The complexation of heavy metals with dissolved organic matter (DOM) in the environment influences the solubility and mobility of these metals. In this paper, we measured the complexation of Cu, Cd, Zn, Ni, and Pb with DOM in the soil solution at pH 3.7-6.1 using a Donnan membrane technique. The results show that the DOM-complexed species is generally more significant for Cu and Pb than for Cd, Zn, and Ni. The ability of two advanced models for ion binding to humic substances, e.g., model VI and NICA-Donnan, in the simulation of metal binding to natural DOM was assessed by comparing the model predictions with the measurements. Using the default parameters of fulvic and humic acid, the predicted concentrations of free metal ions from the solution speciation calculation using the two models are mostly within 1 order of magnitude difference from the measured concentrations, except for Ni and Pb in a few samples. Furthermore, the solid-solution partitioning of the metals was simulated using a multisurface model, in which metal binding to soil organic matter, dissolved organic matter, clay, and iron hydroxides was accounted for using adsorption and cation exchange models (NICA-Donnan, Donnan, DDL, CD-MUSIC). The model estimation of the dissolved concentration of the metals is mostly within 1 order of magnitude difference from those measured except for Ni in some samples and Pb. The solubility of the metals depends mainly on the metal loading over soil sorbents, pH, and the concentration of inorganic ligands and DOM in the soil solution.

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Article (refereed) -postprintTipping, E.; Lofts, S.; Sonke, J.E.. 2011. Humic Ion-Binding Model VII: a revised parameterisation of cation-binding by humic substances. Environmental Chemistry, 8 (3). 225-235. 10.1071/EN11016Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. [1,2] incorporating Humic Ion-Binding Model 55 V [3] or VI [4] permits the calculation of equilibrium chemical speciation for waters and soils in 56 which natural organic matter plays a significant role. The ion-binding models are based on 57 conventional chemical reactions involving O-containing weak acids, with empirical estimation 58 of the influence of soft ligand atoms (N, S) and electrostatic corrections, and are 59 parameterised from laboratory studies with isolated humic and fulvic acids. The NICA 60 model [5] is similarly parameterised and provides an alternative picture based on continuous 61 binding-site distributions. Tipping [2] identified both the Humic Ion-Binding Models and NICA 62 as comprehensive models, meaning that they deal with competitive interactions involving all 63 cations (including H + ), and take account of ionic strength effects and metal-proton exchange 64 ratios. They seek to represent cation-binding by the complex mixtures that comprise natural 65 organic matter as efficiently as possible, with the minimum number of parameters, in order to 66 be useful in addressing chemical processes in the environment. A different approach to 67 these parameterised models, but also potentially comprehensive, is the "forward modelling" 68 developed by Cabaniss [6] in which binding is calculated a priori from the known or assumed 69 distributed chemistry of humic substances. 70 Tipping et al. Humic Ion-Binding Model VII_REVISIONWHAM has been applied in a variety of research and regulatory areas. Examples include 71 the acidification of soils [7][8][9][10][11][12][13][14] and surface waters [15] , trace metal behaviour in soils [16][17][18][19][20][21][22] , surface 72 waters [23][24][25][26][27][28][29][30][31] and groundwaters [32] , lake sediment diagenesis [33,34] , rare earth geochemistry [35-73 37] , iron and manganese geochemistry [38][39][40][41] , radionclide geochemistry [42][43][44][45] , organic matter 74 solubility in soils [46,47] , catchment modelling [48,49] , interactions of metals with biota [50,51] , 75 ecotoxicology [52][53][54][55][56][57][58][59] and Critical Loads [60][61][62] . Given this evident utility, it is worthwhile to 76 continue to improve the humic ion-binding model and incorporate new data into its 77 parameterisation. Here we report on activities undertaken towards these goals, namely 78 modification of assumptions about multidentate binding, the fitting of new data, and the 79 introduction of a procedure to obtain more internally-consistent parameters. 80Changes in binding site formulation were prompte...

Abstract-Metal-based nanoparticles (NPs) (e.g., silver, zinc oxide, titanium dioxide, iron oxide) are being widely used in the nanotechnology industry. Because of the release of particles from NP-containing products, it is likely that NPs will enter the soil compartment, especially through land application of sewage sludge derived from wastewater treatment. This review presents an overview of the literature dealing with the fate and effects of metal-based NPs in soil. In the environment, the characteristics of NPs (e.g., size, shape, surface charge) and soil (e.g., pH, ionic strength, organic matter, and clay content) will affect physical and chemical processes, resulting in NP dissolution, agglomeration, and aggregation. The behavior of NPs in soil will control their mobility and their bioavailability to soil organisms. Consequently, exposure characterization in ecotoxicological studies should obtain as much information as possible about dissolution, agglomeration, and aggregation processes. Comparing existing studies is a challenging task, because no standards exist for toxicity tests with NPs. In many cases, the reporting of associated characterization data is sparse, or missing, making it impossible to interpret and explain observed differences in results among studies. Environ. Toxicol. Chem.

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