An Experimental and Modelling Study of Cu2+ Binding on Humic Acids at Various Solution Conditions. Application of the NICA-Donnan Model

Humic substances are characterized by a strong binding capacity for both metals and organic pollutants, affecting their mobility and bioavailability. The understanding of the mechanisms of proton and metal binding to humic substances is of fundamental importance in geochemical modelling and prediction of cation speciation in the environment. This work reports results on copper binding on humic acids obtained through a thorough experimental and modelling approach. Two humic acids, a reference purified peat humic acid isolated by the International Humic Substances Society (IHSS) and a humic acid from a Greek soil, were experimentally studied at various pH values (4, 6 and 8), humic acid concentrations (ranging from 20 to 200 mg?L^?1) and ionic strength (0.1 and 0.01 M NaNO₃). The binding of copper to humic acids was determined over wide ranges of copper ion concentrations using a copper ion selective electrode. The copper binding isotherms obtained at different conditions have shown that copper binding is dependent on the pH and ionic strength of the solution and on the concentration of both humic acids. Copper binding experimental data were fitted to non-ideal competitive adsorption NICA-Donnan model and the model parameter values were calculated. Both Cu²⁺ and CuOH⁺ species binding to humic acid with different binding affinities were considered. Two sets of the NICA-Donnan parameters have been calculated: one for humic acid concentrations of ??100 mg?L^?1and one for humic acid concentration of 20 mg?L^?1. The meaning of the parameters values for each concentration level is also discussed.     

Addition of sulphuric acid to high organic carbon lake water: Effects on macro-chemistry,aluminium, and iron

A humic lake of pH 5.6 was acidified with H₂SO₄ to pH 4.1. Measurements of total and hollow-fiber ultrafiltered samples were made after three different times of storage, before and after the acid treatment. The nominal molecular weight cutoff of the hollow-fiber membrane was 10 kDalton. Assuming a linear molecular weight distribution of the organic complexes present in solution, the average organic molecule had an average molecular weight of 12.8?08 kDalton (n=6). Not only Ca²⁺ and Mg²⁺, but also detectable amounts of Na⁺ and K⁺ was found to be present on high molecular weight forms. No significant change in the molecular weight distribution of these elements were observed after the pH decrease. Changes in the molecular weight distribution after the acid treatment were only observed for Fe and Al. Significant amounts of SO₄ ^2? were present on high molecular weight forms. A small, but significant increase in the relative amounts of SO₄ ^2? present on high molecular weight forms was observed after the pH lowering. Kinetic constraints were demonstrated for dissolution of Al and Fe. To some extent, kinetic constraints in the equilibrium distribution of cation/anion exchange reactions of Al, Fe, and SO₄ ^2? were also observed. After the acid treatment, the cation exchange capacity (CEC) of the organic pool present was estimated to be at least 18.2±1.4 (n=3) μeq of positive charges per mg C, probably because the negative sites on the organic pool are either not totally protonated or occupied by other cations at pH 4.09. This CEC is of the same order as industrially made cation exchange resins.     

Formation of metal-humic acid complexes by titration and their characterization by differential thermal analysis and infrared spectroscopy

Humic acid (HA) extracted from a Eustis loamy sand (Psammentic Paleudult, Red Yellow Podzolic soil) was flocculated by titration with Al³⁺-, Fe³⁺-, Cu²⁺-, Zn²⁺-, Mn²⁺-, Ba²⁺-, Ca²⁺-, and Mn²⁺-chloride solutions, respectively, to determine possible development of metal-HA complexes, as reported by Flaig et al. (1975), and Tiurin and Kononova (1962). Titration was conducted with HA solutions with an initial pH 11.5 or 7.0. The results indicated that the cations used, except Mg²⁺, yielded insoluble complexes with HA, irrespective of initial pH. After titration, the pH of the metal-HA flocs was 6.0–7.0, which was expected in view of the presence of cation exchange and buffering capacity of HA compounds. More complex formation through electrovalent and covalent bonding by COO^? and phenolic OH groups of the HA molecule was only attained by the use of HA solutions with pH 11.5. On the other hand, less complex formation occurred by the use of HA solutions with an initial pH 7.0, through electrovalent bonding by COO^? groups. Differential thermal analysis (d.t.a.) curves of HA showed shifts in temperatures of the main decomposition peak as a result of flocculation with the different metals. Based on the type of the cations involved, the metal-humic acid flocs could be listed in the following decreasing order of thermal stability: Al³⁺ = Zn²⁺ = Mg²⁺ ≥ HA > Ca²⁺ > Ba²⁺ > Fe³⁺ > Cu²⁺ > Mn²⁺. A systematic relationship could not be found indicating that trivalent ions resulted in the formation of thermally less stable metal-humic acid flocs than divalent ions, as has been reported for HA-metal complexes. Physical mixtures of HA and metal hydroxides exhibited d.t.a. features resembling those of original (nontreated) HA, but not those of the HA-metal flocs.Infrared spectroscopy revealed increased absorption for COO^? vibrations at 1620 and 1400cm^?1 in spectrograms of metal-HA flocs compared to that of original humic acid, a phenomenon explained by many authors to be caused by bonding of the metal ions in hydrated form to the carboxyl or phenolic hydroxyl groups or both of the humic acid molecule. HA-flocs formed from solutions with an initial pH 11.5 had identical i.r. spectra compared with those formed from solutions with an initial pH 7.0.     

Removal of Cu2+ and Cd2+ from Aqueous Solution by Bentonite Clay Modified with Binary Mixture of Goethite and Humic Acid

Pre-modification of bentonite clay with goethite, humic acid, and a binary mixture of goethite and humic acid reagents increased its cation exchange capacity from 95 to 105.32, 120.4, and 125.8 meq/100 g of bentonite clay, respectively. The effective pre-modification of bentonite clay with goethite, humic acid, and goethite–humic acid reagents was confirmed from their Fourier transform infrared spectra which suggested that modification was effective on the AlAlOH and Si–O sites for goethite and humic acid modification and AlAlOH for goethite–humic acid modification. The presence of 0.001 M NaNO₃ electrolyte increased the adsorption capacity of bentonite clay. Temperature was observed to favor the adsorption of Cu²⁺ and Cd²⁺ onto both the raw and modified bentonite clay samples. The goethite–humic acid-modified bentonite gave the best adsorption capacity of ≈10 and 16 mg/g at 30 and 50°C, respectively, for both metal ions. The inner sphere complexation mechanism was suggested for the adsorption of both metal ions onto the modified adsorbents. Modifying bentonite clay with a binary mixture of goethite and humic acid reduced the selectivity of bentonite clay for either Cu²⁺ or Cd²⁺. Preadsorbed goethite and humic acid on bentonite clay will further reduce the mobility of heavy metal ions in soils and in aquatic environments.     

Activite des complexes acides humiques-invertase: Influence des cations floculants

-Humic acid (AH) and invertase (I) may be complexed with one of the following cations Ca²⁺, Co²⁺, Cu²⁺, Mg²⁺, Mn²⁺, Al³⁺ or Fe³⁺ in order to study their catalytic activity and longevity. Provided that enough cations are supplied there is an immediate, almost total flocculation with Cu²⁺, Al³⁺ and Fe³⁺, or a slow flocculation with Ca²⁺, Co²⁺ and Mn²⁺ or no flocculation at all with Mg²⁺, depending on the reactivity between organic matter and cation. The six kinds of complexes have very different enzymatic activities in magnitude and longevity.No correlation was found between enzymatic activity and the atomic weight and valency of the cation used to prepare the complex. There is a pH at which flocs occur that is closely related to the pH of the metal chloride and to the affinity of each cation for precipitate; this pH value largely determines the structure of the humic material and consequently the catalytic activity of the enzymatic complex produced. For Cu²⁺, Al³⁺ and Fe³⁺ complexes, the pH values are low, between 1.7 and 3.2 and correspond to high conversion rates if the AH-I-cation reaction time is short; in cases where the reaction time is more protracted, enzyme molecules bound to the outer surface of the micellae are denatured by these very low pH values and the complexes irreversibly lose much of their activity. For Ca²⁺, Co²⁺ and Mn²⁺, the pH values lie between 4.30 and 5.65 and correspond to nearly ten times lower enzymatic activities; here, however, a prolonged reaction enhances activity. For Mg²⁺, the pH is 6.25–6.45; for this value, the humic material remains highly dispersed in buffer and the complex AH_Mg-I cannot be separated.The possibility of a relation between the number of enzyme molecules retained in complexes and the cationic radii on the one hand, and between the longevity of the complexes and the bound-state of humic acid-flocculating cation, on the other hand, is suggested.     

Sorption of cadmium by complexes of kaolinite with humic acid

Abstract

Levels of cadmium (Cd) in New Zealand pastoral soils have increased due to Cd impurities in applied fertilisers. As there is little information on the interaction of Cd with soil mineral‐organic matter complexes, the sorption of Cd by complexes of kaolinite with humic acid has been investigated. Sorption was measured at pH and ionic strength values typically found for solutions of pastoral soils in New Zealand. Sorption increased with the content of humic acid in the complex, and as the pH of the medium was raised from 4.2 to 6.3. Sorption was also influenced by the ionic strength of the ambient solution, notably by the nature of the cation in the added electrolyte. The experimental data were interpreted in terms of the effect of solution pH and ionic composition on the charge characteristics of kaolinite and humic acid. These factors, in turn, influence clay particle association as well as the clay‐humic and metal‐humic interaction.     

Reactions of nucleic acid bases with inorganic soil constituents

The adsorption at pH's 4, 6 and 8 of adenine, guanine, cytosine, thymine and uracil on clays (montmorillonite, illite and kaolinite), Fe- and Al-oxides (goethite, hematite and gibbsite), a soil, and on a laboratory-prepared fulvic acid-montmorillonite complex was investigated. Portions of the clays and soil were saturated with H⁺, Fe³⁺ and Ca²⁺.Quantitatively, the extent of adsorption of nucleic acid bases by the clays was proportional to their exchange capacities, but the nature of the dominant cation had only minor effects. By contrast, the adsorption was strongly affected by pH, tending to decrease with increase in pH. Adsorption on goethite and gibbsite was lower than that on clays, while adsorption of nucleic acid bases on soils was slightly lower than that on oxides. The fulvic acid-montmorillonite complex adsorbed substantial, although smaller amounts of purines and pyrimidines, than did montmorillonite alone. The main adsorption mechanism at pH 4 appeared to be cation exchange whereas at pH 8 complex formation between the nucleic acid bases and cations on inorganic surfaces seemed to occur.The results of this and earlier work show that both inorganic and organic soil constituents adsorb nucleic acid bases. Which adsorption reaction predominates will depend on the clay and organic matter content and on the pH.     

Stability constants of complexes of metals with humic and fulvic acids under non-acid-conditions

Stability constants of complexes of four divalent metal ions viz. Cu²⁺, Zn²⁺, Mn²⁺ and Ca²⁺ with humic (HA) and fulvic acids (FA) at pH values of 7 and 8 were determined. The log K (logarithm of the stability constant) ranged from 3.09 to 7.77 and from 2.22 to 5.98 for metal-humic and metal fulvic acid complexes, respectively. Sequentially, the order of stability constants were as follows: Cu> Ca> Mn> Zn and Cu> Ca> Zn> Mn for metal -HA and metal-FA complexes, respectively, indicating a higher degree of complexation with Cu metal ion.     

Impacts of Acid Rain on Base Cations,Aluminum, and Acidity Development in Highly Weathered Soils of Thailand

The impacts of simulated acid rain on leachability of major plant nutrients, toxic element [aluminum (Al)], and acidity development in highly weathered tropical soils of Thailand were studied. Leaching experiments were conducted on soil columns with acidic solutions of pH 5.0, 4.0, 3.0, 2.0, and with water of pH 7.0 as a control treatment. Leaching losses of base cations from all soils increased with the decrease in pH associated with simulated acid rain (SAR) additions, and were found to be quite high under SAR with pH 2.0. The leaching removal of these cations was lesser at pH 3.0, 4.0, and 5.0 but greater than that in pH 7.0. The leaching of base cation from the soils depended not only on acid rain pH but also on soil properties, especially cation exchange capacity, soil texture, and initial base content. The significant losses of major plant nutrients [such as potassium (K⁺), calcium (Ca²⁺), and magnesium (Mg²⁺)] from the plant root zone over extended periods could cause nutrient imbalance and lower soil productivity.     

Cation exchange in forest soils: the need for a new perspective

The long‐term sustainability of forest soils may be affected by the retention of exchangeable nutrient cations such as Ca²⁺ and the availability of potentially toxic cations such as Al³⁺. Many of our current concepts of cation exchange and base cation saturation are largely unchanged since the beginnings of soil chemistry over a century ago. Many of the same methods are still in use even though they were developed in a period when exchangeable aluminium (Al) and variable charge were not generally recognized. These concepts and methods are not easily applicable to acid, highly organic forest soils. The source of charge in these soils is primarily derived from organic matter (OM) but the retention of cations, especially Al species, cannot be described by simple exchange phenomena. In this review, we trace the development of modern cation exchange definitions and procedures, and focus on how these are challenged by recent research on the behaviour of acid forest soils. Although the effective cation exchange capacity (CECe) in an individual forest soil sample can be easily shown to vary with the addition of strong base or acid, it is difficult to find a pH effect in a population of different acid forest soil samples. In the very acidic pH range below ca 4.5, soils will generally have smaller concentrations of adsorbed Al³⁺. This can be ascribed to a reduced availability of weatherable Al‐containing minerals and a large amount of weak, organic acidity. Base cation saturation calculations in this pH range do not provide a useful metric and, in fact, pH is modelled better if Al³⁺ is considered to be a base cation. Measurement of exchangeable Al³⁺ with a neutral salt represents an ill‐defined but repeatable portion of organically complexed Al, affected by the pH of the extractant. Cation exchange in these soils can be modelled if assumptions are made as to the proportion of individual cations that are non‐specifically bound by soil OM. Future research should recognize these challenges and focus on redefining our concepts of cation retention in these important soils.     

应用穆斯堡尔(Mössbauer)谱学方法研究Fe+++, Fe++与胡敏酸结合的性质

本文应用穆斯堡尔谱学方法,研究了在不同pH条件下Fe⁺⁺⁺,Fe⁺⁺与胡敏酸结合的性质。计算机拟合的pH3.0⁵⁷Fe-胡敏酸络合物泥浆的穆斯堡尔谱和参数表明,三对四极双峰(图1,AA',BB',CC')是合理的。在pH3.0 Fe⁺⁺⁺以高自旋态存在,它和胡敏酸的结合多于一种环境类型。在pH1.0的⁵⁷Fe-胡敏酸络合物中,观察到了Fe⁺⁺⁺的穆斯堡尔谱信号,但没有Fe⁺⁺的信号,在这个样品离心分离出的液体部分检测到了加入量的59.7%的铁,表明在pH1.0时相当数量的Fe⁺⁺⁺被胡敏酸还原成Fe⁺⁺,Fe⁺⁺并不与胡敏酸牢固结合。在30伏电析过的⁵⁷Fe-胡敏酸络合物样品中(pH2.8)出现了Fe⁺⁺⁺,Fe⁺⁺的穆斯堡尔谱,这一结果指示出,在电析过程中由于⁵⁷Fe⁺⁺⁺-胡敏酸络合物悬浮液pH的降低,一部分Fe⁺⁺⁺还原成Fe⁺⁺,在pH2.8相当数量的Fe⁺⁺与胡敏酸牢固结合。根据在80K记录的穆斯堡尔谱,在pH1—3的范围出现了磁有序成份,但在室温记录的穆斯堡尔谱没有磁分裂。     

Measurement of the potentially available charge and the dissociation behaviour of humic acid from cobaltihexammine adsorption

The use of a new method for characterizing humic acids based on the measurement of the cobaltihexammine cation exchange capacity (CEC) vs. pH is examined. The method was first verified on a macroporous weak-acid cation exchange resin: charge vs. pH curves and pK values from cobaltihexammine adsorption identify with results from discontinuous acid-base titrations at high ionic strength (5 M). Similar agreement was obtained for humic acids from a Podzol soil: the cobaltihexammine-CEC identifies with the macromolecular charge (from continuous acid-base titrations) at ionic strength of 3 and 5 M, but slightly underestimates (5%) the humic acid charge at 1 M ionic strength. The cobaltihexammine method is suitable for determining the potentially available charge in the pH range 2 to 12 and the intrinsic pK values of humic acids.     

A new model for cation exchange equilibrium considering the electrostatic field of charged particles

Purpose

The aim of this study was to establish a new equilibrium model for cation exchange that quantitatively describe the three important effects: (1) the effect of electrostatic field around soil particles on exchange adsorption; (2) the effect of ionic interaction energy on Boltzmann distribution of cations; and (3) the effect of hydration radius of cation species on cationic distribution between adsorption phase and solution phase.

Materials and methods

In this paper, the Li/Na, Li/K and K/Na exchange were studied theoretically and experimentally. Purified montmorillonite was used as the experimental material and it was H-saturated in advance. In experimental study, approximately 2.5?g of the H-saturated sample was weighed in each experiment, then LiOH/NaOH, LiOH/KOH or KOH/NaOH mixture solution was added. In each experiment, the suspension was allowed to equilibrate for 48?h with continuous shaking at 298?K in an incubator shaker, and then 1?mol/l HCl was used to adjust pH to pH?=?7 when equilibrium was reached. Finally, the suspension was centrifuged and the concentration and of Li⁺, Na⁺ or K⁺ in supernatant was determined.

Results and discussion

In theoretical study, firstly, considering the ionic interaction energy in bulk solution, a modified Poisson?CBoltzmann equation was obtained; secondly, considering the electrostatic field around soil particles, the relationship between the selectivity coefficient and surface potential of particles was established; and thirdly, the modification factors were introduced to modify the effective charge of two cation species that involved in exchange because of the difference in hydration radius. Finally, the new models for describing cation exchange equilibrium were developed. Both theoretical and experimental results showed that the electrostatic field, the ionic interaction energy and the difference in hydration radius of two cation species strongly influenced cation distribution or cation exchange equilibrium. The results indicated that the effective charge could be obtained through either experimental determination or theoretical calculation, and the theoretically predicted values met the experimental results well. Therefore, the ion exchange selectivity series of different cation species with the same valence could be evaluated quantitatively.

Conclusions

New equilibrium models for describing cation exchange were established, for which the three important effects was quantitatively taken into account: the electrostatic field on exchange adsorption, the ionic interaction energy on Boltzmann distribution and the hydration radius of cation species. Both theoretical analyses and experimental results demonstrated that the cation hydration radius and the ionic interaction energy strongly influenced the exchange equilibrium considering the electrostatic field of charged particles.     

pH and solubility of aluminium in acidic forest soils: a consequence of reactions between organic acidity and aluminium alkalinity

Data from two Podzol O and E horizons, sampled in 1-cm layers at 13 points within 2 m × 2 m plots, were used to test the hypothesis that the composition of hydrogen ions (H) and aluminium (Al) adsorbed to the solid-phase soil organic matter (SOM) determines pH and Al solubility in organic-rich acidic forest soils. Organically adsorbed Al was extracted sequentially with 0.5 m CuCl₂ and organically adsorbed H was determined as the difference between total acidity titrated to pH 8.2 and Al extracted in 0.5 m CuCl₂. The quotient between fractions of SOM sites binding Al and H (N_Al/N_H) is shown to determine the variation in pH and Al solubility. It is furthermore shown that models in which pH and Al solubility are linked via a pH-dependent solubility of an Al hydroxide and in which cation exchange between Al³⁺ and Ca²⁺, rather than cation exchange between Al³⁺ and H⁺, is the main pH-buffering process cannot be used to simulate pH or Al solubility in O and E horizons. The fraction of SOM sites adsorbing Al increased by depth in the lower O and throughout the E horizon at the same magnitude as sites adsorbing H decreased. The fraction of sites binding the cations Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ remained constant. It is suggested that a net reaction between Al silicates (proton acceptors) and protonated functional groups in SOM (proton donors) is the long-term chemical process determining the composition of organically adsorbed H and Al in the lower part of the O and in the E horizon of Podzols. Thus, in the long term, pH and Al solubility are determined by the interaction between organic acidity and Al alkalinity.     

Cation exchange capacity measurements

Abstract

Soil cation exchange capacity (CEC) measurements are important criteria for soil fertility management, vaste disposal on soils, and soil taxonomy. The objective of this research was to compare CEC values for arable Ultisols from the humid region of the United States as determined by procedures varying widely in their chemical conditions during measurement. Exchangeable cation quantities determined in the course of two of the CEC procedures were also evaluated. The six procedures evaluated were: (1) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity; (2) N Ca(OAc)₂ (pH 7.0) saturation with Mg(OAc)₂ (pH 7.0) displacement of Ca²⁺; (3) N NH₄OAc (pH 7.0) saturation with NaCl displacement of NH₄ ⁺; (4) N MgCl₂ saturation with N KCl displacement of Mg²⁺; (5) compulsive exchange of Mg²⁺ for Ba²⁺; and (6) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus N KCl exchangeable AJ. The unbuffered procedures reflect the pH dependent CEC component to a greater degree than the buffered methods. The compulsive exchange and the summation of N NH₄OAc exchangeable cations plus N KCl exchangeable Al procedures gave CEC estimates of the same magnitude that reflect differences in soil pH and texture. The buffered procedures, particularly the summation of N NH₄OAc exchangeable cations plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity, indicated inflated CEC values for these acid Ultisols that are seldom limed above pH 6.5. Exchangeable soil Ca and Mg levels determined from extraction with 0.1 M BaCl₂ were consistently greater than values for the N NH₄Oac (pH 7.0) extractions. The Ba²⁺ ion is apparently a more efficient displacing agent than the NH₄ ⁺ ion. Also, the potential for dissolving unreacted limestone is greater for the Ba₂ ⁺ procedures than in the NH₄ ⁺ extraction.     

Exchangeable cations and CEC determinations of some highly weathered soils

Abstract

Eight methods to determine exchangeable cations and cation exchange capacity (CEC) were compared for some highly weathered benchmark soils of Alabama. The methods were: (1) 1N NH₄OAc at pH 7.0 by replacement (for CEC only), (2) 1N NH₄OAc at pH 7.0 (summation of basic cations plus 1N KCl extractable Al), (3) 1N NH₄OAc at pH 7.0 (summation of basic cations plus exchangeable H⁺), (4) 0.1M BaCl₂ (summation of basic cations plus exchangeable Mn, Fe and Al), (5) Mehlich 1 (summation of basic cations plus 1N KCl extractable Al), (6) Mehlich 1 (summation of basic cations plus exchangeable H⁺), (7) Mehlich 3 (summation of basic cations plus 1N KCl extractable Al), and (8) Mehlich 3 (summation of basic cations plus exchangeable H⁺). The 0.1M BaCl₂ was chosen as the standard method for the highly weathered soils and the other methods compared to it. The results indicated that the 1N NH₄OAc replacement method gave significantly higher CEC values compared to the summation methods. This was probably due to the overestimation of the field CEC caused by measurement of pH dependent cation exchange sites in these soils. There was, however, close agreement between the BaCl₂ method and the summation methods that included extractable Al. The generally good agreement between these summation methods suggests that the Mehlich 1 and Mehlich 3 extractants, commonly used to determine available nutrients in the southeastem USA, may also be used to measure effective CEC of some acid‐rich sesquioxide benchmark soils of Alabama. However, 1N KCl extractable Al as opposed to exchangeable H⁺ should be included in the computation.     

Complexation of Cu2+ and Pb2+ by peat and humic acid

The binding of metal to humic substances is problematical. The approaches for studying metal binding to organic matter are briefly reviewed. Ion-selective electrodes (Cu²⁺ and Pb²⁺) were used to measure metal complexation by a whole peat and an extracted humic acid (HA) fraction. Scatchard plots and calculation of incremental formation constants were used to obtain values for the binding constants for the metals onto both peat and HA. Both the peat and the humic acid had a larger maximum binding capacity for Pb²⁺ than for Cu²⁺ (e.g. at pH = 5 HA gave 0·188 mmol Cu²⁺ g^?1 and 0·564 mmol Pb²⁺ g^?1: peat gave 0·111 mmol Cu²⁺ g^?1 and 0·391 mmol Pb²⁺ g^?1). Overall, the humic acid had a larger metal binding capacity, suggesting that extraction caused conformational or chemical changes. The binding constants (K₁) for Cu²⁺ increased with increasing pH in both peat and humic acid, and were larger in the peat at any given pH (e.g. at pH = 5 HA gave log K₁= 2·63, and peat gave log K¹= 4·47 for Cu²⁺). The values for Pb²⁺ showed little change with pH or between peat and humic acid (e.g. at pH = 5 HA gave log K¹= 3·03 and peat gave log K¹= 3·00 for Pb²⁺). In the peat, Cu²⁺ may be more able to bind in a 2:1 stoichiometric arrangement, resulting in greater stability but smaller binding capacity, whereas Pb²⁺ binds predominantly in a 1:1 arrangement, with more metal being bound less strongly. Whole peat is considered to be more appropriate than an extracted humic acid fraction for the study of heavy metal binding in organic soils, as this is the material with which metals introduced into an organic soil would interact under natural conditions.     

The involvement of iron and aluminum in the bonding of phosphorus to soil humic acid

Abstract

With a peat soil similar amounts of phosphorus (P) were coprecipitated with humic acid from alkali extracts over a limited range of strongly acidic pH, whereas with a mineral soil the amount was pH dependent. The difference between the two soils relates to the much greater total amounts of inorganic P and aluminum (Al) present in the extract of the mineral soil. In this acid mineral soil, Al rather than iron (Fe) may be involved in the formation of metal bridges in humic acid‐metal‐inorganic P complexes. Neither Al or Fe were implicated in binding of organic P to humic acid. The P species observed in humic acids was dependent on the pH at which they were precipitated from the alkali extracts. In the peat soil the inorganic P was an order of magnitude lower than the organic P.