Reaction kinetics of the adsorption and desorption of nickel, zinc and cadmium by goethite. I. Adsorption and diffusion of metals

The reactions of Ni, Zn and Cd with goethite were studied over a range of initial metal concentrations (10⁻⁶ to 10⁻⁴M), pH values (4 to 8), reaction times (2h to 42d) and temperatures (5 to 35°C). The adsorption of metals increased with pH, reaction time and temperature. Adsorption of Ni increased relative to Zn and Cd with increasing time and temperature. The initially rapid adsorption of metals within a few hours was followed by a much slower reaction linearly related to time^1/2, interpreted as diffusion–controlled penetration of goethite. The pH–dependent relative diffusion rates (Ni > Zn > Cd) were influenced by both affinity for goethite surfaces and by ionic radius. Diffusion coefficients of the three metals ranged from about 10⁻¹⁹to 10⁻²⁰cm²s⁻¹. The corresponding activation energies of diffusion were also calculated (Ni 35, Cd 55, Zn 90 kJ mol⁻¹). Our view about the reactions of heavy metals with goethite involves (i) adsorption of metals on external surfaces, (ii) solid–state diffusion of metals from external to internal binding sites, and (iii) metal binding and fixation at positions inside the goethite particles. The general parameters of these processes are related to the hydrolytic properties (p K values) and the ionic radii of the metals. The results show that goethite may be an efficient sink for trace metals.     

Modeling ferricyanide adsorption on goethite: Basics and applications

Ferricyanide, [Fe^III(CN)₆]^3–, is an anthropogenic and potentially toxic contaminant in soil. Its adsorption on goethite has been previously studied, but not evaluated with a surface complexation model (SCM) considering the effects of pH and ionic strength. Therefore, we carried out batch experiments with ferricyanide and goethite suspensions with different ferricyanide concentrations (0.075 mM and 0.15 mM), ionic strengths (0.01 and 0.1 M), and pH (ranging from 4 to 7.4). Adsorption data were then interpreted with the 1‐pK Stern and the charge distribution model assuming monodentate inner‐sphere ferricyanide surface complexes on goethite (lg K = 10.6), which are known from infrared spectroscopy. Furthermore, we applied the SCM to ferricyanide adsorption in previous studies on ferricyanide adsorption in the presence of sulfate and on the solubility of Fe‐cyanide complexes in a suspension of a loess loam. The SCM correctly reflected ferricyanide adsorption in the batch experiments as well as the effects of pH and ionic strength. The SCM also described ferricyanide adsorption in the presence of sulfate, because the ferricyanide adsorption measured and that modeled were significantly correlated (R² = 0.80). Furthermore, we applied the SCM to a study on the solubility of Fe‐cyanide complexes in soil under varying redox conditions so that ferricyanide adsorption on goethite and precipitation of Fe‐cyanide complexes were considered. The actual ferricyanide concentrations were rather reflected when applying the SCM compared to those modeled in an approach in which exclusively precipitation was taken into account. We conclude that ferricyanide adsorption on goethite should be included into geochemical modeling approaches on the mobility of Fe‐cyanide complexes in subsoils.     

Phosphate reactions with natural allophane, ferrihydrite and goethite

The reactions of phosphate with natural samples of allophane, ferrihydrite, hematite and goethite were measured for up to 30 d. The amount of phosphate sorbed on allophane showed the biggest increase with time whereas the amount sorbed on goethite showed the least increase with time. The total amount of phosphate sorbed either at high levels of phosphate addition or after 10 d followed the order hematite < goethite < ferrihydrite < allophane and was probably related to the specific surface. Si was desorbed as phosphate was adsorbed on the minerals.
The reactions of phosphate on allophane involved rapid, strong adsorption, probably at defect sites, followed by weaker adsorption, followed, probably, by disruption of the allophane structure together with precipitation of aluminium phosphates. Previous suggestions either of diffusive penetration of phosphate into surfaces or about the formation of aluminium phosphate coatings, are unlikely to hold for allophane, if all the Al is at the surface and if the structure can be ruptured.
The reactions of phosphate with iron oxides involved a rapid, strong ligand exchange, followed by weaker ligand exchange, and, probably, by a relatively slow penetration at defect sites and pores. Highly crystalline goethite has virtually no slow reaction and therefore solid-state diffusion of phosphate does not readily occur. The extent of phosphate uptake during the slow penetration reactions probably depends on the degree of crystallinity or porosity of iron oxides.
The most reactive adsorbents, such as allophane, ferrihydrite and Al-humus complexes do not have planar surfaces, and this needs to be considered when modelling phosphate reactions.     

Beta-thujaplicin: new quantitative CZE method and adsorption to goethite

Beta-thujaplicin (beta-TH) is a toxic tropolone derivative present in the heartwood of western red cedar (Thuja plicata) and is used as a preservative and antimicrobial additive in a number of commercial goods. beta-TH released from western red cedar timber used outdoor and from other products containing beta-TH may transfer to soil and leach to groundwater and surface waters. The objective of this study was to quantify the adsorption of beta-TH to goethite as a typical model for geosorbents. Adsorption was studied using pH-adjusted goethite suspensions with solid:solution ratios of 1:500, 0.01 M NaNO(3) electrolyte, and 20 degrees C. beta-TH was determined using a new capillary zone electrophoresis (CZE) method providing a detection limit of 0.21 microM. Near-sorption equilibrium was attained within 48 h. beta-TH showed maximum adsorption at low pH (3.8) and a 70% drop in adsorption from pH 6.2 to 8.8. The Langmuir type adsorption isotherm at pH 5.5 approached a maximum adsorption of 220 micromol/g (= 6.2 micromol/m(2)), which is more than twice the amount of phosphate adsorbed under similar conditions. The affinity of beta-TH for goethite is low as compared with organic ligands such as citrate, oxalate, and 2,4-dihydroxybenzoate. The adsorption data and FTIR analyses indicate that beta-TH is most likely adsorbed as monodentate mononuclear surface complexes at the surface of goethite. Hydrophobic adsorption is thought to contribute to the adsorption, in particular at low pH. The strong adsorption of beta-TH to goethite suggests low mobility in most soil environments, the risk of contamination increasing in soils with high pH (calcareous material), low contents of iron and aluminum oxides, phyllosilicates, and organic matter.     

Adsorption isotherms and hysteresis of proton adsorption by goethite

The adsorption of H⁺ions by goethite is only partially reversible and the adsorption process is faster than desorption. The adsorption isotherm shows two inflexion points at pH values 7.5 and 9.5, which can be attributed to the existence of two or more types of OH groups on the surface with different basic strengths. The lack of reversibility is attributed to partial alterations of the surface by rearrangement of surface atoms although contributions of other phenomena to the overall process cannot be excluded.     

Influence of oxalic acid on the adsorption of cadmium at the goethite surface

The adsorption/desorption of oxalic acid and Cd on goethite has been characterized in function of pH in KNO₃ medium. Electrophoretic mobilities and pH titrations show that while anion desorbs, cation adsorbs, the maxima of adsorption being in significantly different pH ranges. Adsorption-desorption phenomena of oxalic acid in function of pH are identical in absence or presence of Cd. Contrarily, not only adsorption, but also desorption of Cd is enhanced in presence of oxalic acid. Surface sites are shown to never be saturated even in presence of excess of ligand and cation. So, competition between anions and cations for surface sites is minor. Ligand in solution does not compete with surface sites for the Cd, probably due to the low tendency of oxalic acid to form complexes with Cd. Behavior of Cd in presence of oxalate is discussed and results suggest a surface-binding of Cd via an oxalate-bridge between the surface and the metallic cation.     

Effect of organic acids on the adsorption of copper,lead, and zinc by goethite

The adsorption of Cu, Pb, and Zn by synthetic goethite was studied in the absence and presence of oxalic, citric, and glutamic acids at different pH values. It was shown that, in the absence of an acid, the content of adsorbed metals increased with the increasing pH. The content of adsorbed cations at constant pH values decreased in the sequence: Cu > Pb > Zn. The simultaneous addition of metal cations and organic acids to the goethite suspension increased the content of the adsorbed elements. The oxalic and citric acids had similar effects on the adsorption of copper and lead in the studied pH range. The metal: acid concentration ratios significantly affected the adsorption of the heavy metals by goethite. An increase in the metal adsorption was observed to a certain metal: acid ratio, which was followed by a gradual decrease. The adsorption of the metals by goethite also depended on the properties of the metal cations and the organic ligands. The observed tendencies were attributed to the complexation of heavy metals with organic acid anions and the simultaneous sorption of acids at positively charged sites on the goethite surface with the formation of mineral-organic compounds, which significantly modified the surface properties of the mineral. The study of the effect of increasing lead concentrations in the solution on the copper adsorption by goethite in the absence, in the presence, and at the addition of an oxalic acid solution to the goethite suspension one hour before the beginning of the experiment showed that lead decreased the adsorption of copper in all the treatments. The possible mechanisms of the processes occurring in the system were considered.     

Competitive adsorption of humus acids and phosphate on goethite, gibbsite and two tropical soils

Competition in adsorption between humic acid (HA) or fulvic acid (FA) and phosphate on synthetic goethite, gibbsite and two tropical soils was studied. The results for both goethite and gibbsite showed that HA and FA competed strongly with phosphate for adsorption sites at low pH values. The soils showed a similar result with a reduction in phosphate adsorption resulting from the addition of HA at the pH of the soils. The competition between HA and phosphate at different pH levels is illustrated by comparing the adsorption envelopes for phosphate on goethite, gibbsite and the two soils in the presence and absence of HA. The trends observed may be explained by the relative positions of the maximum buffer-power (buffer capacity) of the organic acids and of phosphoric acid which are shown to lie in different pH ranges.     

有机酸对高岭石, 针铁矿和水铝英石吸附镉的影响

Effects of organic acids （oxalic, acetic, and citric） on adsorption characteristics of Cadmium （Cd） on soil clay minerals （kaolinite, goethite, and bayerite） were studied under different concentrations and different pH values. Although the types of organic acids and minerals were different, the effects of the organic acids on the adsorption of Cd on the minerals were similar, i.e., the amount of adsorbed Cd with an initial solution pH of 5.0 and initial Cd concentration of 35 mg L^-1 increased with increasing concentration of the organic acid in solution at lower concentrations, and decreased at higher concentrations. The percentage of Cd adsorbed on the minerals in the presence of the organic acids increased considerably with increasing pH of the solution. Meanwhile, different Cd adsorption in the presence of the organic acids, due to different properties on both organic acids and clay minerals, on kaolinite, goethite, or bayerite for different pHs or organic acid concentrations was found.     

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.     

Comparing the effects of pH on the sorption of metals by soil and by goethite, and on uptake by plants

The effects of soil pH on sorption of cadmium, zinc, nickel and cobalt were studied by changing the pH of a soil and measuring sorption. Results were compared with published results for effects of pH on sorption of cadmium, zinc and nickel by goethite. In a further experiment, the effects of pH on the uptake of zinc and cobalt by subterranean clover were measured. Effects of pH on sorption were described in terms of the concentration of metal ions required to produce equal sorption. Where the metal ions were incubated with the soil, unit increase in pH decreased the concentration of metal ions required about 10-fold for zinc, about 7-fold for nickel, about 6-fold for cobalt, and about 4-fold for cadmium. When the soil was mixed with a large volume of solution, the effects were similar for zinc and cadmium but slightly smaller for cobalt and slightly larger for nickel. In all cases, the magnitude of the effect varied somewhat with pH. Sorption was greater with a dilute background electroiyte than with a concentrated one and the effects of pH were greater. The effects for soil were smaller than effects of pH on sorption by iron oxides for which unit increase in pH can decrease the required concentration of Zinc 35-fold and cadmium 11-fold. These results are consistent with adsorption of divalent ions on a variable charge surface that is negatively charged. They are not consistent with the adsorption of monovalent metal ions on a variable charge surface. This mechanism requires at least a 10-fold effect of pH. They show that the change in electric potential with change in pH is smaller for reacting surfaces in soil than for goethite. The effects of changing pH on the amounts of zinc and cobalt fertilizer required for equal uptake by plants was even smaller with unit increase in pH, causing a 1.4-fold increase in the amount of fertilizer required, that is, a 1.4-fold decrease in fertilizer effectiveness.     

Molecular weight and humification index as predictors of adsorption for plant- and manure-derived dissolved organic matter to goethite

Sorptive retention of organic matter is important in maintaining the fertility and quality of soils in agricultural ecosystems. However, few sorption studies have been conducted that use dissolved organic matter (DOM) characteristic of agricultural amendments. We investigated the sorption to goethite (α‐FeOOH) of DOM extracted from: (i) above‐ground biomass of wheat straw (Triticum aestivum L.), maize residue (Zea mays L.), soybean residue (Glycine max (L.) Merr.), and hairy vetch residue (Vivia billosa L.); (ii) below‐ground biomass from maize, soybean, canola (Brassica napus L.), and green bean (Phaseolus vulgaris L.); and (iii) beef, dairy, poultry, and pig animal manures. The apparent molecular weight (MW_AP) of the DOM was measured by high performance‐size exclusion chromatography and ranged from 312 to 1074 g mol⁻¹. The carboxyl‐group content of the DOM measured by potentiometric titration ranged from 4.84 to 21.38 mmol₍₋₎ g⁻¹ carbon. The humification index (HIX) determined by fluorescence spectrometry varied from 1.15 to 4.33. Sorption was directly related to both MW_AP and HIX values of the DOM. Molecular weight analysis of the solution prior to and after sorption indicated that the DOM molecules > 1800 g mol⁻¹ were preferentially sorbed, resulting in fractionation of the DOM upon reaction with goethite. The multiple regression equation, based only on MW_AP and HIX parameters, explained 76% of the variance in amount of DOM sorbed. The results indicate that MW_AP and HIX are important factors in controlling the sorption of DOM to mineral surfaces. Amendment with materials that release DOM of higher molecular weight and greater humification will result in enhanced initial sorption of DOM to soil solids, thereby contributing to accumulation of a larger soil organic C pool.     

Effect of soil components on the adsorption of heavy metals under technogenic contamination

The adsorption of copper, zinc, and lead by two urban soils with different degrees of contamination was studied. Changes in the sorption capacity of the soils and the binding strength of the metals with the soil were determined after the removal of organic matter and iron minerals with a potassium pyrophosphate solution and Tamm’s reagent. The selectivity of these solutions for the dissolution of soil components was estimated.     

A simple and rapid spectrophotometric method to quantify the herbicide glyphosate in aqueous media. Application to adsorption isotherms on soils and goethite

This article presents a simple, fast and low cost UV–vis spectrophotometric method to quantify glyphosate. This method can be used to perform adsorption isotherms on soils and metal oxides. It comprises a derivatization step and further measurement of the absorbance at 265 nm. The trueness of the results is validated using Ultra Performance Liquid Chromatography with tandem mass spectrometry detection (UPLC-MS/MS) as a reference method. The proposed spectrophotometric method is able to quantify glyphosate in the concentration range from 0.084 to 21.8 mg L^? 1. This range is suitable to construct reliable adsorption isotherms. Examples of adsorption isotherms on goethite at pH 4.5 and a soil sample at pH 4.5, 6.0 and 8.0 are given. Interferences caused by dissolved organic matter can be corrected at least up to an organic matter concentration of 12 mg L^? 1.     

The behavior of mercury in the soil with special emphasis on complexation and adsorption processes - A review of the literature

The behavior of Hg in the soil is mainly controlled by adsorption and desorption processes depending on complexation, the most important ligands in solution being OH^?, Cl^?, and organic anions. Since the solubility of HgCl₂ and Hg(OH)₂ is rather high, the affinity of Hg to these ligands leads to an increased mobility. This is especially true for HgCl₂, whereas the hydrolysis of Hg²⁺ may result in the specific adsorption of Hg on mineral colloids. The high affinity of Hg to S explains the strong binding of Hg to soil organic matter and also the stability of HgS. Further precipitation products than HgS are unlikely to occur, since the activity of Hg²⁺ remains too low to exceed the solubility product of any other defined Hg compound. It is mainly the physical fractioning of soil organic matter (dissolved vs adsorbed) that determines the behavior and distribution of Hg in soils.