Humic Ion-Binding Model VII: a revised parameterisation of cation-binding by humic substances

Abstract

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...

Metal speciation from stream to open ocean: modelling v. measurement

“…We have previously performed three such studies focussing on data from fresh waters, [14] estuarine and coastal waters [15] and the open ocean, [16] and comparing them with predictions obtained using WHAM6 [17] or WHAM7. [18] We found that measurements and model predictions based only on metal binding by humic-type ligands (i.e. DOM) were in broad agreement, but in many cases differences were greater than could be explained by data uncertainty, and there was a tendency for the differences to be larger at low metal concentrations.…”

“…The data set is summarised in Tables 1 and S1, with further details in references. [14][15][16] Chemical speciation modelling We used WHAM [10] incorporating Humic Ion-Binding Model VII [18] to perform the speciation calculations; previously, the open-ocean calculations [16] were done using Model VI. [17] Models VI and VII use structured formulations of discrete, chemically plausible binding sites for protons in humic and fulvic acids (HA, FA), in order to allow the creation of regular arrays of bidentate and tridentate binding sites for metals.…”

“…WHAM constants are derived from the results of numerous studies of proton and metal binding by isolated humic substances, together with linear free-energy relationships. [18] To make WHAM7, the humic ion-binding model is combined with an inorganic speciation model, the species list and constants for which were given by Tipping. [10] The inorganic reactions in this database are restricted to monomeric complexes of metals.…”

Lability, solubility and speciation of Cd, Pb and Zn in alluvial soils of the River Trent catchment UK

“…Speciation of soil pore waters To assess the potential toxicity of the metals in the centrifuged pore waters on soil biota we compared free ion concentrations obtained using WHAM 7 21 against the 'critical limits' published by Lofts et al 18 and de Vries et al 19 . Input data to WHAM 7 40 included solution pH, temperature (277 K) and solution concentrations of Na, Mg, Al, K, Ca, Fe(III), Mn, Ni, Cu, Zn, Cd, Pb, Cl -, NO 3 -, SO 4 2-, CO 3 2-, F -and PO 4 3-.…”

Modelling metal accumulation using humic acid as a surrogate for plant roots

“…In the present study, WHAM VII was used for modelling metal accumulation in 135 plant roots (Tipping, 1994;Tipping et al, 2011). In WHAM, metal sorption to humic 136 substances is simulated by using a structured formulation of discrete, chemically-137 simultaneously because the data were generated in mixture exposures.…”