Characterization of leached phosphorus from soil, manure, and manure-amended soil by physical and chemical fractionation and Diffusive Gradients in Thin films (DGT)

Immobilizing arsenic in soil via amine metal complex: a case study using iron-ethylenediamine

Fe-based nanomaterials have been extensively investigated for their application in mitigating arsenic (As) pollution in groundwater, sediment, and soils. Here, an iron-ethylenediamine (Fe-EDA) complex was synthesized and characterized using Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy before its use as an amendment to ameliorate As-polluted soils. Column leaching tests at three Fe-EDA application rates (1%, 3%, and 5%) were conducted, and their results were compared with those acquired after using nano zerovalent iron (nZVI) and Fe₃O₄, to assess their efficiency to amend As-contaminated paddy soils. After leaching, stabilization efficiency and soil chemical characteristics were determined. Additionally, As fractions were measured using inductively coupled plasma-mass spectroscopy by employing a sequential extraction procedure to evaluate the performance of the treatments and understand the underlying their mechanisms. Compared with the control treatment, the Fe-EDA treatment reduced As release by more than 35.33% in the 2nd leaching cycle, whereas nZVI and Fe₃O₄ decreased the As release by 11.84% and 24.60%, respectively. Moreover, the optimal addition of the Fe-EDA chelate was 5%, which stabilized more than 50% As in the soil from the 7th to 11th leaching cycles. After sequential extraction, the Fe-Mn oxide binding fraction, which was originally 12.65%, increased to 21.5%, 18.23%, and 21.71% after the application of nZVI, Fe₃O₄, and Fe-EDA amendments, respectively. Furthermore, our treatments promoted the binding of the As fraction with crystalline Fe (III) (oxyhydr)oxide (F3); however, other fractions did not increase considerably, suggesting that the Fe-EDA complex could effectively stabilize As through electrostatic attraction between the arsenate anion and EDA, as well as As-O-Fe bond formation via a coordinating reaction. Overall, Fe-EDA was found to be a potent amendment for mitigating As-polluted soil.

Geochemistry and release risk for nutrients in lake sediments based on diffusive gradients in thin films

A comprehensive understanding of the mobility of both nitrogen (N) and phosphorus (P) and the inter-relationships between P, N, and iron (Fe) in sediments is important for controlling the "internal loadings" of nutrients in lakes. In this research, diffusive gradients in thin film (DGT) assemblies with binding layers (ZrO-AT, chelex, and ZrO) were designed for PO₄-P, Fe, ammonium (NH₄-N), and nitrate (NO₃-N) at sediment/water interface (SWI) in Western Lake Taihu (China). The biogeochemical processes of N and P related to the physicochemical properties, the dynamic P transfer, the distribution characteristics of P microniches, and the estimation of the release risks in sediments in Western Lake Taihu were simultaneously revealed by the passive sampling technique-DGT with the high spatial resolutions (millimeter and sub-millimeter). Based on DGT concentration (C_DGT) related to physicochemical properties in sediments, (1) P biogeochemical reactions included P release from Fe-bound P during Fe reduction, algae biomass decomposition, and phosphatase enzyme activity increased by NH₄-N; (2) denitrification and dissimilatory nitrate reduction to ammonium (DNRA) led to exchangeable ammonium (NH₄ex) enrichment and NH₄-N release; anammox depleted NH₄-N transfer; organic matter (OM) mineralization favored NH₄-N release; and (3) aerobic nitrification led to NO₃-N remobilization; denitrification and DNRA reduced NO₃-N release. Redox status, OM, Fe, aluminum, or calcium influenced mobilization of nutrients. The numerical model of DGT-induced fluxes in sediments was used for dynamic P transfers with resupply types ("slow" ~ "fast") controlled by labile P pool, resupply constant, response time, and Dspt rate. The formation of P microniches in two dimensions was revealed. Sediment P release risk index (0.49 ~ 36.85 [lg (nmol cm^-3 d^-1)]) with "light" ~ "high" risks and diffusive fluxes across SWI (µg m^-2 d^-1) of 15.0 ~ 639 (PO₄-P), - 1403 ~ 5010 (NH₄-N), and - 1395 ~ 149 (NO₃-N) were derived and lake management strategies were provided. The DGT technique provides the characterization of the mobilization of nutrients and evidence for biogeochemical processes at the fine spatial scales for control of internal loadings in sediments.

Application of Fourier transform mid-infrared photoacoustic spectroscopy for rapid assessment of phosphorus availability in digestates and digestate-amended soils

Digestate is the anaerobic digestion by-product of biogas production that can be used as a phosphorus (P) fertilizer. To achieve the efficient utilization of digestate as a P fertilizer and evaluate P availability in digestate-amended soils, it is necessary to assess both available P in different digestates and digestate-amended soils. In this study, Fourier transform mid-infrared photoacoustic spectroscopy (FTIR-PAS) combined with multivariate analysis was applied to predict water-extractable P (WEP) in digestates and plant-available P in digestate-amended soils. The plant-available P was determined by the diffusive gradients in thin films (DGT) technique. 45 digestate samples were collected both from laboratory-scale digesters (26 samples) and operating biogas plants (19 samples) in Denmark for WEP determination. Three soils amended with the collected 19 digestate samples from biogas plants (that results to 57 digestate-amended soil samples in total) were deployed for DGT measurement of plant- available P. The WEP predicting model had a coefficient of determination (R²) of 0.80 and a root mean square error of 0.78 g kg^-1 while the plant-available P predicting model exhibited an R² of 0.70 and a root mean square error of 134.09 μg P L^-1. Furthermore, regression coefficients with a significant contribution of the plant-available P predicting model were identified, indicating that FTIR-PAS is capable for correlating spectra information with plant-available P related chemical bonds. In conclusion, FTIR-PAS can be used as a faster and non-destructive alternative for the assessment of both WEP in digestates and plant-available P in digestate-amended soils.

Mobilization and geochemistry of nutrients in sediment evaluated by diffusive gradients in thin films: Significance for lake management

Investigation of in-situ mobilization of both nitrogen (N) and phosphate (PO₄^3-) in sediment is important for lake management strategy. In this paper, diffusion gradients in thin films (DGT) and DGT induced flux in sediments (DIFS) model are newly designed for in-situ measurement of iron (Fe), PO₄^3-, nitrate (NO₃-N) and ammonium (NH₄-N), and nutrients' mobility in sediment in Lake Nanhu (China). According to DGT profiles together with physicochemical properties in sediment, (I) PO₄^3- is released from (i) Fe-bound P plus loosely sorbed P in anoxic sediment and (ii) the loosely sorbed P in oxic sediment; (II) anoxic sediment inhibits nitrification and NO₃-N release, but it favors denitrification and dissimilatory nitrate reduction to ammonium (DNRA), leading to NH₄-N release; (III) Eh and organic matter are two key influence factors on mobility of PO₄^3-, NO₃-N and NH₄-N. According to DIFS calculation, the dynamics of desorption and diffusion at two sites belong to (i) slow rate of resupply and (ii) fast resupply cases, respectively. Internal loadings are estimated to be 92.74 (PO₄^3-), 268.1 (NH₄-N) and -2466 kg a^-1 (NO₃-N), which reflects sediment mainly acts as a source for PO₄^3- and NH₄-N, and a sink for NO₃-N in water. Based on sediment P release risk index (SPRRI), P release risks in lake sediments are estimated, ranging from light to relative high level. DGT and SPRRI aid choice of restoration methods for sediment, including sediment dredging, phytoremediation and in-situ inactivation.

Fluorescence characteristics and biodegradability of dissolved organic matter (DOM) leached from non-point sources in southeastern China

Under the increasingly intensive measures for surface water restoration in China, point source discharge has been strictly regulated; however, for non-point sources, which constitute a large part of surface water pollutants, effective control has been difficult to reach. A comprehensive understanding of the characteristics of non-point source pollutants is essential for surface water improvement programs of cities such as Ningbo, on the southeast coast of China. Ningbo has made tremendous efforts in the past few years to control point source pollutants, but available data and management strategies on the non-point source pollutants are still limited. To this end, leachates of representative non-point source samples from the territory of Ningbo, including cropland and wetland soil, urban channel sediment, and poultry manure, were examined and compared focusing on the fluorescence characteristics and biodegradability of the dissolved organic matter (DOM). Results indicated that biodegradable dissolved organic carbon (BDOC) accounting for the total DOC was 46.7 ± 0.7% for cropland, wetland (56.3 ± 6.8%), non-sewage channel (60.1 ± 0.4%), sewage channel (74.5 ± 1.1%), and poultry manure (62.7 ± 4.5%). The leachates of the studied samples showed significant differences in both the amount and composition of DOM. However, a fluorescence component representing tryptophan-like substances identified by the excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis effectively predicted the BDOC variations among the studied samples. Moreover, under the studied nutrient concentrations, which were equivalent to Grade III water quality in China, nutrient limitation of microbial degradation was not observed. Threats to water quality, especially excessive consumption of dissolved oxygen, could be posed by the non-point source leachates due to their high bioavailability, large distribution, and weak nutrient restraint. Further investigations, including a quantitative evaluation of the non-point source pollution contribution, and pollutant blocking techniques are required.

Phosphorus (P) release risk in lake sediment evaluated by DIFS model and sediment properties: A new sediment P release risk index (SPRRI)

A new sediment P release risk index (SPRRI) for "in-situ" phosphorus (P) release risk in lake sediment, is developed based on diffusive gradients in thin films (DGT) technique, DGT induced flux in sediments (DIFS) model and sediment properties. SPRRI includes three sub-indexes, which contain (1) the labile P pool size, (2) resupply constant (r) and desorption rate (Dspt rate) for P transfer and (3) the molar ratio between iron (Fe) in sequential extraction for sediment P by bicarbonate-dithionite (BD) and aluminum (Al) by NaOH (at 25 °C), i.e. BD(Fe)/Al[NaOH25] in sediment solid. The first sub-index considers P release from (i) sediment with NH₄Cl-P+BD-P pool, i.e. the loosely sorbed P (NH₄Cl-P) plus iron associated P (BD-P), or (ii) sediment with NH₄Cl-P pool, respectively. The second and third sub-indexes reflect kinetic P desorption and resupply ability of solid phase, and the effect of P sequestration by Al hydroxide on P release, in turn. The inner relationship between SPRRI and sub-indexes, and their effects on P release risk are elucidated. SPRRI can be used to evaluate sediment P reactivity by five release risk ranks. For Lake Dianchi (China), P transfer dynamics, labile P pool, resupply ability and Al-P in sediment, and "external P-loading" control and affect P release risk in different regions, which is reflected by the spatial distribution map for SPRRI. The present SPRRI can be applied for lakes with (1) pH range varying from moderate acidity to weak alkalinity in waterbody and (2) NH₄Cl-P or NH₄Cl-P+BD-P pool in sediment solid.

Field-Scale Heterogeneity and Geochemical Regulation of Arsenic, Iron, Lead, and Sulfur Bioavailability in Paddy Soil

A method using miniaturized arrayed DGT-probes (PADDI) for high-frequency in situ sampling with LA-ICPMS and CID analysis was developed to measure the field-scale heterogeneity of trace-element bioavailability. Robust calibrations (R² > 0.99) combined with high-sensitivity (LOD = 0.35 ng cm^-2), multielemental detection, and short measurement times were achieved using a new LA-ICPMS microDGT analysis. In the studied paddy-site (size: ∼2500 m²), total element concentrations across the field were approximately uniform (R.S.D. < 10%), but bioavailability was shown to vary significantly as determined from 864 microgel measurements housed within 72 PADDI arrays. Porewater As measurements were unable to differentiate the top/rhizosphere and bulk/deeper-soil layers. However, dynamic sampling with DGT revealed significant differences. Heterogeneity behaviors varied greatly between the different elements. Arsenic bioavailability was stable laterally across the field, but varied with depth, which was in contrast to the trends for Pb. Fe/S^(-II) change was bidirectional, differing horizontally and vertically throughout the field. The heterogeneity in Pb bioavailability, due to the high frequency of hotspot maxima that were discretely dispersed across the paddy, proved the most difficult to simulate requiring the greatest number of probe deployments to determine a reliable field-average. The DGT-PADDI system provides a new characterization of infield trends for improved trace-inorganics' management in agricultural wetlands.

DGT technique to assess P mobilization from greenhouse vegetable soils in China: A novel approach

Intensive phosphorus (P) inputs to plastic-covered greenhouse vegetable production (PGVP) in China has led to excessive soil P accumulation increasing the potential for leaching to surface waters. This study examined the mobility and hence the potential risk of P losses through correlations between soil solution P (P_Sol) and soil extractable P as determined by conventional soil P test methods (STPs) including degree of P saturations (DPSs), and diffusive gradient in thin-films (DGT P) technique. A total of 75 topsoil samples were chosen from five representative Chinese PGVPs covering a wide range of physiochemical soil properties and cultivation history. Total P and Olsen P contents varied from 260 to 4900, and 5 to 740mgkg^-1, respectively, while P_Sol concentrations were between 0.01 and 10.8mgL^-1 reflecting the large differences in vegetation history, fertilization schemes, and soil types. Overall, DGT P provided the best correlation with P_Sol (r²=0.97) demonstrating that DGT P is a versatile measure of P mobility regardless of soil type. Among the DPSs tested, oxalate extractable Al (DPS_Ox-Al) had the best correlation with P_Sol (r²=0.87). In the STP versus P_Sol relationships, STP break-points above which P mobilization increases steeply were 513μgL^-1 and 190mgkg^-1 for DGT P or Olsen P, respectively, corresponding to P_Sol concentration of 0.88mgL^-1. However, for P_Sol concentration of 0.1mgL^-1 that initiates eutrophication, the corresponding DGT P and Olsen P values were 27μgL^-1 and 22mgkg^-1, respectively. Over 80% of the investigated soils had DGT P and Olsen P above these values, and thus are at risk of P mobilization threatening receiving waters by eutrophication. This paper demonstrates that the DGT extracted P is a powerful measure for soluble P and hence for assessment of P mobility from a broad range of soil types.

Transfer kinetics of phosphorus (P) in macrophyte rhizosphere and phytoremoval performance for lake sediments using DGT technique

DGT (diffusive gradients in thin films) technique and DIFS (DGT induced fluxes in sediment) model are firstly designed for macrophyte-rhizobox system and in-situ macrophytes in Lake Erhai. Dynamics of phosphorus (P) transfer in Zizania latifolia (ZL) and Myriophyllum verticiilatur (MV) rhizosphere is revealed and phytoremediation performance for P in sediment is evaluated. Dynamic transfer process of P at DGT/sediment interface includes (i) diffusion flux and concentration gradients at DGT(root)/porewater interface leading to porewater concentration (C₀) depletion and (ii) P desorption from labile P pool in sediment solid to resupply C₀ depletion. Fe-redox controlled P release from Fe-bound P (BD-P2) and then NH₄Cl-P1 in rhizosphere sediment resupplies porewater depletion due to DGT (root) sink. K_d (labile P pool size in solid phase), r (resupply ratio) and kinetic exchange (Tc and k_-1) lead to change characters of DIFS curves of (1) r against deployment time and (2) C_solu (dissolved concentration) against distance at 24 h. They include two opposite types of "fast" and "slow" rate of resupplies. Sediment properties and DIFS parameters control P diffusion and resupply in rhizosphere sediment. Phytoremoval ability for sediment P in lake is estimated to be 23.4 (ZL) or 15.0 t a^-1 (MV) by "DGT-flux" method.

Development and application of the diffusive gradients in thin films technique for simultaneous measurement of methcathinone and ephedrine in surface river water

In this study, a passive sampling technique, diffusive gradients in thin films (DGT) was developed to simultaneously measure two drugs, methcathinone (MC) and ephedrine (EPH) in surface water. Four types of binding gels and four types of filter membranes were tested for the optimal configuration. XAD18 agarose binding gel and agarose diffusive gel, together with polyethersulfone filter membrane were used for measuring MC and EPH in the DGT device. 5% NH₃ in acetonitrile was used as the elution solvent, with the elution efficiency for MC and EPH higher than 71%. At 25°C, the diffusion coefficients of MC and EPH in the diffusive gel were 7.60×10^-6cm²s^-1 and 6.62×10^-6cm²s^-1, respectively. The DGT was effective in a wide range of pH (4-11) and ionic strength (NaCl: 0.001-0.5M). The DGT device was deployed in Beijing urban surface water for successive 7days to measure the time-weighted concentrations of MC and EPH. Results showed that EPH was detected in all samples, while MC was below its detection limit. DGT concentrations were comparable to the concentrations determined by SPE. This study demonstrated that the developed DGT method was effective to monitor the two drugs in surface water in situ.

Transfer Mechanism, Uptake Kinetic Process, and Bioavailability of P, Cu, Cd, Pb, and Zn in Macrophyte Rhizosphere Using Diffusive Gradients in Thin Films

The transfer-uptake-bioavailability of phosphorus (P), Cu, Cd, Zn, and Pb in rhizosphere of Zizania latifolia (ZL) and Myriophyllum verticiilaturn (MV) cultivated in rhizoboxes in Lake Erhai (China) is evaluated by DGT (diffusive gradients in thin films) technique. DGT induced fluxes in sediments (DIFS) model reveals that resupply ability (r), liable pool size in sediment solid (k_d), kinetic parameter (k_-1), or response time (T_c) control the diffusion-resupply characters of P and Cu (standing for four metals) in rhizosphere interface. The linear fitting curves of element content in ZL or MV roots (C_root) against DGT (C_DGT), porewater (C₀), or sediment concentration demonstrate that C_root for five elements can be predicted by C_DGT more effectively than the other methods. (I) DOC (dissolved organic carbon) in porewater controlled by OM (organic matter) in solid plus pH for Cu and Cd or (II) DOP/DTP ratio in porewater (between dissolved organic P and dissolved total P) for P controlled by Fe-bound P and OM in solid, can affect phytoavailability in rhizosphere. They lead to (I) the larger slope (s) and the linear regression coefficient (R²) in the first part than those for the complete fitting curve (ZL or MV root against C_DGT(Cu) or C₀(Cu) and MV root against C_DGT(Cd)) or (II) the outliers above or below the fitting curve (ZL root (P) against C₀(P) or C_DGT(P)) and the larger R² without outliers. DGT-rhizobox-DIFS should be a reliable tool to research phytoremediation mechanism of macrophytes.

Influence of Controlled Drainage and Liquid Dairy Manure Application on Phosphorus Leaching from Intact Soil Cores

Controlled drainage can reduce nitrate export from tile drainage flow, but its impact on phosphorus (P) loss is largely unknown. We compared P leaching from soil cores treated as free drainage (FD) or controlled drainage (CD) before and after manure application. In August 2012, 16 intact cores (45 cm long, 15 cm diameter) were collected from a grass forage field () located in Chazy, NY, and modified for drainage control and sampling. In Experiment 1 (no manure), initial leachate was defined as FD, and leachate collected 21 d later (valves closed) was considered CD. In Experiment 2, seven cores were randomly assigned to CD or FD. Liquid dairy manure was applied at 1.2 × 10 L ha, followed by simulated rainfall 2 h later. Leachate was sampled on Day 7, 14, and 21. Deionized water was applied at 3.4 cm h over 1 h to mimic a 10-yr rainfall event. Total P (TP), soluble reactive P (SRP), dissolved oxygen, iron (Fe), and pH were measured. Results showed that TP ( = 0.03) and SRP ( = 0.35) were lower for CD prior to manure application. Manure application caused 36- and 42-fold increases in TP and SRP; however, TP was lower for CD at 7 ( = 0.06), 14 ( = 0.003), and 21 d ( = 0.002) of water retention. Mean SRP for CD was nearly 40-fold lower than FD by Day 7 ( = 0.02) and remained low, suggesting CD in the field may reduce P export risk to tile drain flow after manure applications.

Evaluating the mobility of polymer-stabilised zero-valent iron nanoparticles and their potential to co-transport contaminants in intact soil cores

The use of zero-valent iron nanoparticles (nZVI) has been advocated for the remediation of both soils and groundwater. A key parameter affecting nZVI remediation efficacy is the mobility of the particles as this influences the reaction zone where remediation can occur. However, by engineering nZVI particles with increased stability and mobility we may also inadvertently facilitate nZVI-mediated contaminant transport away from the zone of treatment. Previous nZVI mobility studies have often been limited to model systems as the presence of background Fe makes detection and tracking of nZVI in real systems difficult. We overcame this problem by synthesising Fe-59 radiolabelled nZVI. This enabled us to detect and quantify the leaching of nZVI-derived Fe-59 in intact soil cores, including a soil contaminated by Chromated-Copper-Arsenate. Mobility of a commercially available nZVI was also tested. The results showed limited mobility of both nanomaterials; <1% of the injected mass was eluted from the columns and most of the radiolabelled nZVI remained in the surface soil layers (the primary treatment zone in this contaminated soil). Nevertheless, the observed breakthrough of contaminants and nZVI occurred simultaneously, indicating that although the quantity transported was low in this case, nZVI does have the potential to co-transport contaminants. These results show that direct injection of nZVI into the surface layers of contaminated soils may be a viable remediation option for soils such as this one, in which the mobility of nZVI below the injection/remediation zone was very limited. This Fe-59 experimental approach can be further extended to test nZVI transport in a wider range of contaminated soil types and textures and using different application methods and rates. The resulting database could then be used to develop and validate modelling of nZVI-facilitated contaminant transport on an individual soil basis suitable for site specific risk assessment prior to nZVI remediation.

Immobilization of phosphorus in cow manure during hydrothermal carbonization

The surplus of manure phosphorus (P) with increasing livestock production might pose a risk of P loss to the environment due to the high mobility of P in manure. Thus, there is an increasing need to mitigate P loss from manure. This study aimed to investigate the effect of hydrothermal carbonization (HTC) on the immobilization of P in cow manure. The results demonstrated that the P content in cow manure was increased substantially by ∼20% after HTC, while the water-extractable P (WEP) and Mehlich-3-extractable P (MEP) in manure was reduced significantly by >80% and 50%, respectively. The decrease in P solubility might result from the increased apatite P (increased by >85%) and decreased soluble Ca (decreased by ∼50%) after HTC. These results suggested that HTC could be an efficient strategy to immobilize P in cow manure, thereby potentially mitigating the P loss problem from cow manure.