Effect of contaminant carbonaceous matter on the sorption of gold by pyrite

The effect of carbon or graphite coating on the adsorption of gold cyanide on pyrite was investigated with pure pyrite and a pyrite concentrate. In the carbon or graphite contaminated pyrite systems carbon and graphite not only acted as gold sorbents, but also enhanced gold adsorption on pyrite. The carbon coating enhanced gold adsorption on pyrite to a larger extent, in comparison with the graphite coating. The carbon or graphite coating on pyrite reduced the negativity of the pyrite surfaces, and hence improved the physical adsorption of gold cyanide on pyrite. In addition, the highly conductive coating of carbon or graphite on pyrite could enhance electron transfer in the electrochemical reactions occurring in the chemical adsorption of gold and gold reduction on pyrite. The preg-robbing by pyrite or the graphite-coated pyrite was reduced and further eliminated at higher cyanide concentrations. However, gold adsorption on the carbon-coated pyrite could not be prevented even at higher cyanide concentrations due to gold adsorption on the carbon coating. In comparison with pure pyrite, the pyrite concentrate had a higher capacity adsorbing gold, due to the presence of carbonaceous matter in the pyrite concentrate. Fine grinding intensified the smearing of carbon or graphite on the mineral particles, resulting in a larger extent of enhancement in the preg-robbing of the concentrate by the carbon or graphite coating.A diagnostic elution of the preg-robbing pyrite samples indicated that the reduction of gold at the pyrite surfaces was the dominant mechanism for gold adsorption on pyrite, followed by physical and chemical adsorption. Surface topological studies by SEM/EDX showed that gold adsorbed at defect sites on pyrite surfaces. For the pyrite with a 5% carbon coating, gold was observed to adsorb not only at the defect sites, but also at the smooth surfaces with carbon present. For the pyrite with a 5% graphite coating, carbon was also found at the pyrite surfaces, but gold was only detected at the defect sites. XPS studies revealed that part of the gold physically and chemically adsorbed on pyrite or pyrite coated with carbon or graphite. Some gold cyanide was reduced at the pyrite surfaces, with the sulphide ions of pyrite being oxidised to elemental sulphur.     

Effect of contaminant carbonaceous matter on the sorption of gold by pyrite

The effect of carbon or graphite coating on the adsorption of gold cyanide on pyrite was investigated with pure pyrite and a pyrite concentrate. In the carbon or graphite-contaminated pyrite systems carbon and graphite not only acted as gold sorbents, but also enhanced gold adsorption on pyrite. The carbon coating enhanced gold adsorption on pyrite to a larger extent, in comparison with the graphite coating. The carbon or graphite coating on pyrite reduced the negativity of the pyrite surfaces, and hence improved the physical adsorption of gold cyanide on pyrite. In addition, the highly conductive coating of carbon or graphite on pyrite could enhance electron transfer in the electrochemical reactions occurring in the chemical adsorption of gold and gold reduction on pyrite. The preg-robbing by pyrite or the graphite-coated pyrite was reduced and further eliminated at higher cyanide concentrations. However, gold adsorption on the carbon-coated pyrite could not be prevented even at higher cyanide concentrations due to gold adsorption on the carbon coating. In comparison with pure pyrite, the pyrite concentrate had a higher capacity adsorbing gold, due to the presence of carbonaceous matter in the pyrite concentrate. Fine grinding intensified the smearing of carbon or graphite on the mineral particles, resulting in a larger extent of enhancement in the preg-robbing of the concentrate by the carbon or graphite coating.A diagnostic elution of the preg-robbing pyrite samples indicated that the reduction of gold at the pyrite surfaces was the dominant mechanism for gold adsorption on pyrite, followed by physical and chemical adsorption. Surface topological studies by SEM/EDX showed that gold adsorbed at defect sites on pyrite surfaces. For the pyrite with a 5% carbon coating, gold was observed to adsorb not only at the defect sites, but also at the smooth surfaces with carbon present. For the pyrite with a 5% graphite coating, carbon was also found at the pyrite surfaces, but gold was only detected at the defect sites. XPS studies revealed that part of the gold physically and chemically adsorbed on pyrite or pyrite coated with carbon or graphite. Some gold cyanide was reduced at the pyrite surfaces, with the sulphide ions of pyrite being oxidised to elemental sulphur.     

Interactions between sulphide minerals and xanthates,I. The formation of monothiocarbonate at galena and pyrite surfaces

The rate of decomposition of potassium ethyl monothiocarbonate has been determined at pH values between 5 and 10, and its molar absorptivity at 221 nm determined to be 1.24 · 10⁴mol/cm.A novel apparatus for use in the study of reactions between sulphide minerals, oxygen and thiol reagents has been developed, and applied to the reactions of potassium ethyl xanthate with galena and pyrite. It has been shown that both minerals react with ethyl xanthate in the presence of oxygen or oxidation products to form soluble as well as adsorbed xanthate derivatives. The soluble derivative has been identified to be ethyl monothiocarbonate. The adsorbed xanthate at a galena surface, unlike that at a pyrite surface, is gradually converted to a soluble monothiocarbonate under the action of dissolved oxygen. The effect of variables such as pH, the initial xanthate and oxygen concentrations, and the initial state of oxidation of the mineral on the formation of monothiocarbonate has been studied. It is tentatively proposed that an intermediate adsorbed mixed xanthate-hydroxide species is involved in the formation of monothiocarbonate at both galena and pyrite surfaces.The significance of the formation of monothiocarbonate to flotation practice is discussed briefly. The formation of monothiocarbonate represents a wastage of reagent, and could lead to a decrease in flotability of xanthated galena with time of exposure to aerated solutions.     

Electrochemical studies of sulphides,I. The electrocatalytic activity of galena,pyrite and cobalt sulphide for oxygen reduction in relation to xanthate adsorption and flotation

The electrocatalytic activity of galena, pyrite and Co₃S₄ for oxygen reduction has been studied by potentiostatic methods. Open circuit potentials of the sulphide electrodes have also been measured as a function of pH in nitrogen, air and oxygen atmospheres and also in the presence of H₂O₂ and ethyl xanthate. The adsorption of xanthate on sulphides was followed by observing bubble attachment to the electrodes.The catalytic activity for oxygen (or H₂O₂) reduction (the cathodic currents), the electrode potentials and the xanthate adsorption as shown by bubble attachment within certain pH limits, all varied as $\text{Co}{}_3\text{S}{}_4\text{> pyrite (≈ PbS in H}{}_2\text{O}{}_2\text{) ? PbS}$ indicating considerable dependence of the redox processes in flotation on the d - electron character of the sulphides.In the absence of oxygen, xanthate is probably bonded to the water structure of the surface through hydrogen-bonding, thus keeping the surface hydrophilic. Such adsorption reduces the electrode potential and inhibits oxygen reduction.     

The adsorption of gold(I) hydrosulphide complexes by iron sulphide surfaces

The adsorption of gold by pyrite, pyrrhotite, and mackinawite from solutions containing up to 40 mg/kg (8 μm) gold as hydrosulphidogold(I) complexes has been measured over the pH range from 2 to 10 at 25°C and at 0.10 m ionic strength (NaCl, NaClO₄). The pH of point of zero charge, pH_pzc, has been determined potentiometrically for all three iron sulphides and shown to be 2.4, 2.7, and 2.9 for pyrite, pyrrhotite, and mackinawite, respectively. In solutions containing hydrogen sulphide, the pH_pzc is reduced to values below 2. The surface charge for each sulphide is therefore negative over the pH range studied in the adsorption experiments. Adsorption was from 100% in acid solutions having pH < 5.5 (pyrite) and pH < 4 (mackinawite and pyrrhotite). At alkaline pH’s (e.g., pH = 9), the pyrite surface adsorbed 30% of the gold from solution, whereas the pyrrhotite and mackinawite surfaces did not adsorb.The main gold complex adsorbed is AuHS°, as may be deduced from the gold speciation in solution in combination with the surface charge. The adsorption of the negatively charged Au(HS)₂⁻ onto the negatively charged sulphide surfaces is not favoured. The X-ray photoelectron spectroscopic data revealed different surface reactions for pyrite and mackinawite surfaces. While no change in redox state of adsorbent and adsorbate was observed on pyrite, a chemisorption reaction has been determined on mackinawite leading to the reduction of the gold(I) solution complex to gold(0) and to the formation of surface polysulphides. The data indicate that the adsorption of gold complexes onto iron sulphide surfaces such as that of pyrite is an important process in the “deposition” of gold from aqueous solutions over a wide range of temperatures and pressures.     

Decomposition of alkyl dixanthogens in aqueous solutions

Alkyl dixanthogens, (ROCSS)₂, decompose in aqueous solution in the presence of nucleophiles in many ways.It is proposed here that in alkaline solution the principal methods of decomposition of ethyl dixanthogen are by simultaneous attack of OH^? ions on the sulphur-sulphur bond to give products which include xanthate ion (ROCSS^?) and peroxide (H₂O₂) and on the carbon-sulphur bond to give products which include monothiocarbonate ion (ROSCO^?), sulphide ion (S^2?), and sulphur (S⁰). Above pH 12 reaction is complete in a few minutes, and more monothiocarbonate than xanthate is formed. At pH 9 the reaction takes over 20 h and more xanthate than monothiocarbonate is formed.The primary products react further to give various ions which depend in part on the pH of the system. In alkaline solution some of the xanthate and peroxide react to give perxanthate (ROCSSO^?). In acid solution both xanthate and monothiocarbonate decompose rapidly; CS₂ is formed from xanthate and OCS from monothiocarbonate.In the presence of other nucleophiles at pH 9.2, dissolved dixanthogen decomposes much more quickly than with OH^? alone, and other reactions occur. With thiosulphate a higher proportion of xanthate is formed together with some xanthyl thiosulphate and monothiocarbonate but no perxanthate. With sulphite (in the absence of oxygen) or cyanide the products include xanthate and monothiocarbonate but no perxanthate. With sulphite in the presence of oxygen, perxanthate is also formed.Suspensions of dixanthogens react slowly but in a similar fashion to dissolved dixanthogens.Longer-chain dixanthogens are much less soluble than ethyl dixanthogen but, in general, react in a similar way. Higher temperatures increase the rate of decomposition by OH^?.This work has various implications in operating plants.     

An XPS and SEM study of gold deposition at low temperatures on sulphide mineral surfaces: Concentration of gold by adsorption/reduction

Using X-ray photoelectron spectroscopy (XPS or ESCA) and scanning electron microscopy (SEM), we have examined the mechanism of adsorption and reduction of gold solutions on sulphides. KAu^IIICl₄ solutions are quickly adsorbed on the sulphide surfaces, and the Au(III) is quickly reduced to Au(0). Monolayer Au coverage is attained within one minute. The reduction is auto-catalyzed and Au metal grows on the surface. SEM photographs clearly show agglomerates of gold unevenly distributed on the surface. Specific sites of abnormally high Au concentration are found. We propose possible mechanisms for the adsorption and reduction. Our results suggest that adsorption of Au(III) and Au(I) by sulphide minerals, followed by reduction, could play an important role in the deposition of gold in natural systems especially at low temperature and low gold solution concentrations.     

Kinetics and thermochemistry of amyl xanthate adsorption by pyrite and marcasite

The kinetics and thermochemistry of the xanthate adsorption reaction on pyrite and marcasite were evaluated with respect to the existing theory. The rate of xanthate adsorption was studied in a stirred reactor and the xanthate concentration was determined by UV spectrophotometry as a function of time. The heat of the adsorption reaction was measured with a microcalorimeter. The results from both experiments indicate that xanthate adsorption by pyrite or marcasite involves the formation of dixanthogen by an electrochemical reaction at the solid surface which supports the conclusions of other investigators:

$\text{1}\text{2}\text{O}{}_2\text{(aq) =}\text{1}\text{2}\text{O}{}_2\text{(ad) 2X + 2H}{}^{\mathrm{+}}\text{+}\text{1}\text{2}\text{O}{}_2\text{→ X}{}_2\text{(ad) + H}{}_2\text{O}$

The rate of the adsorption reaction was found to be approximately one-half order with respect to the xanthate concentration and to have an activation energy of 7.5 kcal/mole. Additionally, the rate was found to have a slight dependence on pH under certain conditions. In view of these results, it appears that the adsorption reaction is controlled by electrochemical discharge at the pyrite surface. Analysis of the data in terms of an electrochemical kinetic model successfully explained the observed rate phenomena.The measured heat of the adsorption reaction at low pH was found to be between ?63 and ?56 kcal/mole of adsorbed dixanthogen and independent of surface coverage. These experimental heats of adsorption agree with the value of ?57 kcal/mole of dixanthogen calculated for the oxidation of xanthate by oxygen from thermodynamic data reported in the literature.     

Mercury and mercury compounds in surface air, soil gas, soils and rocks

Secondary mercury dispersion haloes were detected and defined above sulphide mineralization by in-situ mercury in soil gas measurements. The meteorological factors controlling the concentration of mercury in soil gas were investigated by long-term experiments. Different mercury compounds in soils and rocks have been determined by a thermal destruction technique. In areas with sulphide mineralization, adsorbed mercury, HgCl₂, HgS, HgSO₄ and organically fixed mercury are the most important mercury compounds. The concentrations, transport and secondary formation of mercury and its compounds is controlled by: (1) the content of organic matter, Fe-oxides/hydroxides and clay minerals of the soils; and (2) the composition of the underlying rocks.The occurrence of mercury-sulphur compounds indicates the topographic influence on down-slope dispersion and the direction of inclination of the ore body. HgS and HgSO₄ are the dominant mercury compounds in the ore; in the bedrock, mercury occurs mainly as adsorbed mercury.     

Interaction of finely ground galena and potassium amylxanthate in relation to flotation, 3. Influence of acid and neutral grinding

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Hallimond tube flotation and microelectrophoresis have been utilized to investigate the reactions in the adsorption-abstraction of K-amylxanthate on finely ground galena. The mineral was ground in a laboratory stainless steel rod mill under controlled conditions (pH 4.0 and 7.0) using HCl as a pH regulator. X-ray photoelectron spectroscopic (XPS) studies have been carried out in order to characterize the surface oxidation products after grinding (weak amounts of S_n and PbS₂O₃). The two-stage adsorption process discovered in previous studies was confirmed. For low concentrations or submonolayer capacity, the layer is formed with 1:1 monocoordinated lead xanthate and dixanthogen. For higher values of surface coverage, it is composed of lead xanthate (stoichiometric at pH 7 and non-stoichiometric at pH 4), amyldixanthogen and amylcarbonate disulphide. In the second stage mainly dixanthogen is formed. This stage corresponds to complete flotation and to a sharp decrease in zeta potentials.     

An electrochemical investigation of pyrite flotation and depression

The oxidation of ethyl xanthate on pyrite electrodes, and the influence of the flotation depressants hydroxide, cyanide, and sulphide, have been investigated using cyclic voltammetry. A layer of a hydrated iron oxide has been identified on pyrite surfaces. Xanthate does not interact with this layer but is oxidized to dixanthogen at positive potentials. An increase in pH results in an increase in the background current due to oxidation of the mineral, and at pH=11 this reaction becomes faster than xanthate oxidation. Cyanide interacts with the electrode to form a surface species which inhibits xanthate oxidation. Sulphide gives rise to an anodic wave preceding the wave due to xanthate oxidation. The flotation and depression of pyrite are interpreted in terms of mixed-potential mechanisms.     

Depression of galena by slime gangue in an agitated pulp

The effect of gangue on mineral behaviour in a miniature batch version of an industrial flotation machine was examined. The flotation systems consisted of a sulphide, galena, with a specific collector, potassium ethyl xanthate, and a particular non-sulphide gangue in each case. Three gangue minerals were selected: corundum, fluorite and quartz. Flotation experiments were augmented with spectrophotometric adsorption and zeta potential measurements. Based on this data, the validity and significance of the slime-coating hypothesis in the context of the environment characteristic of industrial flotation processes were assessed.     

Electrochemical studies of sulphides,II. Measurement of the galvanic currents in galena and the general mechanism of oxygen reduction and xanthate adsorption on sulphides in relation to flotation

Galvanic currents were measured by short circuiting two half cells; PbS  N₂ (g)  KNO₃ | KNO₃  O₂ (g) or H₂O₂  PbS, and then after adding xanthate to the l.h.s. cell. Such addition of xanthate resulted in a 10 fold (with O₂ in the r.h.s. cell) and 100 fold (with H₂O₂ in r.h.s. cell) increase in the short circuit and steady state currents and also lead to bubble attachment on galena in the l.h.s. cell, in nitrogen atmosphere. The results indicated a heterogeneous, two site electrochemical mechanism for the reduction of oxygen and oxidative adsorption of xanthate on galena.     

Adsorption of arsenic by natural aquifer material in the San Antonio-El Triunfo mining area, Baja California, Mexico

 Several experiments of arsenic (As) adsorption by aquifer material of the San Antonio-El Triunfo (SA-ET) mining area were conducted to test the feasibility of this material acting as a natural control for As concentrations in groundwater. This aquifer material is mineralogically complex, composed of quartz, feldspar, calcite, chlorite, illite, and magnetite/hematite. The total iron content (Fe₂O₃) in the fine fraction is ∼12%, whereas Fe₂O₃ in the coarse fraction is <10 wt%. The experimental percent total As adsorbed vs. pH curves obtained match the topology of total As adsorbed onto iron oxi-hydroxides surface (arsenate + arsenite; high adsorption at low pH, low adsorption at high pH). A maximum of about 80% adsorbed in the experiments suggests the presence of arsenite in the experimental solutions. The experimental adsorption isotherm at pH 7 indicates saturation of surface sites at high solute concentrations. Surface titration of the aquifer material indicates a point of zero charge (PZC) for the adsorbent of about 8 to 8.5 (PZC for iron oxyhydroxides =7.9–8.2). Comparison between experimental and modeled results (using the MICROQL and MINTEQA2 geochemical modeling and speciation computer programs) suggests that As is being adsorbed mostly by oxyhydroxides surfaces in the natural environment. Based on an estimated retardation factor (R), the travel time of the As plume from the SA-ET area to La Paz and Los Planes is about 700 to 5000 years. Received: 17 March 1997 · Accepted: 8 September 1997     

Formation and recognition of alkyl xanthyl thiosulphates in sulphide ore flotation liquors

Alkyl xanthyl thiosulphates (R.OCSS.S₂O₃^?) (RXT^?) are formed in solution by mild oxidation (e.g. by I₂) of solutions containing both xanthate and thiosulphate. They can also be formed by reaction of Cu²⁺ with xanthate and thiosulphate, reaction of dixanthogen with thiosulphate, and by reaction of xanthate with tetrathionate; these last three reactions can occur in flotation pulps in slightly acid or alkaline solutions (pH 5–10).Alkyl xanthyl thiosulphates are stable in acid and neutral solution; the solutions have a UV absorption maximum at 289 nm. In strongly alkaline solution (pH 12) RXT^? decomposes within a few minutes to yield a xanthate (mostly) plus a little perxanthate. At pH 10 this decomposition to xanthate takes about 48 h. At pH 7–9 RXT^? is relatively stable. RXT^? is not extracted from aqueous solution with common solvents (chloroform, iso-octane, cyclohexane, or ether). It forms a water-insoluble adduct with cetyltrimethyl-ammonium bromide (CTAB); this adduct can be extracted into chloroform, and the extract has a UV absorption maximum at 296 nm.RXT^? was found in solutions from the gangue-sulphide flotation section at Renison Ltd, the zinc flotation circuit and the copper flotation circuit at Mount Isa Mines Ltd, and the lead flotation section of The Zinc Corporation Ltd. The presence of RXT^? in operating flotation plants has various practical and theoretical implications.     

Magnesia mixture as a regulator in the separation of pyrite from chalcopyrite and arsenopyrite

The aim of this paper is to find an effective method for the separation of the undesirable constituents, namely, chalcopyrite and arsenopyrite from pyrite used for the production of H₂SO₄. A new effective method is developed for co-depressing chalcopyrite with arsenopyrite by AsI₃, followed by the addition of magnesia mixture. This method has been shown to be based on the fact that iron sites exist in the three minerals, whereas copper and arsenic sites exist only in chalcopyrite and arsenopyrite, respectively. This is coupled with the ability of both Cu(I) and Cu(II) to precipitate As(III) in the form of insoluble copper arsenides, namely Cu₃As, Cu₃As₂. In contrast, neither Fe(II) nor Fe(III) form stable arsenides. Consequently, As³⁺ ions are selectively adsorbed onto the surface of chalcopyrite. The facility for oxidizability of As(III) is well known and hence it adsorbs oxygen from the pulp and changes to As(V) of higher valency and smaller size, with ionic potential over 10. Accordingly, it yields a stable complex anion with covalent bonding, namely, [AsO₄]^3?. These newly created arsenate sites on the surface of chalcopyrite, as well as the corresponding original arsenate sites on the surface of arsenopyrite combine with magnesia mixture to form cations leading to the formation of tightly abutting strongly hydrophilic layers of … AsO₄NH₄Mg.6H₂O. The spread of this hydrophilic film on arsenopyrite and chalcopyrite surfaces leads to the screening of their surfaces, making them difficult of access for the collector, ethyl xanthate. Since the pK_a of xanthic acid occurs at pH below 3, xanthate species predominate at pH above 8 and are adsorbed selectively on the pyrite surface in sufficient quantity for its selective flotation and hence for its separation to take place in the pH range 8–9.     

Sulphidizing reactions in the flotation of oxidized copper minerals,II. Role of the adsorption and oxidation of sodium sulphide in the flotation of chrysocolla and malachite

The rate of consumption of sulphide in the sulphidizing reactions of malachite and chrysocolla has been measured. The oxidation of sulphide ions at the surface of sulphidized chrysocolla was shown to take place. The influence of thiosulphate anions on the xanthate flotation of sulphidized malachite and chrysocolla was investigated and it was shown to depress the flotation of chrysocolla strongly.The result suggest, that the presence of thiosulphate as a product of simultaneous oxidation can be one of the reasons for the more difficult flotation of sulphidized chrysocolla.     

Adsorption of organic matter at mineral/water interfaces: I. ATR-FTIR spectroscopic and quantum chemical study of oxalate adsorbed at boehmite/water and corundum/water interfaces

The types and structures of adsorption complexes formed by oxalate at boehmite (γ-AlOOH)/water and corundum (α-Al₂O₃)/water interfaces were determined using in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy and quantum chemical simulation methods. At pH 5.1, at least four different oxalate species were found at or near the boehmite/water interface for oxalate surface coverages (Γ_ox) ranging from 0.25 to 16.44 μmol/m². At relatively low coverages (Γ_ox < 2.47), strongly adsorbed inner-sphere oxalate species (IR peaks at 1286, 1418, 1700, and 1720 cm⁻¹) replace weakly adsorbed carbonate species, and a small proportion of oxalate anions are adsorbed in an outer-sphere mode (IR peaks at 1314 and 1591 cm⁻¹). IR peaks indicative of inner-sphere adsorbed oxalate are also observed for oxalate at the corundum/water interface at Γ_ox = 1.4 μmol/m². With increasing oxalate concentration (Γ_ox > 2.47 μmol/m²), the boehmite surface binding sites for inner-sphere adsorbed oxalate become saturated, and excess oxalate ions are present dominantly as aqueous species (IR peaks at 1309 and 1571 cm⁻¹). In addition to these adsorption processes, oxalate-promoted dissolution of boehmite following inner-sphere oxalate adsorption becomes increasingly pronounced with increasing Γ_ox and results in an aqueous Al(III)-oxalate species, as indicated by shifted IR peaks (1286 → 1297 cm⁻¹ and 1418 → 1408 cm⁻¹). At pH 2.5, no outer-sphere adsorbed oxalate or aqueous oxalate species were observed. The similarity of adsorbed oxalate spectral features at pH 2.5 and 5.1 implies that the adsorption mechanism of aqueous HOx⁻ species involves loss of protons from this species during the ligand-exchange reaction. As a consequence, adsorbed inner-sphere oxalate and aqueous Al(III)-oxalate complexes formed at pH 2.5 have coordination geometries very similar to those formed at pH 5.1.The coordination geometry of inner-sphere adsorbed oxalate species was also predicted using quantum chemical geometry optimization and IR vibrational frequency calculations. Geometry-optimized Al₈O₁₂ and Al₁₄O₂₂ clusters with the reactive surface Al site coordinated by three oxygens were used as model substrates for corundum and boehmite surfaces. Among the models considered, calculated IR frequencies based on a bidentate side-on structure with a 5-membered ring agree best with the observed frequencies for boehmite/oxalate/water samples at Γ_ox = 0.25 to 16.44 μmol/m² and pH 2.5 and 5.1, and for a corundum/oxalate/water sample at Γ_ox = 1.4 μmol/m² and pH 5.1. Based on these results, we suggest that oxalate bonding on boehmite and corundum surfaces results in 5-coordinated rather than 4- or 6-coordinated Al surface sites.     

ATR-FTIR spectroscopic study of the adsorption of desferrioxamine B and aerobactin to the surface of lepidocrocite (γ-FeOOH)

The adsorption of two model siderophores, desferrioxamine B (DFOB) and aerobactin, to lepidocrocite (γ-FeOOH) was investigated by attenuated total reflection infrared spectroscopy (ATR-FTIR). The adsorption of DFOB was investigated between pH 4.0 and 10.6. The spectra of adsorbed DFOB indicated that two to three hydroxamic acid groups of adsorbed DFOB were deprotonated in the pH range 4.0-8.2. Deprotonation of hydroxamic acid groups of adsorbed DFOB at pH values well below the first acid dissociation constant of solution DFOB species (pK_a = 8.3) and well below the point of zero charge of lepidocrocite (pH_PZC = 7.4) suggested that the surface speciation at the lower end of this pH range (pH 4) is dominated by a surface DFOB species with inner-sphere coordination of two to three hydroxamic acids groups to the surface. Maximum adsorption of DFOB occurred at approximately pH 8.6, close to the first pK_a value of the hydroxamic acid groups, and decreased at lower and higher pH values.The spectra of adsorbed aerobactin in the pH range 3-9 indicated at least three different surface species. Due to the small spectral contributions of the hydroxamic acid groups of aerobactin, the interactions of these functional groups with the surface could not be resolved. At high pH, the spectral similarity of adsorbed aerobactin with free aerobactin deprotonated at the carboxylic acid groups indicated outer-sphere complexation of the carboxylate groups. With decreasing pH, a significant peak shift of the asymmetric carboxylate stretch vibration was observed. This finding suggested that the (lateral) carboxylic acid groups are coordinated to the surface either as inner-sphere complexes or as outer-sphere complexes that are strongly stabilized at the surface by hydrogen bonding at low pH.     

Adsorption and desorption characteristics of ammonium in eight loams irrigated with reclaimed wastewater from intensive hogpen

There are many reports of NO₃ ^? violating safety standards in the neighboring areas of concentrated animal feeding operations (CAFOs), which have become the bottleneck of the CAFOs development. The high concentration of ammonium nitrogen (NH₄ ⁺-N), which transforms into nitrate nitrogen (NO₃ ^?-N) through nitrification, and then leaches into the groundwater, is a potential threat to the environment. Adsorption and desorption characteristics of ammonium can reduce the amount of NH₄ ⁺-N in soils, which effectively prevents or slows down the nitrate leaching. Researches on the adsorption and desorption of ammonium mainly focus on the simple NH₄ ⁺ solution. Researches on the adsorption and desorption from hogpen wastewater are few, which is a complex system coexisting with many ions. In this paper, ammonium was selected as the object of pollutant, a batch of equilibration experiments was conducted to evaluate the adsorption–desorption and its kinetics in eight loams, typically found in Northern China, irrigated with original wastewater (OW) and reclaimed wastewater (RW) from intensive hogpen and a simple one consisting of clean water (CW). This study showed that the Freundlich and Langmuir model described the ammonium adsorption properties very well in multi-ion coexistensive system of hogpen wastewater; the ammonium adsorbed amount in the corresponding matrices followed by OW < RW < CW tendency, although the adsorption model parameters had great diversity. The adsorbed amount increased as the adsorption time went on and then approached to a stable state. CW had the shortest reaction time to reach equilibrium, whereas OW had the longest. The normal adsorption kinetics equation could not depict the adsorption behavior of loams but characterized by the ExpAssoc equation well. The study could provide references for the wastewater treatment and recycling, and rural water pollution controlling.