ADSORPTION ON HYDROUS OXIDES: II. OXALATE, BENZOATE AND PHOSPHATE ON GIBBSITE

Adsorption isotherms, the infrared spectra of adsorbed species, and the amount of hydroxyl released when oxalate is adsorbed, all indicate that adsorption of oxalate, benzoate and phosphate at low solution concentrations occurs only on A1(OH)(H₂ O) sites exposed on edge faces of the platy gibbsite crystals studied. This is confirmed by the finding that the vibrations of surface hydroxyl groups on the principal plate faces are unaffected when excess oxalate is present, and further supported by the low affinity for oxalate of imogolite, a tubular mineral with a very large gibbsite-like surface, but with a very low concentration of edge sites. Infrared spectra indicate that oxalate is adsorbed in bidentate form. Near neutrality and at low solution concentrations, gibbsite adsorbs more oxalate than does goethite of comparable surface area.     

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.     

不同pH、电解质浓度条件下草酸对针铁矿吸附Cd(II)的影响及机制

用吸附平衡法研究了不同草酸浓度、体系pH对针铁矿 (G)吸附Cd2+的影响与机制以及电解质 (KNO3)浓度对针铁矿、草酸化针铁矿 (G+40 )吸附Cd2+的影响差别及原因。结果表明 ,低浓度草酸 (1mmolL-1 )促进Cd2+的吸附 ;高浓度草酸 (1mmolL-1/sup )抑制Cd2+的吸附。已吸附在针铁矿表面的草酸对Cd2+ 吸附的影响与液相中草酸的影响不同 ,这主要与草酸引起的针铁矿表面电荷性质的变化、草酸在固液两相间的分配、草酸与Cd2+的配合作用和竞争作用有关。电解质 (KNO3)浓度对针铁矿和草酸化针铁矿吸附Cd2+的影响明显不同 ,随KNO3 浓度的提高 ,针铁矿的Cd吸附率由 44.5%增至 95%以上 ,而草酸化针铁矿吸附率由 29%降至6.2% ,这主要决定于二者的电荷零点 (PZC)和体系pH变化的不同。     

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 and noncompetitive adsorption of silicate and phosphate by two acid Si-deficient soils and their effects on P and Si extractability

The effects of pH on the adsorption of silicate and phosphate, either singly or in competition, by two acid soils were investigated. Both soils adsorbed two to three times more P than Si and adsorption isotherms at pH 5.0, 5.5, 6.0 and 6.5 showed that increasing pH greatly increased Si adsorption but decreased that of P. Silicate adsorption was very low below pH 5.0, increased rapidly up to pH 9–10 before decreasing again. Adsorption of P was at a maximum at pH 2.0, decreased slowly up to pH 7.0 and then more rapidly above pH 7.0. When Si and P were added at equimolar concentrations, the presence of P decreased Si adsorption between pH 6.0 and 8.0 while the presence of Si decreased P adsorption in the pH region 6.0 and 11. Addition of calcium silicate at rates equivalent to 300, 600 and 1200 kg Si ha^?1 resulted in a progressive increase in soil pH. Separate samples of soil were treated with Ca(OH)₂ to give the same pH values so that the effect of Si could be identified. The highest rate of Si (1200 kg ha^?1 which gave a pH of 6.5) caused a significant decrease in P adsorption (as determined by adsorption isotherms) and an increase in resin-extractable P but the lower rates had little effect. Addition of P to the soil as calcium phosphate at rates equivalent to 30, 60 and 100 kg P ha^?1 all caused a decrease in Si adsorption capacity and an increase in CaCl₂-extractable Si. It was concluded that the strategy of adding Si to lower P requirements in acid soils is not likely to be effective while addition of fertilizer P may well lower Si adsorption and promote Si desorption and its increased mobility.     

ADSORPTION ON HYDROUS OXIDES. IV. MECHANISMS OF ADSORPTION OF VARIOUS IONS ON GOETHITE

Infrared spectroscopy has been used to investigate the complexes that are formed when acids are evaporated onto goethite. It is probable that, like HPO^2?₄, the anions SO^2?₄, SeO^2?₃, and oxalate are adsorbed by ligand exchange and form binuclear bridging complexes where two singly coordinated A-type OH groups are replaced by two oxygen atoms of one ligand. There is evidence that HPO^2?₄ and oxalate are likely to be present in this form in wet environments, and this is probably also true for SO^2?₄ and SeO^2?₃. Fluoride ions can completely replace the singly coordinated A-type OH groups but do not replace C- or B-type OH groups that are coordinated, respectively, to two and three Fe³⁺ ions. The other halides, nitrate, and benzoate partially replace the A-type OH groups, benzoate being adsorbed as a monodentate ligand. Copper ions do not appear to react with A-type OH but zinc ions are probably adsorbed on the goethite (100) face in conjunction with carbonate or bicarbonate.     

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.     

PHOSPHATE ADSORPTION BY SYNTHETIC AMORPHOUS ALUMINOSILICATES

Phosphate adsorption isotherms were determined for four synthetic amorphous aluminosilicate gels with A1: A1 + Si molar ratios of 0.29 to 0.88. The concomitant silicate release and acid consumed to maintain the pH of the suspensions constant were also measured. The adsorption isotherms were analysed applying a two-term Langmuir equation–assuming two types of sites. The experimental points fitted the predicted adsorption curves only up to a certain amount of phosphate adsorbed. The deviation at high phosphate adsorption values suggested the presence of more than two types of adsorption site. A comparison of phosphate adsorbed with the silicate released and acid consumed to maintain the pH constant indicated that, for a 3 h reaction time at concentrations below about 10 μmol cm^?3, phosphate exchanges mainly with aquo and hydroxo ligands and with adsorbed silicate. At higher concentrations phosphate is adsorbed (i) on sites arising from the disruption of hydroxy aluminium polymers in the gels and (ii) by the displacement of structural silicate.     

草酸/草酸盐对森林暗棕壤的磷释放效应

模拟森林凋落物淋洗液中的草酸/草酸盐浓度范围,设计了不同浓度草酸/草酸盐溶液一次性浸提和多次连续浸提系列实验,其中的草酸(阴离子)载荷量为0~200 mmol kg-1。结果表明,草酸能显著促进暗棕壤A1层(腐殖质层)磷的释放,土壤磷溶出量随草酸溶液浓度升高而线性增加;但对B层土壤磷的释放效应相对较弱,草酸浓度低于5mmol L-1时B层磷的释放不明显。pH 5.16草酸钠溶液比相同浓度的草酸溶液具有更高的解磷效率,在设置二者浓度为0.5~20.0 mmol L-1时,前者的解磷量是后者的1.51~2.98倍,推断草酸盐溶液或凋落物淋洗液中草酸(盐)类物质促进暗棕壤磷释放的主要机理在于草酸阴离子(C2O42-)配位反应。草酸盐对暗棕壤磷的释放效应具有一定累加性,土壤磷释放量主要由草酸阴离子累积载荷量决定,而与其加入方式(多次或一次性)关系不大;当以pH5.16草酸钠溶液加入时,土壤磷释放量Y(mgkg-1)与草酸阴离子累积载荷量X(mmol kg-1)间的回归方程为Y=-0.000 4X2 0.176 6X 0.425 3,R2=0.990 2。仅以凋落物层溶出的草酸(阴离子)量进行估计,由此增加的A1层土壤磷释放量达2.40 kg hm-2a-1,大约相当于中龄林年吸收磷量的1/3~1/5,因此其实际作用是不可忽视的。     

Desorption Characteristics of Three Mineral Oxides and a Non-crystalline Aluminosilicate for Supplying Phosphate in Soilless Root Media

The objective was to evaluate phosphate desorption characteristics of synthetic hematite, goethite, and allophane and commercial alumina after loading at maximum adsorbed phosphate levels to determine their potential to release phosphate at a constant, low level to sustain plant growth in soilless media and reduce phosphate leaching. Desorption isotherms were measured at pH 6.4 ± 0.1 using a continuously stirred-flow reactor. The time period during which dissolved phosphate was maintained within the range of 5–0.2 mg·L^?1 phosphate-P decreased in the order: allophane (12.4 d) > alumina (4.6 d) > goethite (3.6 d) > hematite (1.9 d). Allophane released the most phosphate during the desorption process (40% of maximum adsorbed phosphate; 12.7 mg?g^?1) followed by alumina and goethite (19–20%; ≈2.5 mg?g^?1) and lastly hematite (5%; 0.1 mg?g^?1). Allophane demonstrated the greatest potential as a phosphate-charged source for soilless root media, in amount and duration of phosphate release.     

ADSORPTION OF SELENITE, PHOSPHATE AND SULPHATE ON HYDROUS ALUMINA

Selenite was adsorbed on a positively charged hydrous alumina at a solution pH of 5.0, and the OH ^? released and changes in the surface charge were measured. The adsorption isotherm levelled off at high concentrations suggesting a definite adsorption maximum. The OH^? released yielded a curvilinear relationship of increasing slope with the selenite adsorbed. The positive charge on the alumina surface was neutralized by the adsorbed selenite and the net charge became close to zero as the adsorption of selenite was near maximum. The molar ratio of the ligands (OH^?+ OH₂) displaced over selenite adsorbed was more than one at surface saturations of <0.5, but it was approaching one with increasing adsorption of selenite. The results are explained in terms of preferential adsorption of divalent (SeO²₃^?) and monovalent (HSeO^?₃ selenite on a highly positively and less positively charged surface respectively, in agreement with the VSC^? VSP model of Bowden et al. (1973; 1977). The supply of SeO²₃^? for adsorption is considered to be favoured by an expected lower H activity in solution adjacent to the positively charged surface than that in the bulk solution. A general discussion is given on the ligand exchange adsorption of selenite, phosphate and sulphate on hydrous alumina.     

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.     

Adsorption/Desorption of Organic Anions in Brazilian Oxisols

Abstract

Organic anions affect solute mobility in soils. This study evaluated citrate and oxalate adsorption (0 to 4 mmol L^?1, soil–solution 1∶100, pH 5.5, ionic strength 30 mmol L^?1 as NaCl, 72‐h reaction) and desorption (pH 5.5, 30‐mmol L^?1 NaCl, 72 h) on A‐ and B‐horizon samples of two Brazilian Oxisols. Langmuir and Freundlich isotherms were used to assess adsorption maximum, distribution coefficients (Kf, Ku), and buffer index. Adsorption maximums (mol kg^?1) for red Latossol‐A, red Latosol‐B, red‐yellow Latosol‐A, and red‐yellow Latosol‐B horizons follow: citrate 0.0318, 0.0272, 0.0289, 0.0392; oxalate 0.0641, 0.0329, 0.0538, 0.0380. Kf (mol^1?1/n kg^?1 L^1/n) follows: citrate 0.3550, 0.3781, 0.4211, 0.2024; oxalate 1.0916, 0.0637, 1.8228, 0.0922. Buffer index (mol kg^?1)(mol kg^?1)^?1 follows: citrate 0.0841, 0.0756, 0.0738, 0.0264; oxalate 0.3787, 0.0862, 0.3233, 0.1082. Both anions showed great affinity for variable‐charge soils. The distribution curves for Ku showed higher adsorption energy in B‐ than in A‐horizons.     

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.     

ADSORPTION OF THE HERBICIDE 2,4-D ON GOETHITE

Adsorption, of the herbicide 2,4-Dichlorophenoxyacetic acid (2,4-D) on an iron oxide, goethite, was studied in aqueous suspensions as a function of solution pH, ionic strength of the medium, and initial 2,4-D concentration. The 2,4-D anion was reversibly adsorbed on positively charged goethite surfaces, maximum adsorption being observed near the pK_a of 2,4-D (2.73) and at low ionic strength. Within certain levels of adsorption (5–22 mg 2,4-D adsorbed/g goethite) the complex became hydrophobic and floated to the liquid surface. This flotation effect disappeared on further adsorption. It is suggested that adsorbed 2,4-D anions are orientated with their hydro-phobic aromatic ends directed towards the solution, the carboxyl groups being weakly bound to positive sites on the oxide surface. At high levels of adsorption, some of the anions are orientated in the opposite direction by π–π interaction, with the first adsorbed layer and the surface reverts to its hydrophilic nature.     

FLUORIDE ADSORPTION BY ILLINOIS SOILS

Fourteen surface and 6 subsurface horizons of Illinois soils adsorbed significant amounts of F^? with release of OH^?. At low concentrations, adsorption was described by both Langmuir and Freundlich isotherms. The calculated Langmuir adsorption capacities were related to pH, clay, organic carbon, and amorphous aluminum contents. Two soils with different gross chemical properties behaved in essentially the same manner, with adsorption maxima occuring between pH 5.5 and 6.5. The similarity between adsorption at different pH values for the soils and those for bauxite, allophane and synthesized ‘soil chlorite’, and the lack of adsorption maxima between pH 5.5 and 6.5 for pure kaolinite and montmorillonite, suggest that F^? adsorption in the soils is due primarily to the presence of amorphous aluminum oxyhydroxides which are common weathering products in these soils.     

THE MECHANISM OF PHOSPHATE ADSORPTION BY KAOLINITE, GIBBSITE, AND PSEUDOBOEHMITE

The adsorption isotherms (20°C) of phosphate on two potassium kaolinites and two aluminium oxides have been determined at pH values from 3 to 10 and at concentrations ranging from 10^?4 to 10^?2M. The reversibility of the adsorption with respect to pH and concentration has also been examined. The isotherms result from at least three types of adsorption site (regions I, II, and III) of widely different reactivities. The number of adsorption sites increases to a limit with decrease in pH for regions I and II. The behaviour of region III is more complex. The different adsorbents behave in essentially the same manner and differ only in the number of adsorption sites. It is tentatively suggested that regions I and II are located on an edge –Al(OH)₂ of the adsorbents, while region III results from penetration into some amorphous region of the crystal surface.     

Effects of crystallinity of goethite: II. Rates of sorption and desorption of phosphate

Eight samples of goethite ranging in surface area from 18 to 132 m² g^-1 were mixed with phosphate at a range of pH values for periods which ranged from 0·5 h to 6 weeks. The sample with a surface area of 18 m² g^-1 had been hydrothermally treated to improve its crystallinity. Its rate of reaction with phosphate depended on pH but was complete within a day. Its maximum observed reaction was close to the theoretical maximum for surface adsorption of 2·5 μmol m^?2. For the other samples, phosphate continued to react for up to 3 weeks and exceeded the value of 2·5 μmole m^?2. The duration and extent of the reaction depended on the crystallinity of the goethite. The results were closely described by a model in which the phosphate ions were initially adsorbed on to charged external surfaces. The phosphate ions then diffused into the particles. This was closely described using equations for diffusion into a cylinder. Samples of goethite which had been loaded with phosphate dissolved more slowly in HCl, and had a longer lag phase, than phosphate-free goethite. For the hydrothermally treated goethite, HCl removed much of the phosphate when only a small proportion of the iron had been dissolved. For a poorly crystallized goethite, it was necessary to dissolve much more of the iron to obtain a similar removal of phosphate. Brief treatment with NaOH removed most of the phosphate from the hydrothermally treated goethite but only half the phosphate from a poorly crystallized goethite. These results are consistent with the idea that phosphate ions were not only bound on external surface sites but had also penetrated into meso- and micro-pores between the domains of the goethite crystals and were then adsorbed on internal surface sites. This penetration tied the domains together more firmly thus increasing the lag phase for dissolution. Differences between sites for phosphate adsorption are therefore caused mainly by their location on either external or internal sites. Models that ignore this are incomplete.     

Radiation Synthesis of Poly(Acrylamide-Acrylic Acid-Dimethylaminoethyl Methacrylate) Resin and Its Use for Binding of Some Anionic Dyes

Poly(acrylamide-acrylic acid-dimethylaminoethyl methacrylate) P(AAm-AA-DMAEMA) resin was prepared by the template copolymerization. PAAm was used as a template for the copolymerization of DMAEMA and AA in aqueous solution using gamma rays. The adsorption of indigo carmine and eriochrome black-T anionic dyes from aqueous media on P(AAm-AA-DMAEMA) has been investigated. The adsorption behavior of this resin has been studied under different adsorption conditions: dye concentrations (50?C500 mg l^?1), contact times, temperature (30?C55°C), and pH values (2?C7). The amount of dye adsorbed increased with increasing resin content, but it had a little change with temperature and decreased slightly with increasing pH. Adsorption data of the samples were modeled by the pseudo-first-order and pseudo-second-order kinetic equations in order to investigate dye adsorption mechanism. It was found that the adsorption kinetics of the resin followed a pseudo-second-order model with rate constant (k ₂) of 2.5?×?10^?3 and 1.8?×?10^?2 g (mg^?1 min^?1) for indigo carmine and eriochrome black-T, respectively. Equilibrium isotherms were analyzed using the Langmuir and Freundlich isotherms. It was seen that the Freundlich model fits the adsorption data better than the Langmuir model.     

磷和柠檬酸共存对高岭石和针铁矿吸附铅的影响

通过平衡吸附试验及矿物电动电位(Zeta电位)的变化分析,研究了磷(P)和柠檬酸(CA)共存对针铁矿和高岭石吸附铅的影响。结果表明:(1)针铁矿和高岭石对铅的吸附量随柠檬酸浓度的升高呈现"峰形"曲线变化,铅吸附量达到峰值的柠檬酸浓度均为0.5 mmol L~(-1),不同浓度磷存在下柠檬酸对矿物吸附Pb2+量有不同程度增加。(2)随着磷添加浓度的增加,两种矿物对铅吸附量均呈增加趋势,磷添加浓度分别为1 mmol L~(-1)和0.6 mmol L~(-1)时,针铁矿和高岭石吸附铅量达到平衡;当处理中添加不同浓度柠檬酸,两种矿物均表现为对铅的吸附量增加,且随着柠檬酸浓度增加促进铅吸附的作用增强,说明在磷及试验浓度柠檬酸存在下促进了矿物对铅的吸附。(3)高岭石体系中,加入磷或(和)柠檬酸后,Zeta电位-pH曲线向负值方向位移,降低程度大小顺序为1.0 mmol L~(-1) P+0.5mmol L~(-1) CA0.5 mmol L~(-1) CA1.0 mmol L~(-1) P,说明高岭石表面增加的负电荷也部分增加了其对铅的电性吸附;添加磷和柠檬酸处理针铁矿的Zeta电位显著降低,且随着体系pH的升高其Zeta电位没有明显变化,表明磷和柠檬酸均主要是通过吸附到针铁矿表面而增加对铅的专性吸附。