Iodine dynamics in soil

The principal aim of this investigation was to understand the transformation and reaction kinetics of iodide and iodate added to soil in relation to soil properties. In addition, to integrate the data into a predictive model of iodide and iodate sorption kinetics parameterised by soil properties. Solid phase fractionation coupled with solution phase speciation (HPLC-ICPMS) was used to follow the assimilation of 129I- and 129IO3- spikes into ‘steady state’ soil microcosms. The extraction efficiency of tetra-methyl ammonium hydroxide (TMAH) for soil iodine, and the effects of experimental procedures and conditions on the speciation of extracted iodine were tested. Moreover, the possibility of extracting ‘reactive’ inorganic iodine forms sorbed on soil metal oxides by competition with PO43- ions was investigated. Results showed that changing TMAH concentration, extraction time, extraction temperature or soil particle size did not generally affect the concentrations of total iodine extracted. The ratio of iodide to total iodine in the TMAH extracts varied with the extraction conditions which led to the conclusion that part, or all, of the measured iodide is possibly produced by hydrolysis of organic iodine forms. This conclusion was also confirmed by the detection of high concentrations of iodide in TMAH extracts of a humic acid. Only iodide was measured in the phosphate extracts of soil and it constituted up to 33% of the total iodine in the KH2PO4 extracts which indicates that most of the iodine mobilised by KH2PO4 is organically bound. When soil / KH2PO4 suspensions were spiked with 129I- and 129IO3-, at least 50% of 129I- and 15% of 129IO3- were recoverable after 72 hours of reaction. The lowest recoveries were observed with the highest concentration of KH2PO4, which also mobilised the greatest concentrations of DOC, indicating that although KH2PO4 is capable of releasing sorbed iodide and iodate in soil, it may also promote iodide and iodate reaction with soil organic matter. Iodine content of soil biomass was determined following chloroform fumigation of soil. The concentrations of total iodine in fumigated soil samples were only marginally higher than iodine concentration in the control samples indicating that microbial biomass iodine constitutes only a small fraction of total soil iodine (0.01 – 0.25 %). The change in iodine (129I) solubility and speciation in nine soils with contrasting properties (pH, Fe/Mn oxides, organic carbon and iodine contents), incubated for nine months at 10oC and 20oC, was also investigated. The rate of 129I sorption was greater in soils with large organic carbon contents, low pH and at higher temperatures. Loss of iodide (129I-) from solution was extremely rapid, apparently reaching completion over minutes-hours; iodate (IO3-) loss from solution was slower, typically occurring over hours-days. In all soils an apparently instantaneous sorption reaction was followed by a slower sorption process for IO3-. For iodide a faster overall reaction meant that discrimination between the two processes was less clear. Instantaneous sorption of IO3- was greater in soils with high Fe/Mn oxide content, low pH and low organic content, whereas the rate of time dependent sorption was greatest in soils with higher organic contents. Phosphate extraction (0.15 M KH2PO4) of soils, ~100 h after 129I spike addition, indicated that concentrations of sorbed inorganic iodine (129I) were very low in all soils suggesting that inorganic iodine adsorption onto oxide phases has little impact on the rate of iodine assimilation into humus. Transformation kinetics of dissolved inorganic 129IO3- and 129I- to sorbed organic forms was modelled using a range of reaction and diffusion based approaches. Irreversible and reversible first order kinetic models, and a spherical diffusion model, adequately described the kinetics of both IO3- and I- loss from the soil solution but required inclusion of a distribution coefficient (Kd) to allow for instantaneous adsorption. A spherical diffusion model was also collectively parameterised for all the soils studied by using pH, soil organic carbon concentration and combined Fe + Mn oxide content as determinants of the model parameters (Kd and D/r2). From the temperature-dependence of the sorption data the activation energy (Ea) for 129IO3- transformation to organic forms was estimated to be ~43 kJ mol-1 suggesting a reaction mechanism slower than pore diffusion or physical adsorption, but faster than most surface reactions

Chemical and isotopic fractionation of lead in the surface soils of Egypt

Chemical fractionation via sequential extraction (SEP) combined with isotopic analysis of Pb was used to investigate the origins and reactivity of Pb in 66 topsoil samples collected from 12 different locations in Egypt. The total soil Pb concentrations (TPb) covered a wide range (∼80–16,000 mg kg−1), but were only elevated in four industrial and urban locations within Cairo and Alexandria. In all the other locations values of TPb were generally low and were close to the average crustal Pb concentration of 14 mg kg−1. The largest Pb fraction in all soils, with the exception of two industrial locations, was the ‘residual’ fraction (38–63% of TPb) followed by Pb bound to ‘organic’ and ‘metal oxide’ phases. The Pb isotopic signatures (206Pb/207Pb vs 208Pb/207Pb) of all samples in all SEP fractions were highly variable, suggesting a heterogeneous mix of Pb contamination sources; however, they aligned closely to a binary mixing line between geogenic and petrol Pb sources. There were similar Pb isotopic patterns across all of the non-residual fractions with measureable data (F2 – F4) suggesting that the non-residual anthropogenic-Pb and geogenic-Pb have been assimilated into common pools within the soil. Binary and ternary source-apportionment models based on Pb isotopic ratios and abundances showed that the relative contribution of petrol-Pb and geogenic-Pb can be ascribed with reasonable certainty. However, the contribution of further sources can only be accounted for if the isotopic abundance of all end-members are known and are at the periphery of the soils dataset

Iodine binding to humic acid

The rate of reactions between humic acid (HA) and iodide (I-) and iodate (IO3-) have been investigated in suspensions spiked with 129I at concentrations of 22, 44 and 88 µg L-1 and stored at 10oC. Changes in the speciation of 129I-, 129IO3- and mixed (129I-+129IO3-) spikes were monitored over 77 days using liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). In suspensions spiked with 129I- 25% of the added I- was transformed into organic iodine (Org-129I) within 77 days and there was no evidence of 129IO3- formation. By contrast, rapid loss of 129IO3- and increase in both 129I- and Org-129I was observed in 129IO3--spiked suspensions. However, the rate of Org-129I production was greater in mixed systems compared to 129IO3--spiked suspensions with the same total 129I concentration, possibly indicating IO3-—I- redox coupling. Size exclusion chromatography (SEC) demonstrated that Org-129I was present in both high and low molecular weight fractions of the HA although a slight preference to bond with the lower molecular weight fractions was observed indicating that, after 77 days, the spiked isotope had not fully mixed with the native 127I pool. Iodine transformations were modelled using first order rate equations and fitted rate coefficients determined. However, extrapolation of the model to 250 days indicated that a pseudo-steady state would be attained after ~ 200 days but that the proportion of 129I incorporated into HA was less than that of 127I indicating the presence of a recalcitrant pool of 127I that was unavailable for isotopic mixing

Kinetic study of time-dependent fixation of UVI on biochar

Biochar, a by-product from the production of biofuel and syngas by gasification, was tested as a material for adsorption and fixation of UVI from aqueous solutions. A batch experiment was conducted to study the factors that influence the adsorption and time-dependent fixation on biochar at 20oC, including pH, initial concentration of UVI and contact time. Uranium (UVI) adsorption was highly dependent on pH but adsorption on biochar was high over a wide range of pH values, from 4.5 to 9.0, and adsorption strength was time-dependent over several days. The experimental data for pH > 7 were most effectively modelled using a Freundlich adsorption isotherm coupled to a reversible first order kinetic equation to describe the time-dependent fixation of UVI within the biochar structure. Desorption experiments showed that UVI was only sparingly desorbable from the biochar with time and isotopic dilution with 233UVI confirmed the low, or time-dependent, lability of adsorbed 238UVI. Below pH 7 the adsorption isotherm trend suggested precipitation, rather than true adsorption, may occur. However, across all pH values (4.5–9) measured saturation indices suggested precipitation was possible: autunite below pH 6.5 and either swartzite, liebigite or bayleyite above pH 6.5

Historical trends in iodine and selenium in soil and herbage at the Park Grass experiment, Rothamsted Research, UK

Long term trends in iodine and selenium retention in soil, and uptake by herbage, were investigated in archived samples from the Park Grass Experiment, initiated in 1856 at Rothamsted, UK. Soil (0-23 cm) and herbage samples from plots receiving various mineral fertilisers and organic manures, with and without lime, were analysed for Se and iodine (I) to assess the effect of soil amendment, annual rainfall, crop yield and changes in soil chemistry from 1876 to 2008. Comparing soil from limed and un-limed control (unfertilized) plots, TMAH-extractable Se and I concentrations both diverged, with time, with greater retention in un-limed plots; differences in concentration amounted to 92 and 1660 µg kg-1 for Se and I respectively after 105 yr. These differences were broadly consistent with estimated additions from rainfall and dry deposition. Offtake of both elements in herbage was negligible compared to soil concentrations and annual inputs (<0.003% of total soil I and <0.006% of total soil Se). A positive correlation was observed between I and Se concentrations in herbage, suggesting some common factors controlling bioavailability. A growth-dilution effect for I and Se was suggested by the positive correlation between growing season rainfall (GSR) and herbage yield together with soil-to-plant transfer factors decreasing with yield. Phosphate and sulphate fertilizers reduced I and Se herbage concentrations, both through ion competition and increased herbage yield. Results suggest that in intensive agriculture with soil pH control, the I requirement of grazing animals is not likely to be met by herbage alone

Integration of renewable energy and the benefit of storage from a grid and market perspective - results from Morocco and Egypt case studies

This paper presents results from case studies of the future power systems in Morocco and Egypt, with a high increase in renewable generation capacity. Datasets representing 2030 scenarios have been generated and studied with a simplified grid-market model that takes into account variable renewable generation, energy storage and electricity grid constraints. Simulation results for Morocco and Egypt are studied and compared, with emphasis on the benefit of energy storage.acceptedVersio

Data for: Sorption kinetics of isotopically labelled divalent mercury (196Hg2+) in soil

Raw, Processed and Modelling data spreadsheet

Iodine dynamics in soil

The principal aim of this investigation was to understand the transformation and reaction kinetics of iodide and iodate added to soil in relation to soil properties. In addition, to integrate the data into a predictive model of iodide and iodate sorption kinetics parameterised by soil properties. Solid phase fractionation coupled with solution phase speciation (HPLC-ICPMS) was used to follow the assimilation of 129I- and 129IO3- spikes into ‘steady state’ soil microcosms. The extraction efficiency of tetra-methyl ammonium hydroxide (TMAH) for soil iodine, and the effects of experimental procedures and conditions on the speciation of extracted iodine were tested. Moreover, the possibility of extracting ‘reactive’ inorganic iodine forms sorbed on soil metal oxides by competition with PO43- ions was investigated. Results showed that changing TMAH concentration, extraction time, extraction temperature or soil particle size did not generally affect the concentrations of total iodine extracted. The ratio of iodide to total iodine in the TMAH extracts varied with the extraction conditions which led to the conclusion that part, or all, of the measured iodide is possibly produced by hydrolysis of organic iodine forms. This conclusion was also confirmed by the detection of high concentrations of iodide in TMAH extracts of a humic acid. Only iodide was measured in the phosphate extracts of soil and it constituted up to 33% of the total iodine in the KH2PO4 extracts which indicates that most of the iodine mobilised by KH2PO4 is organically bound. When soil / KH2PO4 suspensions were spiked with 129I- and 129IO3-, at least 50% of 129I- and 15% of 129IO3- were recoverable after 72 hours of reaction. The lowest recoveries were observed with the highest concentration of KH2PO4, which also mobilised the greatest concentrations of DOC, indicating that although KH2PO4 is capable of releasing sorbed iodide and iodate in soil, it may also promote iodide and iodate reaction with soil organic matter. Iodine content of soil biomass was determined following chloroform fumigation of soil. The concentrations of total iodine in fumigated soil samples were only marginally higher than iodine concentration in the control samples indicating that microbial biomass iodine constitutes only a small fraction of total soil iodine (0.01 – 0.25 %). The change in iodine (129I) solubility and speciation in nine soils with contrasting properties (pH, Fe/Mn oxides, organic carbon and iodine contents), incubated for nine months at 10oC and 20oC, was also investigated. The rate of 129I sorption was greater in soils with large organic carbon contents, low pH and at higher temperatures. Loss of iodide (129I-) from solution was extremely rapid, apparently reaching completion over minutes-hours; iodate (IO3-) loss from solution was slower, typically occurring over hours-days. In all soils an apparently instantaneous sorption reaction was followed by a slower sorption process for IO3-. For iodide a faster overall reaction meant that discrimination between the two processes was less clear. Instantaneous sorption of IO3- was greater in soils with high Fe/Mn oxide content, low pH and low organic content, whereas the rate of time dependent sorption was greatest in soils with higher organic contents. Phosphate extraction (0.15 M KH2PO4) of soils, ~100 h after 129I spike addition, indicated that concentrations of sorbed inorganic iodine (129I) were very low in all soils suggesting that inorganic iodine adsorption onto oxide phases has little impact on the rate of iodine assimilation into humus. Transformation kinetics of dissolved inorganic 129IO3- and 129I- to sorbed organic forms was modelled using a range of reaction and diffusion based approaches. Irreversible and reversible first order kinetic models, and a spherical diffusion model, adequately described the kinetics of both IO3- and I- loss from the soil solution but required inclusion of a distribution coefficient (Kd) to allow for instantaneous adsorption. A spherical diffusion model was also collectively parameterised for all the soils studied by using pH, soil organic carbon concentration and combined Fe + Mn oxide content as determinants of the model parameters (Kd and D/r2). From the temperature-dependence of the sorption data the activation energy (Ea) for 129IO3- transformation to organic forms was estimated to be ~43 kJ mol-1 suggesting a reaction mechanism slower than pore diffusion or physical adsorption, but faster than most surface reactions.EThOS - Electronic Theses Online ServiceGBUnited Kingdo