Al-substituted Goethite Research Articles

Effect of Al Substitution on Visible Short-Wave Infrared Reflectance Spectroscopy (VSWIR) of Goethite and Ferrihydrite

Goethite and ferrihydrite are the two major iron hydroxides, essential mineral constituents in the terrestrial surface system. Aluminum (Al) is the most common substituent in iron hydroxides, and it may significantly change the bulk and surficial physicochemical properties of iron hydroxides. Consequently, a practical and convenient approach is needed to efficiently identify the Al substitution degrees of iron hydroxides in natural occurrences. This study presents a comprehensive investigation of the VSWIR characteristics of laboratory-synthesized Al-substituted goethite and ferrihydrite, to establish diagnostic VSWIR parameters for the identification and quantification of Al substitution levels in iron hydroxides. The findings revealed that Al substitution can affect the band positions (P) of goethite and ferrihydrite at ~650 nm, ~900 nm, and ~1400 nm. The relationships between the Al substitution of ferrihydrite and VSWIR parameters can be expressed as P900 = −0.43 × Al(%) + 931 and P1400 = −0.07 × Al(%) + 1428, while that of goethite can be expressed as P650 = 0.42 × Al(%) + 657 and P900 = 2.29 × Al(%) + 936. The peak fitting results showed that the absorption intensity at 480–550 nm linearly decreases with increased Al substitution. The obtained VSWIR spectra of Al-substituted goethite and ferrihydrite provide a critical supplement to the spectral library for (Al) iron hydroxides, and these VSWIR parameters can be utilized for the semi-quantitative determination of Al substitution in natural iron hydroxides

Efficient Nitrate Adsorption from Groundwater by Biochar-Supported Al-Substituted Goethite

Groundwater nitrate contamination is challenging and requires efficient solutions for nitrate removal. This study aims to investigate nitrate removal using a novel adsorbent, biochar-supported aluminum-substituted goethite (BAG). The results showed that an increase in the initial Al/(Al + Fe) atomic ratio for BAGs from 0 to 20% decreased the specific surface area from 115.2 to 75.7 m2/g, but enhanced the surface charge density from 0.0180 to 0.0843 C/m2. By comparison, 10% of Al/(Al + Fe) led to the optimal adsorbent for nitrate removal. The adsorbent’s adsorption capacity was effective with a wide pH range (4–8), and decreased with increasing ionic strength. The descending order of nitrate adsorption inhibition by co-existing anions was SO42−, HCO3−, PO43−, and Cl−. The adsorption kinetics and isotherms agreed well with the pseudo-first-order equation and Langmuir model, respectively. The theoretical maximum adsorption capacity was 96.1469 mg/g. Thermodynamic analysis showed that the nitrate adsorption was spontaneous and endothermic. After 10-cycle regeneration, the BAG still kept 92.6% of its original adsorption capacity for synthetic nitrate-contaminated groundwater. Moreover, the main adsorption mechanism was attributed to electrostatic attraction due to the enhancement of surface charge density by Al substitution. Accordingly, the BAG adsorbent is a potential solution to remove nitrate from groundwater.

Reading the climate signals hidden in bauxite

The dynamics of tropical weathering through time with the formation and long-term evolution of laterite associated with climatic and geodynamic forcing is still a poorly explored issue. To better access this, we focused on lateritic-bauxitic duricrusts from the well-explored, 300 m high Kaw mountain in northeastern French Guiana. Macroscopically homogeneous Fe (oxyhydr)oxides-rich subsamples were separated from 10 bulk samples. Bulk- and subsamples were investigated by mineralogical and geochemical analyses and (U-Th)/He geochronology. The samples show a large heterogeneity on the macro- and microscopic scale reflecting different stages of duricrust formation and evolution through processes of dissolution and (re)precipitation of Fe (oxyhydr)oxides. The 284 (U-Th)/He ages obtained for goethite α-(Fe, Al)OOH and hematite α-Fe2O3 range from 30.5 ± 3.1 to < 0.8 Ma and show large variability within a sample. The oldest hematite and Al-poor goethite subsamples precipitated since 30 Ma and formed while kaolinite was stable. Precipitation of Fe-minerals increased since ∼ 14–12 Ma but still happened under ferruginous lateritic, non-bauxitic conditions. The dissolution and (re)precipitation of Fe minerals, Al-substitution in goethite, and the overall enrichment in Th, as well as gibbsite precipitation indicate an intensification of weathering and a shift towards bauxitic conditions since the end of the Miocene. The Th-, U-, and Al-rich Fe phases formed in this late episode of intense weathering partially replace the oldest, often Th- and U-poor phases leading to a considerable age spread. We claim that this episode of intensified weathering, which had its peak at ca. 6–2 Ma, corresponds to the bauxitization of the lateritic cover of Kaw mountain.Our proposed model of lateritization since at least 30 Ma and bauxitization since the late Miocene provides new constraints on the timescale and intensity of weathering at Kaw mountain. The onset of lateritization in French Guiana (Kaw region) is potentially older than ca. 30 Ma and was possibly synchronous with the development of bauxites in neighboring countries, and differences in the Paleogene weathering intensity might be due to different drainage capacities of the parental material. Late Neogene bauxitization suggests a regional increase in precipitations and drainage potentially linked to increased incision owing to local uplift.Finally, we confirm that a detailed combination of geochronological results coupled to mineralogical and geochemical analyses improves our understanding of tropical weathering processes and duricrust formation by placing mineralogical and geochemical processes into a temporal framework.

Low-temperature thermal conversion of Al-substituted goethite in gibbsitic bauxite for maximum alumina extraction.

The conversion of Al-substituted goethite (Al-goethite) to hematite in gibbsitic bauxite is conducive to alumina extraction during the Bayer process and the enrichment of iron minerals in red mud. In this work, mineralogical characteristics of gibbsitic bauxite were identified by AMICS analysis, and the low-temperature thermal conversion behavior of both synthetic Al-goethite and natural Al-goethite in gibbsitic bauxite were investigated through thermal gravity analysis, phase transformation, and microstructure studies. Results show that the proportion of aluminum in Al-goethite reached 12.68% of the total aluminum content in gibbsitic bauxite. The conversion of synthetic Al-goethite to hematite starts at ∼280 °C, while that of natural Al-goethite starts at ∼320 °C, and the addition of NaOH can accelerate the conversion. The formed hematite inherits the needle-like appearance of the original Al-goethite, has many holes on the surface due to dehydroxylation, and no migration of aluminum elements occurs during the roasting process, indicating that Al-goethite transformed into porous Al-substituted hematite (Al-hematite), which is beneficial to the extraction of the aluminum retained in the hematite structure during Bayer digestion. To confirm the above results, digestion experiments (without or with roasting for typical Bayer digestion or low-temperature roasting-Bayer digestion) were carried out with gibbsitic bauxite and the one roasted at 400 °C for 30 min as raw materials, respectively. Compared to the typical Bayer digestion, the relative alumina recovery of low-temperature roasting-Bayer digestion increased from 90.06% to 95.65%, the red mud yield decreased from 36.32% to 34.08%, and the grade of Fe in red mud increased from 48.45% to 52.88% at 270 °C for 60 min. Enhanced transformation of Al-goethite significantly improves alumina recovery and the resultant iron-rich red mud can be easily co-processed in the steel industry, thus significant emission reduction of red mud from the Bayer system might be achieved.

Microstructure of Al-substituted goethite and its adsorption performance for Pb(II) and As(V)

Naturally occurring goethite commonly undergoes Al-substitution, while how changes in microstructure induced by Al-substitution affect the interactive reaction of Pb(II) or As(V) at the goethite-water interface remains poorly understood. This study reveals the structural properties of Al-substituted goethite and its adsorption behavior for Pb(II) and As(V) by multiple characterization techniques and Charge Distribution-Multisite Surface Complexation (CD-MUSIC) modeling. Al-substitution caused an obvious decrease in the length-to-width ratio in goethite particles and a slight decrease in the proportion of (110) facets. The presence of Al-O sites and higher surface roughness induced by Al-substitution contributed to a higher inner Stern layer capacitance (C1) and surface charge density of goethite. CD-MUSIC modeling results further revealed that the affinity constant of Pb(II) complex (log KPb) at the goethite-water interface and the adsorption capacity of goethite for Pb(II) decreased with increasing amount of Al-substitution, while an opposite tendency was observed for As(V) adsorption. The dominant species of both Pb(II) and As(V) on goethite were bidentate complexes, and Al-substitution had a minor impact on the abundance of Pb(II) and As(V) complexes on the surface of goethite. Overall, these experimental and modeling results provide new and important insights into the interfacial reactivity of Al-substituted goethite and facilitate the prediction of the environmental fate of heavy metals.

Effects of aluminum substitution on the surface charge of colloidal goethite particles: experiments and MUSIC modeling.

Proton adsorption behavior on the surface of Al-substituted goethites as a function of pH and ionic strength was investigated and simulated with the multisite surface complexation (MUSIC) model. In addition, X-ray diffraction, X-ray photoelectron spectroscopy, and field emission scanning electron microscope were used to characterize the crystal structure, chemical composition, micromorphology, and surface properties of the Al-substituted goethite. Al substitution was found to affect the crystal structure and micromorphology of goethite. The morphological differences did not result in significant differences in PZC value but largely affected the surface charge values. Goethite surface charge capacity increased progressively with increasing amount of Al substitution, which was attributed to increases in the density of surface coordinated sites due to the increase in (021)/(110) face ratio. The optimization calculations enabled a satisfactory fitting of the titration data of both pure goethite and Al-substituted goethite, and the MUSIC model facilitated a more specific understanding of the charging behavior of Al-substituted goethite. The singly (≡FeOH-0.5 + ≡AlOH-0.5) and triply coordinated (≡Fe3O-0.5 + ≡AlFe2O-0.5) surface groups were most likely responsible for the basic charging behavior of goethite in the pH range of 4-10. All results indicate that the MUSIC model has excellent performance in characterizing Al-substituted goethite, and the model has promising application prospect in other substituted metal (hydr)oxides.

Preferential phosphate sorption and Al substitution on goethite

The Al-substitution affected not only the basal spacing between adjacent lattice planes but also the structural arrangement in goethite which results in the Al-preferential PO4 sorption on Al-substituted goethite.

The transformation of α-(Al, Fe)OOH in natural fire: Effect of Al substitution amount on fixation of phosphate

The transformation of Al-substituted goethite in oxidative and reductive atmosphere, which models natural fire, was investigated in detail. Various characterization results indicated that Al-substituted goethite transformed into hematite in an oxidative atmosphere and magnetite followed by zero-valent iron in a reductive atmosphere. Interestingly, the substitution of Al for Fe not only changed the crystal morphology affecting crystal surface reactivity, but also restrained the transformation of goethite into hematite, magnetite and zero-valent iron. In addition, the Al in the goethite was embedded into the crystal structure of thermally formed hematite and magnetite, considerably influencing their surface reactivity. The macroscopic adsorption results indicated that the substitution of Al for Fe increased the adsorption capacity of goethite and the corresponding derivatives except zero-valent iron. For the same Al amount, the adsorption capacity followed an order of goethite > hematite > magnetite, implying a loss of phosphate in goethite-rich soil after experiencing natural fire. However, with increasing in temperature in the reductive atmosphere, Al-magnetite transformed into a mixture of Al-magnetite and zero-valent iron which displayed excellent phosphate adsorption capacities increasing to 1.21–5.96 mg/g. The phosphate adsorption behaviors to thermally formed products were fitted well by surface complexation modeling with five complexation sites, which were more obvious for goethite than for hematite and magnetite. These findings presented in this study represent significant progress toward an understanding of the migration, enrichment and transformation of phosphate in Al-substituted goethite-rich soil in cases experiencing natural fire.

Phosphate speciation on Al-substituted goethite: ATR-FTIR/2D-COS and CD-MUSIC modeling

Natural goethite commonly encounters Al substitution for Fe, which alters the surface reactivity of goethite.

Imaging of Al/Fe ratios in synthetic Al-goethite revealed by nanoscale secondary ion mass spectrometry.

Aluminium (Al)-substituted goethite is ubiquitous in soils and sediments. The extent of Al-substitution affects the physicochemical properties of the mineral and influences its macroscale properties. Bulk analysis only provides total Al/Fe ratios without providing information with respect to the Al-substitution of single minerals. Here, we demonstrate that nanoscale secondary ion mass spectrometry (NanoSIMS) enables the precise determination of Al-content in single minerals, while simultaneously visualising the variation of the Al/Fe ratio. Al-substituted goethite samples were synthesized with increasing Al concentrations of 0.1, 3, and 7 % and analysed by NanoSIMS in combination with established bulk spectroscopic methods (XRD, FTIR, Mössbauer spectroscopy). The high spatial resolution (50-150nm) of NanoSIMS is accompanied by a high number of single-point measurements. We statistically evaluated the Al/Fe ratios derived from NanoSIMS, while maintaining the spatial information and reassigning it to its original localization. XRD analyses confirmed increasing concentration of incorporated Al within the goethite structure. Mössbauer spectroscopy revealed 11 % of the goethite samples generated at high Al concentrations consisted of hematite. The NanoSIMS data show that the Al/Fe ratios are in agreement with bulk data derived from total digestion and demonstrated small spatial variability between single-point measurements. More advantageously, statistical analysis and reassignment of single-point measurements allowed us to identify distinct spots with significantly higher or lower Al/Fe ratios. NanoSIMS measurements confirmed the capacity to produce images, which indicated the uniform increase in Al-concentrations in goethite. Using a combination of statistical analysis with information from complementary spectroscopic techniques (XRD, FTIR and Mössbauer spectroscopy) we were further able to reveal spots with lower Al/Fe ratios as hematite.

Adsorption of Europium on Al-substituted goethite

Europium (Eu(III)) has been extensively used as an analogue of trivalent lanthanide in the radioactive wastewater from nuclear industry. The adsorption of Eu(III) on the Al-substituted goethite was investigated by characteristic and batch techniques. The characteristic results illustrated that the Al-substituted goethite was synthesized successfully and the acicular-like structure goethite was transformed to multi-domain jagged crystal with the increase of Al content, respectively. The adsorption amount of Eu(III) on Al-substituted goethite was increased with an increase of pH from 4.0 to 8.0. The thermodynamic parameters revealed that the adsorption of Eu(III) on Al-substituted goethite was a spontaneous and endothermic process. The increase of Al content increased the adsorption capacity of Al-substituted goethite to Eu(III). The maximum Eu(III) adsorption capacity was determined to be 6.75mg/g for α-(Fe0.85Al0.15)OOH at pH=5.5 and T=293K using Langmuir model. Furthermore, XPS (X-ray photoelectron spectroscopy) analysis verified that oxygen-containing functional groups were responsible for the major adsorption mechanism and the effect of ionic strength on the adsorption displayed the inner-sphere surface complexation of Al-substituted goethite dominated the adsorption process.

Kinetics and mechanism of Sr(II) adsorption by Al-Fe2O3: Evidence from XPS analysis

The Al-Fe2O3 with different aluminum contents were synthesized by heating Al-substituted goethite and were characterized by FESEM, XRD, FT-IR and XPS techniques. The characteristic results showed that the Fe3+ ion in hematite was partly substituted by Al3+ ion, leading to the change of crystal structure. The batch experiments indicated that the adsorptions of Sr(II) on Al-Fe2O3 slowly increased with increasing pH starting from 2 to 11. The pseudo-second-order kinetic and the Langmuir model gave the better fits to the adsorption kinetics and isotherms, respectively. The thermodynamic parameters indicated that the adsorption of Sr(II) on Fe2O3 was an endothermic and spontaneous process. The XPS analysis demonstrated that the oxygen-containing functional groups were responsible for the Sr(II) adsorption. These findings indicated that Al-Fe2O3 may be a valuable candidate for pre-concentration and solidification of radionuclides from aqueous solutions.

Studies on Adsorption Characteristics of Al-Free and Al-Substituted Goethite for Heavy Metal Ion Cr(VI)

In the present paper, α-FeOOH and α-Fe(Al)OOH were prepared, and the adsorption of Cr(VI) on the two samples was investigated. The influence of pH, initial concentration, and some anions such as SO4 2−, H2PO4 −, C2O4 2−, CO3 2−, and SiO3 2− on the adsorption of Cr(VI) on α-FeOOH and α-Fe(Al)OOH was studied by batch techniques. The results show that the adsorption capacity of Cr(VI) on α-Fe(Al)OOH increases with the introduction of aluminum, but decreases with the increase of pH. The adsorption irreversibility of Cr(VI) on α-Fe(Al)OOH is much higher than that on pure α-FeOOH. The adsorbed Cr(VI) species mainly exists in the form of *Fe(wk)-OHCrO4 2− on the surface of the samples. With the presence of SiO3 2−, CO3 2−, C2O4 2−, SO4 2−, and H2PO4 −, the binding of Cr(VI) is inhibited by different degree. The inhibition of those anions is larger in the pure goethite than that in the Al-substituted goethite system. After Al was introduced into α-FeOOH, Cr(VI) ions are preferentially adsorbed on Al sites rather than Fe sites on α-Fe(Al)OOH.

Multi‐Metal‐Substituted‐Goethite as an Effective Catalyst for Azo Dye Wastewater Oxidation

Different samples of goethite modified with cobalt, manganese, or aluminum were synthesized and characterized by means of X‐ray diffraction, chemical analysis, scanning electron microscopy, zeta potential measurements, and N2 adsorption–desorption isotherm analysis. The goethite oxides were tested as efficient catalysts for the degradation of methyl orange (MO, sodium 4‐[(4‐dimethylamino)phenylazo]benzenesulfonate), a sulfonated azo dye indicator, employing potassium persulfate (PS) in water, at pH 3. The Mn, Co, and Al substitution in goethite gave rise to active sites for PS activation. The most effective catalyst reached 93% of degradation at 120 min of reaction. The increase of temperature led to an expected conversion enhancement, and kinetic parameters were calculated for one of the evaluated catalysts. MO degradation curves best fitted to pseudo‐second order kinetics. The extent of mineralization, measured as total removed organic carbon (TOC), was also monitored. A significant degree of mineralization was achieved and confirmed by TOC analysis. A mechanism for MO oxidation with radicals was discarded and a pathway involving peroxymonosulfate species was proposed. The results indicate that the goethite‐substituted oxides are effective catalysts for the studied azo dye degradation in aqueous solution.

Ferralsols in the Cameroon plateaus, with a focus on the mineralogical control on their cation exchange capacities

Land use intensity in the Cameroon plateaus, where Ferralsols and the associated highly weathered soils are dominant, has been steadily increasing, so that soil fertility status and its controlling factors on these plateaus should be elucidated to establish long-term land management strategies. Although it is known that Ferralsols have small, but variable CEC values, it has not been fully determined which clay components are the main source of CECclay in Ferralsols. Here, soils in three Cameroon plateau regions of different altitude (the lower South Cameroon (SC) Plateau with an altitude of 300–600m, SCL; the upper SC Plateau with an altitude of 600–900m, SCH; and the Adamawa Plateau with an altitude above 900m, ADM) were collected, and their physicochemical and mineralogical properties were investigated. The relative abundance of illitic minerals, which are a minor but the most promising source of CECclay in Ferralsols, was determined by using three indicators: the relative abundances of 1.0nm-thick structures as represented by XRD peak intensity (XRD1.0nm), structural K in illitic minerals as represented by clay-K content, and the quantity of intermediate interlayer sites between illite and vermiculite structures (i.e., frayed edge sites) as represented by the radiocesium interception potential (RIP). The relative abundances of two low-activity clay components, kaolinite and gibbsite, were determined by differential thermal analysis, and the degree of Al-substitution in goethite was determined by differential x-ray diffraction analysis. All three quantitative indicators for illitic minerals decreased along the altitude gradient, SCL>SCH>ADM, and were positively correlated with CECclay. While kaolinite also showed a decreasing trend at higher altitude, it did not show any significant correlation with CECclay. In contrast to the decrease in these Si-bearing minerals, gibbsite and Al-substituted goethite increased along the same altitude gradient. These associative changes strongly indicate that illitic minerals were the clay component largely controlling CECclay and that de-silication or aluminization may proceed at higher altitude or on stable surfaces with older formation ages in the Cameroon plateau regions. Thus, mineralogical properties well explained the distribution of Ferralsols; they were marginal in the SCL region, while those with relatively high CECclay were dominant in the SCH region and those with lower CECclay were dominant in the ADM region.

Dissolution of Al-Substituted Goethite in the Presence of Ferrichrome and Enterobactin at pH 6.5

Naturally occurring goethites often show Al for Fe substitution approaching 33 mol% Al. This substitution has potential to influence the rate of goethite dissolution and therefore the supply of bioavailable Fe. Siderophores such as ferrichrome and enterobactin have considerable potential to dissolve Fe from Fe3+ rich minerals, including Al-substituted goethites. Here, we show that Al substitution in synthetic goethites (0.021 ≥ x ≥ 0.098) gives rise to a significant increase in both ferrichrome- and enterobactin-mediated dissolution rates. In the presence of ferrichrome, Al-goethite (x = 0.033) yields a dissolution rate of 19.0 × 10−3 µmol m−2 h−1, nearly twice that of pure goethite, whereas dissolution of the most highly substituted Al-goethite (x = 0.098) is 36.9 × 10−3 µmol m−2 h−1, more than threefold greater than the pure mineral. Similarly, in the presence of enterobactin, the dissolution rate of Al-goethite increases with increasing Al substitution. Ferrichrome is a less effective ligand than enterobactin in its dissolution of both pure goethite and the range of Al-goethites, an observation we ascribe to the lower affinity of the hydroxamate functional groups of ferrichrome for both Fe3+ and Al3+. Despite greater affinity of both ferrichrome and enterobactin for Fe3+ over Al3+, we observe a broadly congruent dissolution of all our Al-goethites.

Iron oxides and rare earth elements in the clay fractions of a soil chronosequence in southern Spain

Mineralogical (XRD), crystallochemical (DXRD), geochemical (ICP‐ms and ICP‐AES) and spectroscopic colour (DRS) properties of the clay fractions confirmed that five soils (a Fluvisol, two Calcisols and two Luvisols) on fluvial terraces in southern Spain form a genuine chronosequence that extends back to 600 ka. Analyses showed that goethite was more abundant than haematite. Both iron (hydr)oxide contents and Al substitution in goethite (mole %) increased with soil age. The Fe in the (hydr)oxides came from the transformation of illites to smectite + mixed‐layer phyllosilicates. Rare earth element contents correlated with a mafic alteration index and with soil age. Soil colours reflected variation in the proportions of calcite and iron (hydr)oxides, and became redder with age. Most soil properties were fitted by logarithmic chronofunctions that approach asymptotes at 70–100 ka.HighlightsA chronosequence is confirmed from the soil of fluvial terraces in southern Spain. Goethite exceeds haematite throughout and increases with soil age. Rare earth elements correlate with a mafic alteration index and soil age. Most soil attributes show logarithmic chronofunctions with asymptotes at 70–100 ka.

The associations of heavy metals with crystalline iron oxides in the polluted soils around the mining areas in Guangdong Province, China

In current selective sequential extraction (SSE) methods, heavy metals associated well-crystallized iron oxyhydroxides are not well defined and considered, and usually fall into residual forms, which actually make their mobility and environmental risk underestimated. This study compared various fractions of heavy metals in samples of mining tailings and nearby soils under different land uses in Guangdong Province of China. Iron oxides in these soils were mainly Al-substituted goethites and/or hematites. Independent of the land uses, the percentages of various Cd fractions in the soils decrease in the order of well-crystallized iron oxyhydroxides-associated (27.2–91.2%) > residual (2.1–37.0%) > reducible FeMn oxides (3.2–45.7%) > exchangeable (1.0–28.2%) > organic-associated form (0–5.1%). Pb mainly existes in the residual fraction (25.9–74.5%), followed by crystalline iron oxyhydroxides-associated (0.4–69.0%) and reducible fractions (2.4–40.4%). Zn mainly exists in the residual fraction (25.4–85.9%), followed by the well-crystallized iron oxyhydroxides-associated fraction (6.4–55.0%). However, Cu mainly existes in the residual form and the fractionation of Cu varies obviously between samples. The association of these metals with crystalline iron oxyhydroxides in the soils decreases in the sequence of Cd > Pb > Zn > Cu. These results suggest that in heavy-metal-polluted weathered soils in tropical and subtropical areas enriched with iron oxides, it is essential to assess potential bioavailability and toxicity of heavy metals bound to well-crystallized iron oxyhydroxides with appropriate procedures in the fractionation measurement. The modified Tessier method present here can be used to quantify the fractions of heavy metals in such areas.

Effects of Al3 + doping on the structure and properties of goethite and its adsorption behavior towards phosphate

Al substitution in goethite is common in soils, and has strong influence on the structure and physicochemical properties of goethite. In this research, a series of Al-doped goethites were synthesized, and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The adsorption behavior of these samples towards PO43− was also investigated. Characterization results demonstrated that increasing Al content in goethite led to a reduction in crystallinity, increase in specific surface area (SSA), and morphology change from needle-like to granular. Rietveld structure refinement revealed that the lattice parameter a remained almost constant and b slightly decreased, but c was significantly reduced, and the calculated crystal density increased. EXAFS analysis demonstrated that the Fe(Al)–O distance in the structure of the doped goethites was almost the same, but the Fe–Fe(Al) distance decreased with increasing Al content. Surface analysis showed that, with increasing Al content, the content of OH groups on the mineral surface increased. The adsorption of phosphate per unit mass of Al-doped goethite increased, while adsorption per unit area decreased owing to the decrease of the relative proportion of (110) facets in the total surface area of the minerals. The results of this research facilitate better understanding of the effect of Al substitution on the structure and properties of goethite and the cycling of phosphate in the environment.

Arsenic adsorption onto aluminium-substituted goethite

Environmental contextGoethite, commonly found in soils, is often partially substituted by Al and strongly influences the mobility of arsenic in the environment. The adsorption of AsV onto goethites with increasing Al substitution was explored, finding that Al incorporation decreases AsV sorption per gram of adsorbent, and that a low level of Al incorporation enhances the adsorption per unit area. Structures of the complexes formed between AsV and the oxy(hydr)oxide surface, at different pH values, are proposed by studying the changes in the surface charges of the adsorbed and non-adsorbed substituted and non-substituted goethites. AbstractAluminium and iron oxy(hydr)oxides in nature are often partially substituted by other elements and strongly influence the mobility of arsenic in the environment. Because goethite is commonly found in soils, and the oxide is easily substituted, in the present work, the adsorption of AsV onto several Al-substituted goethites was explored in order to determine how substitution affects the adsorption process. Three samples with increasing Al content (GAl0, GAl3.78 and GAl7.61) were prepared and fully characterised. The variations in AsV adsorption under different conditions, as well as the variations of the particle surface charge, were analysed. The results showed that the removal capacity of Al-goethites is determined by the Al content. The adsorption maxima per gram followed the trend GAl0&gt; GAl3.78&gt; GAl7.61, indicating that Al incorporation decreases AsV sorption. Adsorption per surface area decreased in the order GAl3.78&gt; GAl0&gt; GAl7.61, implying that a small incorporation of Al enhances the adsorption properties of the surface. The stoichiometry of the probable surface complexes formed with the contaminant at different pH values is proposed, by analysis of all the experimental results obtained before and after AsV adsorption. These surface complexes were used to fit the experimental data with good agreement, and the formation and acidity constants were also estimated.