Insights into the changes in the surface properties of goethite with Ni in the lattice in the presence of salicylhydroxamic acid:Experimental and density functional theory studies

Comparative experiments and theoretical analysis of the surface chemistry changes of goethite(GT)and goethite containing Ni(GTN)in the lattice in the presence of salicylhydroxamic acid(SA)were performed.It was revealed that in the presence of 100 g·t^(-1)of SA,the flotation recovery of GTN and GT increased with increasing pH,achieving a maximum recovery of 98.9%for both minerals at p H 8.3 and decreasing beyond that pH,with GTN having a slightly higher recovery than GT,except at pH 8.3.This was further confirmed by the higher complexation energies of GTN···SA(-883.87 kJ·mol^(-1))compared with GT···SA(-604.23 kJ·mol^(-1))resulting from covalent,closed-shell,and conventional hydrogen bonding.The higher adsorption of SA onto GTN relative to GT was due to the formation of aπ-hole in GTN,thereby promoting a higher interaction of the collector with the mineral.Thus,the presence of Ni in the GT lattice improves and decreases the adsorption and desorption of SA onto and from the mineral,respectively,compared with those onto and from GT.

Comparison of the effects of Ti-and Si-containing minerals on goethite transformation in the Bayer digestion of goethitic bauxite

Goethitic bauxite is a widely used raw material in the alumina industry.It is an essential prerequisite to clarify the effect of Ti-and Si-containing minerals on goethite transformation in the Bayer digestion process,which could efficiently utilize the Fe-and Al-containing minerals present in goethitic bauxite.In this work,the interactions between anatase or kaolinite with goethite during various Bayer digestion processes were investigated using X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),transmission electron microscopy (TEM),and scanning electron microscopy (SEM).The results showed that anatase and kaolinite hindered the transformation of goethite.Anatase exerted more significant effects than kaolinite due to the dense sodium titanate layer on the goethite surface after reacting with the sodium aluminate solution.Adding the reductant hydrazine hydrate could eliminate the retarding effect by inducing the transformation of goethite into magnetite.In this process,titanium was embedded into the magnetite lattice to form Ti-containing magnetite.Furthermore,the weakening of the interaction between magnetite and sodium aluminosilicate hydrate reduced the influence of kaolinite.As a validation of the above results,the reductive Bayer method resulted in the transformation of goethite into goethitic bauxite with 98.87% relative alumina digestion rate.The obtained red mud with 72.99wt% Fe2O3could be further utilized in the steel industry.This work provides a clear understanding of the transformative effects of Ti-and Si-containing minerals on iron mineral transformation and aids the comprehensive use of iron and aluminum in goethitic bauxite subjected to the reductive Bayer method.

Negative effects of dissolved organic compounds on settling performance of goethite in Bayer red mud

Goethite, a typical iron-containing monomineral in red mud, was synthesized under the simulated Bayer digestion condition during the alumina production. The effects of dissolved organic compounds including sodium formate, sodium acetate, sodium oxalate, sodium salicylate and disodium phthalate on the settling performance of goethite slurries were studied. The settling performance of the slurries was also investigated with the addition of self-made hydroxamated polyacrylamide flocculant （HCPAM）. The adsorption mechanism of dissolved organic compounds on the goethite surfaces was studied by FT-IR and XPS, respectively. The results show that the addition of organic compounds lowers the settling performance of the slurries and a deterioration in settling performance is observed in the order of sodium oxalate 〉 sodium salicylate （~ disodium phthalate） 〉 sodium formate 〉 sodium acetate. Moreover, HCPAM can efficiently eliminate the negative effects of sodium formate, sodium acetate and sodium oxalate on the settling performance of the goethite slurries, but it can only partially improve the settling performance of the goethite slurries containing sodium salicylate or disodium phthalate. FT-IR and XPS results show that these organic compounds are chemically adsorbed on the goethite surface.

FT-IR, XPS, and DFT Study of Adsorption Mechanism of Sodium Acetohydroxamate onto Goethite or Hematite

The adsorption of sodium acetohydroxamate on the goethite or hematite surface was investi- gated by Fourier transform infrared spectroscopy （FT-IR）, X-ray photoemission spectroscopy and periodic plane-wave density functional theory （DFT） calculations. The core-level shifts and charge transfers of the adsorbed surface iron sites calculated by DFT with periodic in- terfacial structures were confronted to the X-ray photoemission experiments. FT-IR results reveal that the interracial structure of sodium acetohydroxamate adsorbed on the goethite or hematite surface may be assigned to a five-membered ring complex. In agreement with the adsorption energies determined by the DFT calculations, a five-membered ring complex is formed via bonding of one surface iron atom of goethite （101） or （100） to both oxygen atoms of hydroxamate group, and these two oxygen atoms of the hydroxamate group correspond- ingly attach to two neighboring iron atoms of the goethite surface. But a five-membered ring complex between two oxygen atoms of the hydroxamate group and one surface iron atom of hematite （001） is formed without any extra attachments. The calculated core-level shifts of Fe2p for the interracial structures are correspondingly in good agreement with the experimental observed one, which confirmed the reliability of the calculated results.

Competitive and cooperative adsorption of arsenate and citrate on goethite

The fate of arsenic in natural environments is influenced by adsorption onto metal （hydr）oxides. The extent of arsenic adsorption is strongly affected by coexisting dissolved natural organic acids. Recently, some studies reported that there existed competitive adsorption between arsenate and citrate on goethite. Humic acid is known to interact strongly with arsenate by forming complexes in aqueous solution, hence it is necessary to undertake a comprehensive study of the adsorption of arsenate/citrate onto goethite in the presence of one another. The results showed that at the arsenate concentrations used in this study （0.006-0.27 mmol/L）, citrate decreased arsenate adsorption at acidic pH but no effect was observed at alkaline pH. In comparison, citrate adsorption was inhibited at acidic pH, but enhanced at alkaline pH by arsenate. This was probably due to the formation of complex between arsenate and citrate like the case of arsenate with humic acid. These results implied that the mechanism of the adsorption of arsenate and citrate onto goethite in the presence of one another involved not only competition for binding sites, but the cooperation between the two species at the watergoethite interface as well.

A Study of Chromium Adsorption on Natural Goethite Biomineralized with Iron Bacteria

Goethite, especially biogenic goethite, has high specific surface area and great capacity for the adsorption of many contaminants including metal ions and organic chelates. Chromium is a redox actively toxic metal ion that exists as either Cr^Ⅲ or Cr^Ⅵ in nature, and as such it is essential to understand its behavior of adsorption on natural goethite mineralized by iron bacteria, as Gallionella and Leptothrix in water body. The adsorption of Cr^3＋ and Cr^Ⅵ on naturally biomineralized goethite is studied in this paper. The results show that both Langmuir and Freundlich adsorption isothermal models are able to accurately describe the adsorption of these two ions. Investigation of SEM/EDS, TEM/EDS indicates that the two ions do not adsorb homogeneously on goethite owing to the different microstructures of goethite, and that the microspherical goethite has a greater adsorption capacity for chromium ions than the helical one. XPS data show that redox reaction of chromium on the surface of biomineralized goethite takes place in the adsorption of both Cr^3＋ and Cr^Ⅵ. The CrvI adsorbed on biogoethite is much easier to transform into CrIII than the oxidization of Cr^Ⅲ on the bio-goethite.

Oxidation of As^Ⅲ by Several Manganese Oxide Minerals in Absence and Presence of Goethite

Oxidation of As^Ⅲ by three types of manganese oxide minerals affected by goethite was investigated by chemical analysis, equilibrium redox, X-ray diffraction （XRD） and transmission electron microscopy （TEM）. Three synthesized Mn oxide minerals of different types, birnessite, todorokite, and hausmannite, could actively oxidize As^Ⅲ to Asv, and greatly varied in their oxidation ability. Layer structured birnessite exhibited the highest capacity of As^Ⅲ oxidation, followed by the tunnel structured todorokite. Lower oxide hansmannite possessed much low capacity of As^Ⅲ oxidation, and released more Mn^2＋ than birnessite and todorokite during the oxidation. The maximum amount of Asv produced during the oxidation of As^Ⅲ by Mn oxide minerals was in the order： birnessite （480.4 mmol/kg） 〉 todorokite （279.6 mmol/kg） 〉 hansmannite （117.9 mmol/kg）. The oxidation capacity of the Mn oxide minerals was found to be relative to the composition, crystallinity, and surface properties. In the presence of goethite oxidation of As^Ⅲ by Mn oxide minerals increased, with maximum amounts of Asv being 651.0 mmol/kg for birnessite, 332.3 mmol/kg for todorokite and 159.4 mmol/kg for hansmannite. Goethite promoted As^Ⅲ oxidation on the surface of Mn oxide minerals through adsorption of the Asv produced, incurring the decrease of Asv concentration in solutions. Thus, the combined effects of the oxidation （by Mn oxide minerals）-adsorption （by goethite） lead to rapid oxidation and immobilization of As in soils and sediments and alleviation of the As^Ⅲ toxicity in the environments.

Incorporation of silica into the goethite structure: a microscopic and spectroscopic study

Quartz and iron(hydr)oxide are reactive surface phases that are often associated with one another in soils and sediments. Despite the several studies on the coating of quartz with iron oxides, the reactivity of dissolved species(Si) leached from quartz with iron(hydr)oxides has received limited attention. In this study, goethite synthesized on quartz substrates were characterized using field emission scanning electron microscopy, X-ray diffraction(XRD), transmission electron microscopy, and Fouriertransform infrared(FT-IR) spectroscopy. The SEM characterization revealed that bundles of thin parallel aligned goethite rods were formed at p H [ 10, while large pseudohexagonal crystals of twinned goethite needles were synthesized at p H B 10 after dehydration and hydration in the alkaline media. TEM analysis showed expanded and distorted lattice spacing of the crystal structure of iron(hydr)oxide due to silica incorporation. The characterization showed that silica increased the crystallite size of the goethite and transformed its acicular texture to a larger,twinned needle structure. FT-IR and XRD analyses revealed band shifts in crystal bonds as well as new bond formations, which indicate the presence of changes in the chemical environment of Fe–O and Si–O bonds. Thus, the presence of sorbed silicates modifies the crystal and lattice structure of goethite.

Goethite Morphologies of Some Soils in South of Central China

The morphologies of goethites in latosol, red soils, yellow-brown soil and the paddy soils developed from red soils were studied in comparison with the morphology of synthetic goethite by means of the X-ray distraction, transmission electron microscopy and energy dispersive X-ray analysis. The synthetic goethite displayed acicular particles elongated in the c-direction. The goethites in the latosol, red soils and yellowbrown soil were platy particles stretched in two directions or isodimensional particles, and those in the paddy soils from red soils were acicular, short columnar, platy or isodimensional particles. Various morphologies of the goethites probably suggested their different dominant crystal faces, surface charge distribution and surface adsorption characteristics.

Improvement of visible light-induced photocatalytic performance by Cr-doped SrTiO_3-carbon nitride intercalation compound (CNIC) composite

Novel organic-inorganic composite photocatalyst offers new opportunities in the practical applications of photocatalysis. Novel visible light-induced Cr-doped Sr Ti O3–carbon nitride intercalation compound(CNIC) composite photocatalysts were synthesized. The composite photocatalyst was characterized by X-ray diffraction(XRD), field-emission scanning electron microscopy(FESEM), high-resolution transmission electron microscopy(HRTEM), Fourier transform infrared(FT-IR) spectroscopy, UV-vis diffuse reflection spectroscopy, photoluminescence(PL) spectroscopy, and BET surface area analyzer. The photocatalytic oxidation ability of the novel composite photocatalyst was evaluated using methyl orange(MO) as a target pollutant. The photocatalysts exhibited a significantly enhanced photocatalytic performance in degrading MO. For maximizing the photodegradation activity of the composite photocatalysts, the optimal CNIC content was determined. The improved photocatalytic activity of the as-prepared Cr-doped Sr Ti O3–CNIC composite photocatalyst may be attributed to the enhancement of photo-generated electron–hole separations at the interface.

Study on Mechanisms of Cd^(2+) Sorption on Goethite byMicrocalorimetry

A study on energy changes and mechanisms of Cd sorbed on goethite was performed using the technique ofmicrocalorimetry. The data of the amount of Cd sorption (Aq) and concentration of Cd in equilibrium solution(Ce), and the data of Aq and the heat effect (AH) caused by Cd^2+ sorption on goethite all fitted Langmuirisotherm. The amount of heat released from Cd sorption on goethite increased with the amount of Ce or Aq.The reaction process of Cd sorption on goethite may be divided into five stages and three plateaus, dependingon the variation of enthalpy change (.Hm) of Cd sorption with Aq, which implied three mechanisms ofinteraCtion between Cd and goethite. The experimental results showed that the microcalorimetry may beuseful for determination of microcalorie variation in soil.

Visible Light-induced Cr-doped SrTiO_3-g-C_3N_4 Composite for Improved Photocatalytic Performance

Novel visible light-induced Cr-doped Sr Ti O3-g-C3N4 composite photocatalysts were synthesized by introducing polymeric g-C3N4. The composite photocatalyst was characterized by X-ray diffraction（XRD）, high-resolution transmission electron microscopy（HRTEM）, Fourier transform infrared（FT-IR） spectroscopy, UV-vis diffuse reflection spectroscopy, photoluminescence（PL） spectroscopy and BET surface area measurements. The photocatalytic oxidation ability of the novel composite photocatalyst was evaluated using methyl orange（MO） as a target pollutant. The photocatalysts exhibited a significantly enhanced photocatalytic performance in degrading MO. The optimal g-C3N4 content for the photodegradation activity of the composite photocatalysts was determined. The as-prepared composite photocatalyst exhibits an improved photocatalytic activity due to enhancement of photo-generated electron-hole separation at the interface.

Synthesis and Electrochemical Properties of Cr-doped Li_3V_2(PO_4)_3 Cathode Materials for Lithium-ion Batteries

Cr-doped Li3V2（PO4）3 cathode materials Li3V2-xCr（PO4）3 were prepared by a carbothermal reduction（CTR） process. The properties of the Cr-doped Li3V2（PO4）3 were investigated by X-ray diffraction （XRD）, scanning electron microscopic （SEM）, and electrochemical measurements Results show that the Cr-doped Li3V2（PO4）3 has the same monoclinic structure as the undoped Li3V2（PO4）3, and the particle size of Cr-doped Li3V2（PO4）3 is smaller than that of the undoped Li3V2（PO4）3 and the smallest particle size is only about 1 1μm. The Cr-doped Li3V2（PO4）3 samples were investigated on the Li extraction/insertion performances through charge/discharge, cyclic voltammogram （CV）, and electrochemical impedance spectra（EIS）. The optimal doping content of Cr was that x=0.04 in the Li3V2-xCrx（PO4）3 samples to achieve high discharge capacity and good cyclic stability. The electrode reaction reversibility was enhanced, and the charge transfer resistance was decreased through the Cr-doping. The improved electrochemical performances of the Cr-doped Li3V2（PO4）3 cathode materials are attributed to the addition of Cr^3＋ ion by stabilizing the monoclinic structure.

Flocculation performance and mechanism of hydroxamate flocculants on synthetic hematite or goethite suspension

Suitable amounts of Al(OH)3, Na OH and synthetic hematite or goethite were taken to be digested under the simulated condition of the Bayer process and subsequently diluted with simulative Bayer red mud lotion, then the synthetic hematite or goethite suspension was obtained. The flocculation effect of self-made modified poly(acrylic acids) flocculants containing hydroxamic acid groups(abbreviated as HPAA) on the synthetic hematite or goethite suspension was studied. The experimental results show that with the increase of the dosages of added flocculants, the average settling rate of the synthetic hematite or goethite suspension in the first 1min speeds up, the supernatant solids decrease. It could also be found that there is generally a decrease in the average settling rate of the synthetic hematite or goethite suspension in the first 1 min with increasing solid content of the suspension. When the solid content of hematite or goethite of the suspension is 50 g/L with 240 g/t dosage of HPAA, HPAA has a better flocculation effect on the synthetic goethite suspension than on the synthetic hematite suspension. The adsorption mechanism of HPAA on the surface of hematite or goethite was investigated by Fourier transform infrared spectroscopy(FT-IR) and X-ray photoelectron spectroscopy(XPS).All the results suggest that HPAA is adsorbed on the hematite or goethite surface by a chemisorption, and it has a stronger adsorption on the goethite surface than on the hematite surface.

Using Goethite as a Heterogeneous Fenton Catalyst for The Removal of Tetracycline Hydrochloride: Effects of Its Adsorptive and Reductive Activities

The removal of the antibiotic compound tetracycline hydrochloride (TC) was investigated by using goethite/H2O2 as a heterogeneous Fenton reagent. Five principle operational parameters, especially solution pH value, were taken into account to investigate how the heterogeneous Fenton process factors mediated the TC removal. This process was effective but seriously impacted by the pH value and temperature, as well as the dosages of α-FeOOH, TC and H2O2. Very interestingly, the acidic and alkaline aqueous medium conditions were both very favorable due to the occurrence of transformation of Fe(III) to Fe(II) on goethite surfaces reduced by TC at pH 3.0~4.0 even though with a low adsorption capacity of TC because its maximum adsorption of negatively charged form occurred at pH around 8.0[1], thereby greatly promoting the TC Fenton oxidative elimination. However, a rapid initial TC decay was observed at the first 5 min, followed by a much slower retardation stage, which was likely because the reductive transformation of Fe(III) to Fe(II) by TC in the solution was inhibited as the Fenton reaction proceeded. Moreover, the hydroxyl radical scavenger t-butanol addition can decrease the removal rate of TC in the goethite/H2O2 system to a certain extent. This further indicated that the main reactive species in this process were hydroxyl radicals[2]. All the goethite-catalysed heterogeneous Fenton reactions are responsible for the TC removal following the Langmuir-Hinshelwood model, were well fitted to pseudo-first order kinetics (R2】0.99), and their apparent activation energy (E) for this Fenton-like reaction was 31.86 kJ mol 1, a low value that is highly consistent with the ease of TC decay greatly enhanced by the temperature rise, indicated that the interfacial controlling interactions such as a proton induced solubilization and a reductive dissolution of goethite can clearly improve its Fenton catalytic activity[3], and these dissolution processes may not be effective in some cases, while the TC adsorption process may always play an important role to control the TC removal rate during the Fenton reaction.

Biomass Roasting Reduction of Goethite Ores

Effective use of low-grade goethite ores in steel industry is necessary to achieve cost reduction and solve the problem of resource shortage. Biomass as heating and reducing agent attract much more attention for utilization in ironmaking process due to its low-carbon, energy-saving, emission-cutting and low-cost. We investigate three types of biomass (corn straw, pine sawdust, rice husk powders) roasting reduction mechanism and the magnetism of the roasting products. Structure analysis indicates that 15% dosage of each biomass mixed with goethite ores roasting at 550-600 °C for 1h could be effectively converted into strong magnetic product, i.e. maghemite (γ-Fe2O3). Weak magnetic separation shows that under the magnetic field of 200 kA/m, goethite ores roasted by 15% of pine sawdust could achieve TFe 61.64% with the recovery of 79.75%, TFe 61.75% with the recovery of 80.16% for roasting with rice husk, and TFe 61.47% with the recovery of 81.28% for roasting with corn straw.

Turning Industrial Residues into Resources： An Environmental Impact Assessment of Goethite Valorization

Goethite is a metals-rich residue that occurs during zinc production. The feasibility of metal recovery from goethite has been demonstrated, but is not economically viable on an industrial scale. Therefore, goethite is landfilled with considerable economic costs and environmental risks. The goal of this study is to evaluate the environmental performance of a new valorization strategy for goethite residues from zinc production, with the aims of： ① recovering the valuable zinc contained in the goethite and ② avoiding the landfilling of goethite by producing a clean byproduct. The presented goethite valoriza- tion strategy consists of a sequence of two processes： ① plasma fuming and ② inorganic polymerization of the fumed slag. Plasma fuming recovers the valuable metals by fuming the goethite. The metals-flee fumed slag undergoes a process of inorganic polymerization to form inorganic polymers, that can be used as a novel building material, as an alternative to ordinary Portland cement （OPC）-based concrete. Life- cycle assessment （LCA） is used to compare the environmental performance of the inorganic polymer with the environmental performances of equivalent OPC-based concrete. The LCA results show the tradeoff between the environmental burdens of the fuming process and inorganic polymerization versus the environmental benefits of metal recovery, OPC concrete substitution, and the avoidance of goethite land- filling. The goethite-based inorganic polymers production shows better performances in several environ- mental impact categories, thanks to the avoided landfilling of goethite. However, in other environmental impact categories, such as global warming, the goethite valorization is strongly affected by the high-energy requirements of the plasma-fuming process, which represent the environmental hotspots of the proposed goethite recycling scheme. The key elements toward the sustainability of goethite valorization have been identified, and include the use of a clean electric mix, more effective control of the fumed gas emissions, and a reduced use of fumed slag through increased efficiency of the inorganic polymerization process.

Cr-Doped TiO_(2)Thin Films Prepared by Means of a Magnetron Co-Sputtering Process:Photocatalytic Application

This paper deals with the effect of Cr content on photocatalytic activity of TiO2 thin films deposited on quartz and intrinsic silicon substrates by using the RF magnetron co-sputtering process. Some physical investigations on such sputtered films were made by means of X-ray Diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy as well as UV-Vis-IR absorption techniques. The heat treatment under oxygen atmosphere at 550°C reveals that the crystalline structure of TiO2: Cr depends on Cr content. Anatase-to-rutile phase transformation occurs at a Cr content of about 7%. On the other hand, the band gap energy value of annealed TiO2: Cr films varies in terms of Cr doping and a transition around 7% of Cr is accrued. The photocatalytic activity of undoped and doped TiO2 films was evaluated by photo-degrading of the amido black under UV light irradiation. Modification of the chemical structure of titanium dioxide by Cr doping allows moving the photocatalytic activity of titanium dioxide towards visible light. The results indicate that films doped with 2% Cr exhibit the highest UV and visible light photocatalytic activity.

Synthesized Goethite and Natural Iron Oxide as Effective Absorbents for Simultaneous Removal of Co(II) and Ni(II) Ions from Water

This study reports on the adsorption efficiency of a natural iron oxide from Mballam-Cameroon in comparison with synthesized goethite to simulta-neously remove cobalt and nickel ions from aqueous solutions. Chemical analysis on the natural iron oxide sample revealed iron as the main element and hematite (58.52%) goethite (19.42%), kaolinite (12.69%) and quartz (7.79%) as the component phases in the iron oxide sample. The iron oxide was found to be microporous (BET surface area 43.27 m2/g) with fairly spherical polydisperse particles. Results show maximum absorption for Co(II) and Ni(II) ions for both adsorbents occurred at an equilibrium contact time of 80 mins, dose rate of 0.1 g/L, and pH = 7. Goethite was slightly more efficient at removing target metal ions with maximal adsorbed quantities at 117.8 mg/g of Co(II) and 100.6 mg/g of Ni(II), and 103.9 mg/g of Co(II) and 85.2 mg/g of Ni(II) ions for natural iron oxide. Equilibrium modelling presented the Freundlich isotherm as the best fit model for both adsorbents and metal ions, indicating heterogeneity of the surface binding sites during adsorption. The pseudo-second order kinetic model was the best-fit model, indicating chemical adsorption between the adsorbent surface and metal ions, hence a good correlation between equilibrium and kinetics. The findings indicate that the efficacy of the natural iron oxide from Mballam is almost equivalent to that of synthetic goethite, validating its applicability for the simultaneous removal of cobalt and nickel ions from aqueous solution.

Adsorption Mechanism of Water Molecule on Goethite(010) Surface

Abstract Goethite widely exists among ocean sediments; it plays an important role in fixing heavy metals and adsorbing organic contaminants. So the understanding of the adsorbing process of water molecule on its surface will be very helpful to further reveal such environmental friendly processes. The configuration, electronic properties and interaction energy of water molecules adsorbed on pnma goethite （010） surface were investigated in detail by using density functional theory on 6-31G （d,p） basis set and projec- tor-augment wave （PAW） method. The mechanism of the interaction between goethite surface and H20 was proposed. Despite the differences in total energy, there are four possible types of water molecule adsorption configurations on goethite （010） surface （Aa, Ab, Ba, Bb）, forming coordination bond with surface Fe atom. Results of theoretical modeling indicate that the dissociation process of adsorbed water is an endothermic reaction with high activation energy. The dissociation of adsorbed water molecule is a proton transportation process between water＇s O atoms and surface. PDOS results indicate that the bonding between 1-120 and （010） surface is due to the overlapping of water＇s 2p orbitals and Fe＇s 3d orbitals. These results clarify the mechanism on how adsorbed water is dissociated on the surface of goethite and potentially provide useful information of the surface chemistry of goethite.