluble reactive phosphorus (SRP) and soluble Fe(II) concentrations in the top 20 mm sediment layer on a 3-h time interval, at different phases of harmful algal bloom (HAB) development. The results showed that the algal blooms prevailed up to 15 days after incubation, after which the process of bloom collapse proceeded until the 70th day. The concentrations of pore-water soluble Fe(II) and SRP increased throughout the incubation period. Compared to day 1, maximum increases of 214% in soluble Fe(II) and 387% in SRP were observed at night during the bloom and collapse periods, respectively. The diffusive fluxes of Fe and P at the sediment-water interface (SWI) generally corresponded to their changes in concentrations. Hourly fluctuation in soluble Fe(II) and SRP concentrations were observed with two distinct concentration peaks occurred at 21:00 p.m. and 06:00 a.m. (or 03:00 a.m.), respectively. These findings suggest that Fe-P coupling mechanisms are responsible for the release of P from sediments. During the collapse period, soluble Fe(II) concentrations were suppressed by the increase of labile S(-II) at night. Meanwhile, SRP concentrations were decoupled from Fe cycling with small fluctuations (<11% RSD) on an hourly timescale, and the decomposition of algae was a dominant source contributing to the release of P from sediments. These results significantly improved the understanding of processes and mechanisms behind the stimulated release of P from sediments during HABs.