Inhibition of phosphorus sorption to goethite, gibbsite, and kaolin by fresh and decomposed organic matter

The direct effects of dissolved organic matter (DOM) on the sorption of orthophosphate onto gibbsite, goethite, and kaolin were examined using a one-point phosphorus sorption index and the linear Tempkin isotherm model. DOM extracted from fresh and decomposed agricultural residues, as well as model organic and humic acids, were used. Changes in the chemical and sorptive characteristics of the DOM in the absence and presence of added orthophosphate (50 mg l⁻¹) were also determined. For residue-derived materials, DOM sorption to all minerals correlated well with percent hydrophobicity, apparent molecular weight, and phenolic acidity in the absence of added orthophosphate. Sorption of DOM to goethite and gibbsite was significantly decreased in the presence of added P. The correlation coefficient values of percent hydrophobicity, apparent molecular weight, and phenolic acidity to sorption also declined in the presence of added P. Thus, the addition of P substantially lowered fractionation of DOM after sorption to goethite and gibbsite. In contrast, few significant P sorption-induced differences were observed in the kaolin system. According to one-point P sorption results, DOM in the form of Aldrich humic acid, oxalate, and decomposed clover and corn residue, significantly inhibited P sorption to goethite at concentrations of 50 and 200 mg total soluble carbon (C_TS l⁻¹). Phosphorus sorption to gibbsite was significantly inhibited by 50 mg C_TS l⁻¹ derived from decomposed corn residue, fresh dairy manure residue, and oxalate solution. At 200 mg C_TS l⁻¹, all DOM solutions were found to inhibit P sorption to gibbsite. This study suggests that DOM inhibition of P sorption depends on the chemical properties of both the sorbent and the DOM itself. In general, DOM from decomposed organic materials inhibited P sorption to a greater extent than did DOM derived from fresh materials. This stronger inhibition highlights the importance of microbial processes in the release of soluble soil P, a key determinant of P availability to plants.     

Retention of cobalt by an oxisol in relation to the content of iron and manganese oxides

Abstract

The study aims at determining the cobalt retention properties of various soil components. Therefore, cobalt (Co) sorptions and extractions were carried out using an Oxisol sample before (untreated) and after successive removal of organic matter and active manganese (Mn) oxides (H₂O₂‐treated) and iron (Fe) oxides (H₂O₂+CBD‐treated). A synthetic goethite was included for comparison. Sorption of the four sorbents was determined over a range of Co concentrations (initially 10^‐8 M to 10^‐4 M), pH values (3 to 8) and reaction times (2 hours to 504 hours). The Co species sorbed was Co(ll), since oxygen exclusion during sorption had no effect on the amount sorbed. The pH‐dependent sorption curve (sorption edge) was shifted to lower pH at decreasing initial Co concentration and increasing reaction time. The displacements, in particular of the sorption edges corresponding to the lowest initial Co concentrations, to successively higher pH following removal of Mn oxides, organic matter and Fe oxides could be attributed to sorption onto sites of decreasing Co affinity [Mn oxides (and organic matter) > Fe oxides > kaolinite]. Extractions of sorbed Co at pH 5.5–7.5 with 2 M HCI showed that the extractability decreased with increasing sorption time and decreasing initial Co concentration. The untreated and H₂O₂‐treated soil samples retained sorbed Co at least as firmly as the synthetic goethite, whereas the H₂O₂+CBD‐treated sample (kaolinite) was clearly less effective. The results emphasized the importance of the soil Mn and Fe oxides for Co retention in soils but also the necessity of taken interior sorption sites into consideration.     

Simple modelling of the sorption of iron‐cyanide complexes on ferrihydrite

The sorption of the iron‐cyanide complexes ferricyanide, [Fe(CN)₆]^3—, and ferrocyanide, [Fe(CN)₆]^4—, on ferrihydrite was investigated in batch experiments including the effects of pH (pH 3.5 to 8) and ionic strength (0.001 to 0.1 M). The pH‐dependent sorption data were evaluated with a model approach by Barrow (1999): c = a exp(bS)S/(S_max‐S), where c is the solution concentration; S is the sorbed amount; S_max is maximum sorption; b is a parameter; and a is a parameter at constant pH. Ferricyanide sorption was negatively affected by increasing ionic strength, ferrocyanide sorption not at all. More ferricyanide than ferrocyanide was sorbed in the acidic range. In the neutral range the opposite was true. Fitting the pH‐dependent sorption to the model resulted in a strong correlation for both iron‐cyanide complexes with a common sorption maximum of 1.6 μmol m^—2. Only little negative charge was conveyed to the ferrihydrite surface by sorption of iron‐cyanide complexes. The sorption of iron‐cyanide complexes on ferrihydrite is weaker than that on goethite, as a comparison of the model calculations shows. This may be caused by the lower relative amount of high‐affinity sites present on the ferrihydrite surface.     

Dissolved organic matter sorption on sub soils and minerals studied by 13C-NMR and DRIFT spectroscopy

Sorption on the mineral matrix is an important process restricting the movement of dissolved organic matter (DOM) in soils. In this study, we aimed to identify the chemical structures responsible for the retention of DOM by sorption experiments with total DOM and acidic humic substances (AHS), containing humic and fulvic acids, on soil samples and minerals (goethite, ferrihydrite, and amorphous Al(OH)₃). The AHS remaining in solution after sorption were studied by ¹³C nuclear magnetic resonance (NMR) analysis, and total DOM and AHS for bed on the surfaces of minerals by diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy. The soil samples were taken from strongly sorbing Bw horizons of two Inceptisols rich in pedogenetic Fe (29 and 35 g kg ^?1) and containing little C (7 and 22 g kg^?1). The ¹³C-NMR spectra showed that sorption causes a preferential removal of aromatic and carboxyl C from the solution, whereas alkyl-C accumulates in the solution. No change was observed for O-alkyl C. The DRIFT spectra of sorbed total DOM and AHS showed a relative increase of the band intensity of carboxyl groups compared to DOM in the initial solution, confirming the importance of those groups for the sorption to mineral surfaces. The spectra also indicated reactions of carboxyl groups with metals at the mineral surfaces. The extent to which the carboxyl groups are bound depended on the surface coverage with DOM and the type of mineral.     

Adapted DAX‐8 fractionation method for dissolved organic matter (DOM) from soils: development,calibration with test components and application to contrasting soil solutions

Most methods to fractionate natural dissolved organic matter (DOM) rely on sorption of acidified DOM samples onto XAD‐8 or DAX‐8 resin. Procedural differences among methods are large and their interpretation is limited because there is a lack of calibration with DOM model molecules. An automated column‐based DOM fractionation method was set up for 10‐ml DOM samples, dividing DOM into hydrophilic (HPI), hydrophobic acid (HPOA) and hydrophobic neutral (HPON) fractions. Fifteen DOM model components were tested in isolation and in combination. Three reference DOM samples of the International Humic Substances Society were included to facilitate comparison with other methods. Aliphatic low‐molecular‐weight acids (LMWAs) and carbohydrates were classified as HPI DOM, but some LMWAs showed also a partial HPO character. Aromatic LMWAs and polyphenols partitioned in the HPOA fraction, menadione (quinone) and geraniol (terpenoid) in HPON DOM. Molecules with log K_ow > 0.5 had negligible HPI fractions. The HPO molecules except geraniol had specific UV absorbance (SUVA, measure for aromaticity) >3 litres g⁻¹ cm⁻¹ while HPI molecules had SUVA values <3 litres g⁻¹ cm⁻¹. Distributions of DOM from eight soils ranged from 31 to 72% HPI, 25 to 46% HPOA and 2 to 28% HPON of total dissolved organic carbon. The SUVA of the HPI DOM was consistently smaller compared with the HPOA DOM. The SUVA of the natural DOM samples was not explained statistically by fractionation and the variation coefficient of SUVA among samples was not reduced by fractionation. Hence, fractionation did not reduce the variability in this DOM property, which casts some doubts on the practical role of DOM fractionation in predicting DOM properties.     

黑土、潮土和红壤可溶性有机质的光谱特征及结构差异

为探究不同类型土壤可溶性有机质(DOM)含量和结构的差异性,选取黑土、潮土和红壤3种土壤的表层土壤(0～20 cm)为研究对象,提取其中的DOM,应用紫外–可见光谱、荧光光谱等技术,分析土壤中DOM的数量和光谱特征.结果显示:3种不同类型土壤中可溶性有机碳(DOC)含量及其与土壤有机碳(SOC)的比值(SOC/DOC)...     

Sorption of iron‐cyanide complexes on goethite investigated in long‐term experiments

The iron‐cyanide complexes ferrocyanide, [Fe^II(CN)₆]^4–, and ferricyanide, [Fe^III(CN)₆]^3–, are anthropogenic contaminants in soil. We investigated their sorption on goethite, α‐FeOOH, in batch experiments in a time range from 1 d to 1 yr, their desorption by phosphate and chloride as well as their surface complexes on goethite by Fourier‐transform infrared spectroscopy (FTIR). The sorption of both complexes continued over the whole time range. Percent desorption of ferricyanide by phosphate decreased, whereas that of ferrocyanide increased until it amounted to approximately 87% for both complexes. By FTIR spectroscopy inner‐sphere complexation of both complexes on the goethite surface was indicated. With both complexes, a Berlin‐Blue‐like layer (Fe₄[Fe(CN)₆]₃) was formed initially on the goethite surface which disappeared with increasing reaction time. After at least 30 d reaction time, ferricyanide was the only sorbed iron‐cyanide complex detected even when ferrocyanide was initially added. This resulted from slow oxidation of ferrocyanide, most probably by dissolved oxygen. Based on all results, we propose that ferricyanide forms monodentate inner‐sphere complexes on the goethite surface.     

Stabilization of dissolved organic matter by sorption to the mineral soil

The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We quantified mineralized C by frequent CO₂ measurements in the headspace of the incubation vessels and calculated mean residence times by a double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence) and isotope (¹³C, ¹⁴C) measurements after hot-water extraction of OM.The fraction of sorbed organic C mineralized during the incubation was only one-third to one-sixth of that mineralized in solution. The mean residence time of the most stable OM sample was estimated to increase from 28 years in solution to 91 years after sorption. For highly degradable DOM samples, the portion of stable C calculated by a double exponential model nearly doubled upon sorption. With less degradable DOM the stability increased by only 20% after sorption. Therefore, the increase in stability due to sorption is large for labile DOM high in carbohydrates and relatively small for stable DOM high in aromatic and complex molecules. Nevertheless, in terms of stability the rank order of OM types after sorption was the same as in solution. Furthermore, the extent of sorption of recalcitrant compounds was much larger than sorption of labile compounds. Thus, sorptive stabilization of this stable DOM sample was four times larger than for the labile ones. We conclude that stabilization of OM by sorption depends on the intrinsic stability of organic compounds sorbed. We propose that the main stabilization processes are selective sorption of intrinsically stable compounds and strong chemical bonds to the mineral soil and/or a physical inaccessibility of OM to microorganisms. The UV, fluorescence and ¹³C measurements indicated that aromatic and complex compounds, probably derived from lignin, were preferentially stabilized by sorption of DOM. The ¹³C and ¹⁴C data showed that degradation of the indigenous OM in the mineral soil decreased after sorption of DOM. We estimated DOM sorption stabilizes about 24 Mg C ha⁻¹ highlighting the importance of sorption for accumulation and preservation of OM in soil.     

Retention of Ni,Zn and Cd by Si-associated goethite

Synthetic goethite used to study the effects of reaction time and temperature on the pH-dependent sorption of Ni, Zn and Cd was associated with amorphous silica. Ni interacted with dissolved Si and formed a Ni/Si precipitate on the goethite surface. Individual metals added at a concentration of 0.5 μmol g^?1 and sorbed during a reaction period of 504 hours (21 days) at 35°C were extracted by 0.7 M HNO₃ for 14 days. At the end of this period 11,28 and 40 percent of Ni, Cd and Zn, respectively, were not extracted whereas 20 percent of the total Fe content of the goethite and 39 percent of the associated Si were dissolved. During the sorption process metals became immobilized in the goethite particles. This effect can be related to a diffusion of metal ions into micropores. A total mobilization of sorbed metals can only be achieved by a complete dissolution of the goethite. The strong fixation of Ni, Zn and Cd by goethite suggests that additions of this Fe oxide could be used to ameliorate highly contaminated sludges or soils.     

Rate of desorption of eight heavy metals from goethite and its implications for understanding the pathways for penetration

We compared rates of desorption of heavy metals from goethite, an important soil constituent, in order to understand the mechanisms of sorption and desorption better. Samples of goethite were reacted with salt solutions of heavy metals for 2 hours or for 8 weeks. The metals were Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb. Desorption was then induced by successive treatments with 0.7 m HNO₃ for up to 360 hours followed by complete dissolution of the goethite particles. After brief sorption (2 hours), a large proportion of the sorbed metals was desorbed by brief treatment (15 minutes) with HNO₃. This suggests that the metals were mainly sorbed on, or close to, external goethite surfaces. After longer sorption (8 weeks), even 360 hours of extraction with 0.7 m HNO₃ did not recover all of the sorbed metals. Complete dissolution of the goethite particles was necessary. This suggests that the eight heavy metals had penetrated the goethite particles deeply. When desorption was summarized using a pragmatic equation, it was only for brief sorption followed by brief desorption that indices of the rate of desorption were well correlated with measures of the rate of sorption as obtained in an earlier work. When desorption was described with a mechanistic model, observed desorption was often faster than predicted and the discrepancy was greatest with the shorter periods of desorption. The discrepancy was marked for Ni, Cr and Co. We think this was because large portions of these metals were incorporated into the goethite structure by lattice diffusion and were therefore held close to the surface. When the acid dissolved some of the goethite these metals were released. For long‐term desorption and for most metals, the ratio of observed to predicted desorption decreased with increasing ionic radius. We think that this reflects a changing balance between lattice and pore diffusion, with lattice diffusion more important for metals with radii near that of Fe (Cr, Co, Ni), and pore diffusion more important for the larger cations of Cd and Pb. Manganese was an exception. Desorption was at first faster than predicted but then declined to be close to predicted values. This suggests that Mn mostly penetrated more deeply by pore diffusion.     

Sorptive stabilization of organic matter by microporous goethite: sorption into small pores vs. surface complexation

Stabilization of organic matter (OM) by sorption to minerals is thought to be due to (i) sorption into small pores (Ø < 50 nm) that prevents hydrolytic enzymes approaching and decomposing the organic substrate or (ii) reduced availability of organic molecules after formation of strong multiple bonds by complexation of organic ligands at mineral surfaces. We tested these two potential mechanisms by studying the binding of dissolved OM to microporous goethite (α‐FeOOH). The size of organic molecules dissolved prior to and after equilibration with goethite was determined using atomic force microscopy (AFM). The goethite–OM complexes were analysed for bulk and surface elemental composition (by X‐ray photoelectron spectroscopy, XPS), specific surface area (SSA) and mesopore and micropore volumes (by N₂ adsorption/desorption), by scanning electron microscopy (SEM), and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The absolute density of goethite–OM complexes was determined by gas pycnometry and the sorbed OM’s apparent density was calculated by assuming no major changes in the volumes of the goethite upon sorption of OM. The stability of the OM–mineral interactions was tested in desorption experiments and by treatment with NaOCl. Surface accumulation of OM by sorption decreased the N₂‐accessible SSA of the goethite, mostly because micropores (Ø < 2 nm) were rendered inaccessible to N₂. The decrease in accessibility of micropores was most pronounced at small surface OM concentrations. The majority of dissolved organic molecules detected with AFM prior to interaction with goethite were globular with a diameter of 4–10 nm, the rest were mainly linear, 20–100 nm long and 4–8 nm thick. After contact with goethite, the latter type of molecules dominated, which suggests preferential sorption of globular molecules. Their size exceeded or equalled the size of micropores and small mesopores (Ø < 10 nm) and so sorption therein is unlikely. Also, the changes in volumes of pores with a size of 2–50 nm were smaller than the estimated volume of the OM sorbed. The apparent density of sorbed OM always exceeded that of the freeze‐dried OM and was largest at small surface concentrations. DRIFT spectroscopy showed that most carboxyl groups at the goethite surface were in their complexed form. The proportion of complexed carboxyl groups dropped at larger surface concentrations, parallel to the decrease in micropore volume. Thus, micropores seem to favour the formation of multiple complex bonds per molecule. Scanning electron microscopy showed that at small surface concentrations, OM coated the goethite crystals and crystallites tightly, while at larger surface concentrations bulky accumulations of OM were more abundant. Even strongly desorbing reagents such as NaOH and Na pyrophosphate released only part of the sorbed OM. Treatment with NaOCl removed mainly bulky accumulations of OM; the OM tightly bound to goethite crystals was hardly affected by NaOCl. We conclude that molecules tightly bound via multiple complex bonds, probably at the mouths of small pores, are barely desorbable and resist the attack of chemical reagents and probably also of enzymes.     

Abiotic solubilization of soil organic matter,a less-seen aspect of dissolved organic matter production

Dissolved organic matter (DOM) is a small but reactive pool of the soil organic matter (SOM) that contributes to soil dynamics including the intermediary pool spanning labile to resistant SOM fractions. The solubilization of SOM (DOM production) is commonly attributed to both microbially driven and physico-chemically mediated processes, yet the extent to which these processes control DOM production is highly debated. We conducted a series of experiments using ¹³C-ryegrass residue or its extract (¹³C-ryegrass-DOM) separately under sterile and non-sterile conditions to demonstrate the importance of DOM production from microbial and physico-chemical processes. Soils with similar properties but differing in parent material were used to test the influence of mineralogy on DOM production. To test the role of the source of C for DOM production, one set of soils was leached frequently with ¹³C-ryegrass-DOM and in the other set of soils ¹³C-ryegrass residue was incorporated at the beginning of the experiment into the soil and soils were leached frequently with 0.01 mol L⁻¹ CaCl₂ solution. Leaching events for both treatments occurred at 12-d intervals over a 90-day period. The amount of dissolved organic C and N (DOC and DON) leached from residue-amended soils were consistently more than 3 times higher in sterile than non-sterile soils, decreasing with the time. Despite changes in the concentration of DOC and DON and the production of CO₂, the proportion of DOC derived from the ¹³C-ryegrass residue was largely constant during the experiment (regardless of microbial activity), with the majority (about 70%) of the DOM originating from native SOM. In ¹³C-residue-DOM treatments, after successive leaching events and regardless of the sterility conditions i) the native SOM consistently supplied at least 10% of the total leached DOM, and ii) the contribution of native SOM to DOM was 2–2.9 times greater in ¹³C-residue-DOM amended soils than control soils, suggesting the role of desorption and exchange reactions in DOM production in presence of fresh DOM input. The contribution of the native SOM to DOM resulted in higher aromaticity and humification index. Our results suggest that physico-chemical processes (e.g. exchange or dissolution reactions) can primarily control DOM production. However, microbial activity affects SOM solubilization indirectly through DOM turnover.     

Contribution of dissolved organic matter to carbon storage in forest mineral soils

Dissolved organic matter (DOM) is often considered the most labile portion of organic matter in soil and to be negligible with respect to the accumulation of soil C. In this short review, we present recent evidence that this view is invalid. The stability of DOM from forest floor horizons, peats, and topsoils against microbial degradation increases with advanced decomposition of the parent organic matter (OM). Aromatic compounds, deriving from lignin, likely are the most stable components of DOM while plant‐derived carbohydrates seem easily degradable. Carbohydrates and N‐rich compounds of microbial origin produced during the degradation of DOM can be relatively stable. Such components contribute much to DOM in the mineral subsoil. Sorption of DOM to soil minerals and (co‐)precipitation with Al (and probably also with Fe), especially of the inherently stable aromatic moieties, result in distinct stabilization. In laboratory incubation experiments, the mean residence time of DOM from the Oa horizon of a Haplic Podzol increased from <30 y in solution to >90 y after sorption to a subsoil. We combined DOM fluxes and mineralization rate constants for DOM sorbed to minerals and a subsoil horizon, and (co‐)precipitated with Al to estimate the potential contribution of DOM to total C in the mineral soil of a Haplic Podzol in Germany. The contribution of roots to DOM was not considered because of lack of data. The DOM‐derived soil C ranges from 20 to 55 Mg ha^–1 in the mineral soil, which represents 19%–50% of the total soil C. The variation of the estimate reflects the variation in mineralization rate constants obtained for sorbed and (co‐)precipitated DOM. Nevertheless, the estimates indicate that DOM contributes significantly to the accumulation of stable OM in soil. A more precise estimation of DOM‐derived C in soils requires mineralization rate constants for DOM sorbed to all relevant minerals or (co‐)precipitated with Fe. Additionally, we need information on the contribution of sorption to distinct minerals as well as of (co‐)precipitation with Al and Fe to DOM retention.     

Phosphorus exchange properties of European soils and sediments derived from them

The sorption and desorption of phosphorus (P) from eroding soil particles in land runoff are important processes contributing to agriculturally‐driven eutrophication. We investigated the P‐exchange properties and related chemical characteristics of contrasting European agricultural soils and sediment material eroded from them under indoor (small‐scale) and outdoor (larger‐scale) rainfall simulations. Quantity‐intensity (Q/I) relationships revealed large variation in equilibrium P concentrations at zero net P sorption (EPC₀) (0–10.3 mg l⁻¹) and instantly labile P (−Q₀, the amount of P to be desorbed to obtain a P equilibrium concentration of 0 mg l⁻¹) (2–75 mg kg⁻¹), both correlating closely with Al‐bound P and the P saturation degree of Al oxides (DPS_Alox). Maximum P sorption (Q_max) (43–515 mg kg⁻¹) also correlated most closely with Al_ox. The indoor and outdoor rainfall simulations produced sediments with different P sorption properties: in the indoor simulation (less kinetic energy, constant slope), the sediments had larger EPC₀ values, and usually larger −Q₀ values, than the sediments in the outdoor simulation (greater kinetic energy, variable slopes). Furthermore, the P exchange properties of the sediments differed from those of the bulk soil depending on the enrichment of soil P‐sorption components (Fe/Al oxides, clay). The outdoor simulation indicated that sites with gentle slopes produced sediments that were more enriched with Al_ox, Fe_ox, Mn_ox and organic C than those with steeper slopes. In this study, when the bulk soil had an initial EPC₀ greater than 1.3 mg l⁻¹, the outdoor rainfall simulation produced sediment with smaller EPC₀ and vice versa, indicating that, depending on the P status of the bulk soil, the sediment material was acting as source or sink for P during transport. However, on the basis of their EPC₀ values, most eroding sediments might be expected to desorb, rather than adsorb, P when entering surface water.     

Chlorophenol binding to dissolved and particulate soil organic matter determined in controlled equilibrium systems

We determined the sorption of 2,4‐dichlorophenol (DCP), 2,4,5‐trichlorophenol (TCP) and pentachlorophenol (PCP) to dissolved (DOM) and particulate soil organic matter (POM) from the same soil in controlled equilibrium systems, using ¹⁴C‐labelled chlorophenols in combination with reversed‐phase high‐performance liquid chromatography (RP‐HPLC) and liquid scintillation. Associations of DCP, TCP and PCP to DOM and POM were satisfactorily described by linear adsorption isotherms. Together with the absence of substantial competition between DCP and TCP for binding sites, this indicates a hydrophobic partitioning mechanism. The organic carbon normalized partitioning coefficient (K_OC) for the binding of DCP was similar in magnitude for POM (K_POC) and for DOM (K_DOC), whereas K_POC for the more hydrophobic compounds TCP and PCP were approximately one order of magnitude greater than K_DOC. On the basis of the relationships between log K_OC and the organic carbon normalized partitioning coefficient (log K_OW), the extent of association to POM increases more with the hydrophobicity of the chlorophenol than the extent of association to DOM. This holds for our data obtained for DOM and POM of similar origin, as well as for various sources of POM and DOM reported in the literature. Differences in the magnitude of K_POC and K_DOC in our study could not be accounted for by differences in gross carbon chemistry of POM and DOM, as determined by nuclear magnetic resonance (¹³C‐NMR) and X‐ray photoelectron spectroscopy (XPS). Thus, other factors such as the average size and capacity of hydrophobic moieties could explain differences in chlorophenol association between POM and DOM. We conclude that K_POC and K_DOC need to be determined explicitly, when the transport and retention of chlorophenols is modelled, and not calculated from relationships between log K_OC and log K_OW.     

Effect of soil microorganisms on the sorption of zinc and lead compounds by goethite

The objectives of this study were (1) to determine the effect of microorganisms during in‐vitro incubation on the amount of Zn and Pb from solution retained on goethite precipitated as coatings on a sand matrix and (2) to evaluate accumulation of heavy metals in the biomass of soil microorganisms in the fresh soil samples using an extractive approach. A mixture of colonies of cultivated microorganisms extracted from a Haplic Luvisol (Russia) and an Antropi‐urbic Regosol (Germany) were used to prepare the cell and the microbial‐debris suspensions. The concentrations of Zn and Pb in the studied solutions supplied with microbial suspensions and/or goethite coated sand were 0.1 mM (130.8 and 414 mg kg^–1 of sand, respectively). Exchangeable forms of metals were determined by extraction with 10 mL of 1.0 M KNO₃. Nonexchangeable forms of Zn and Pb were recovered using 40 mL of 0.3 M NH₂OH‐HCl in 1 M HNO₃. Concentrations of Pb increased in the solutions and decreased on the surface of the Fe‐mineral due to living microorganisms. In comparison to incubation of heavy‐metal solutions with goethite only, the absolute concentrations of nonexchangeable forms of metal were reduced by microbial suspension to a greater extent than those of the exchangeable forms, whereas the relative content of both fractions decreased by a factor of almost two. Sorption of Pb by goethite was inversely correlated with the concentration of organic C in the solution. Microorganisms clearly influenced the Zn sorption by goethite at concentrations of C_org > 400 mg L^–1. The amount of Zn retained was decreased primarily due to decreasing Zn portions in the exchangeable fraction. Microbial debris prepared by autoclaving reduced the Pb sorption by goethite similar to the results for living cells. Living microorganisms accumulated more Zn than did microbial debris. The data of this paper show that a direct determination of heavy‐metal accumulation in soil microorganisms by extraction with 2.0 M KCl as well as by extraction with 1 M CH₃COONH₄ at the natural pH of the soils after chloroform fumigation of fresh soils samples with different concentrations of organic C was not possible.     

SORPTION OF INORGANIC PHOSPHATE BY IRON- AND ALUMINIUM- CONTAINING COMPONENTS

The amounts of inorganic P sorbed by a range of Fe- and Al- containing components varied appreciably and decreased in the order allophane > fresh Al gel > Fe gel pseudoboehmite > aged Al gel > dried Fe gel > Fe-coated kaolinite > haematite > goethite > akaganeite > gibbsite = ground kaolinite > dispersed kaolinite. Al gel sorbed 30 to 70 times more P than gibbsite, and Fe gel sorbed approximately 10 times more P than its crystalline analogues (haematite, goethite. and akaganeite). Despite large differences in the extent of P sorption, the form of the isotherm was essentially the same for each sorbent. The ability of freshly-prepared Al gel suspensions to sorb P decreased with ageing, a property not shown by Fe gel. Drying of Fe gel at 80°C, however, caused an approximately 4-fold decrease in P sorption. Precipitation of Fe gel (2% Fe) on the surface of kaolinite increased P sorption by a factor of 10. The occurrence of Fe gel as a coating apparently presents more sorption sites to solution per unit weight of Fe gel than Fe gel alone. A linear relationship (r= 0.98) was obtained between the amount OH^? sorbed per unit increase in pH value (‘hydroxyl buffering’) and the overall P sorption maximum for each sorbent. Hydroxyl buffering provided a better index of P sorption potential than specific surface area. Except for the crystalline Fe sorbents, isotherms obtained by plotting fractional sorption saturation against final solution P concentration for the sorbents were essentially coincident with those for several contrasting soils. For crystalline Fe components a lower relative amount of weaker sorption, as opposed to chemisorption, of the overall sorption maximum was obtained. Differences in the extent of P sorption. however, appear to be primarily related to the number of functional M-OH groups presented at the solid-solution interface.     

Sorption of iron-cyanide complexes on goethite

Iron‐cyanide complexes are present in soils on sites of former gas plants and coke ovens. We have studied the sorption of the complexes ferricyanide, [Fe(CN)₆]^3–, and ferrocyanide, [Fe(CN)₆]^4–, on goethite in batch experiments, including the effects of concentration, time, ionic strength, pH, and the extent of reversibility. The sorption of ferricyanide showed features of both outer‐sphere and inner‐sphere complexation: its extent decreased with increasing pH; it depended on ionic strength; it was quickly and completely reversible; and it induced a change in surface electric potential. In contrast, sorption of ferrocyanide depended on pH to a lesser extent and was not affected by ionic strength at different pHs. The desorption was slower and incomplete. For ferrocyanide we conclude that sorption involves inner‐sphere complexation and precipitation of a Berlin‐Blue‐like phase on the goethite surface.     

Specific sorption of trace amounts of cadmium by soils

Abstract

Sorption of trace quantities of Cd in four soils of different chemical and mineralogical properties, was studied. Initial Cd concentrations were between 15 to 150 μg. 1^?1. The sorption isotherms were linear and had a positive intercept in three of the soils, indicating a constant partition‐high affinity sorption isotherm (Giles et. al⁶). The data also followed the Freundlich sorption isotherm, and the Freundlich K parameter was taken as a measure of the relative affinity of the different soils for the Cd metal sorbed. Cadmium sorbed was extracted by IN‐NH₄C1 followed by 0.1N HC1, and the fraction remaining in the soils was considered specifically sorbed Cd. This fraction also followed a linear sorption isotherm, and was around 30% for the four soils studied. The sorption order for the amount of specifically sorbed Cd showed that the Boomer soil (kaolinite‐iron oxides) had the lowest affinity for specific sorption of this metal. This was taken as evidence that kaolinite and iron oxides have a lower capacity for retaining cadmium through specific sorption mechanism(s) than the materials present on the other soils (2:1 layer silicates and humic substances). The existence of specific mecha‐nism(s) responsible by the sorption of trace quantities of Cd in soil solutions has important implications on soil‐plant relationships, Cd mobility in soil profiles and control of Cd activity in soil solutions.