Presence Of Goethite Research Articles

Elucidating Phosphate and Cadmium Cosorption Mechanisms on Mineral Surfaces with Direct Spectroscopic and Modeling Evidence.

The simultaneous sorption of cations and anions at the mineral-water interface can substantially alter their individual sorption characteristics; however, this phenomenon lacks a mechanistic understanding. Our study provides direct spectroscopic and modeling evidence of the molecular cosorption mechanisms of the cadmium ion (Cd2+) and phosphate (P) on goethite and layered manganese (Mn) oxide of birnessite, through in situ attenuated total reflection Fourier-transform infrared (ATR-FTIR), P K-edge X-ray absorption near-edge structure (XANES) spectroscopy, and surface complexation modeling. Phosphate synergistically cosorbed with Cd on goethite predominantly through P-bridged ternary complexes (≡Fe-P-Cd) and electrostatic interactions at wide pH conditions. Likewise, P and Cd exhibited synergistic cosorption on birnessite by forming P-bridged ternary complexes (≡Mn-P-Cd) and weak competitive sorption at the layer edge sites. As pH and Cd loading increased, the surface P species transitioned from a binary complex to a ternary complex and/or Cd3(PO4)2 precipitate for both goethite and birnessite. Compared to that in solution at pH 8, the formation of Cd3(PO4)2 was inhibited by the presence of goethite and birnessite, ascribed to the specific adsorption of P and Cd, more pronounced in birnessite due to the stronger sorption of Cd at its vacant sites. The discovered cosorption mechanisms of P and Cd have important implications for understanding and predicting their mobility and availability in Cd-contaminated settings.

Reflectance spectrometry and multispectral data for mapping fractures and hydrothermal alterations in the northern edge of the eastern High Atlas, Morocco

The Marginal Folds, in the north-western axis of Moroccan eastern High Atlas, are mainly composed of the Infra-Cenomanian terrains. These latter contain continental detrital deposits (red beds) and are characterized by a network of fractures. These marginal folds host significant concentrations of metallic minerals, such as Cu, Pb, Ag, and Zn, particularly at the Merija (Cu) and Bou Sellam (Pb-Zn) sites.Several techniques were applied to the Landsat-8 Oli images to improve the clarity and visibility of the linear features. Radiometric and atmospheric corrections, color compositions, directional filters, and principal component analysis were adopted. The lineaments resulting from automatic extraction, as well as their statistical analysis were validated by visual interpretation and field investigations. Statistical analysis revealed three dominants fracture systems: N-S, NE-SW, and E-W; the first two systems show a high density in the study area.The mineral alteration of the outcropping rocks is well described by ASTER multispectral imaging techniques. In this work, the effectiveness of this techniques for mineral alteration mapping was demonstrated on the Merija area. Several occurrences of mineral alteration were highlighted, using a combination of false color and band ratios, the principal component analysis (PCA), and the band ratios. This research was completed by reflectance spectrometry, covering the visible, near-infrared (VNIR), and short-wave infrared (SWIR) domains, to identify mineral alteration.Hydrothermal alteration in the Merija region includes clay, phyllite, carbonate, and phyllosilicate alteration, such as kaolinite, illite, montmorillonite, palygorskite, and vermiculite. Regarding to oxidation, as evidenced by the presence of goethite, jarosite, and hematite, it is widespread in most of the analyzed samples, resulting from the alteration of pyrite, followed by jarosite and goethite.

Effect of iron minerals on formation of hydroandradite during alkali-thermal process

The hydroandradite (HA) is an efficient and ideal desilication product to deal with the bauxite and red mud based on the alkali-thermal process, while the selection of iron source is crucial for the HA generation. This work investigates the effects of various natural iron minerals on the alkali-thermal formation HA in aluminate solution by comparing with synthetic sodium ferrate. The Gibbs free energy changes of HA formation with different iron oxides were calculated based on the complex silicate theory, and the preferential conversion order for HA formation was obtained thermodynamically. The solubility data of various iron minerals in aluminate solution were determined and correlated by the Apelblat model. The contribution order of iron minerals for HA formation was obtained, and the goethite is the best alternative to replace sodium ferrite by comparing to hematite and magnetite. The phase transformation of HA is above 80% in the presence of goethite, and the alkali content in reaction product is as low as 0.20%. Moreover, the effect mechanism of goethite on HA formation was discussed and established.

Biotransformation of Chlorpyrifos Shewanella oneidensis MR-1 in the Presence of Goethite: Experimental Optimization and Degradation Products.

In this study, the degradation system of Shewanella oneidensis MR-1 and goethite was constructed with chlorpyrifos as the target contaminant. The effects of initial pH, contaminant concentration, and temperature on the removal rate of chlorpyrifos during the degradation process were investigated. The experimental conditions were optimized by response surface methodology with a Box-Behnken design (BBD). The results show that the removal rate of chlorpyrifos is 75.71% at pH = 6.86, an initial concentration of 19.18 mg·L-1, and a temperature of 30.71 °C. LC-MS/MS analyses showed that the degradation products were C4H11O3PS, C7H7Cl3NO4P, C9H11Cl2NO3PS, C7H7Cl3NO3PS, C9H11Cl3NO4P, C4H11O2PS, and C5H2Cl3NO. Presumably, the degradation pathways involved are: enzymatic degradation, hydrolysis, dealkylation, desulfur hydrolysis, and dechlorination. The findings of this study demonstrate the efficacy of the goethite/S. oneidensis MR-1 complex system in the removal of chlorpyrifos from water. Consequently, this research contributes to the establishment of a theoretical framework for the microbial remediation of organophosphorus pesticides in aqueous environments.

In-situ studies of rust layer formed on OCTG N80 steels alloyed with rare earth elements

In this study, the effect of rare earth (RE) alloying on the rust formed on N80 steels was investigated with in-situ micro-Raman spectroscopy and optical microscopy. Under potentiostatic tests at potentials representing accelerated conditions, there was formation of goethite phase on N80RE, while this protective and stable rust layer was not found on N80. The presence of goethite might explain the smaller corrosion rate of N80RE than N80 at later stages of corrosion, although the initial corrosion rate of N80RE was greater, which was probably due to RE modified inclusions.

Evidence and mineralogical and physico-chemical properties of chernozem and chernozem-like soils in Croatia

The aim was to determine possible local differences between the parent materials of recent leoss-derived soils in eastern Croatia (Dalj, Zmajevac). Furthermore, it highlights the existence of chernozem and chernozem-like soils in Croatia and describes their basic physical, chemical and mineral properties. For this purpose, two soil profiles (P-3 and P-6) south of the Dalj settlement and one soil profile (P-10) near the Zmajevac settlement were excavated. The investigation included a detailed pedological analysis, a modal analysis of the heavy and light mineral fraction and a mineralogical analysis of bulk samples (the < 2 mm fraction) and the fraction < 2 μm. By comparing the obtained results with the criteria of the Croatian Soil Classification and the World Reference Base for Soil Resources, the soil profiles P-3 and P-6 can be defined as Chernozem on Loess or Hortic Calcic chernozem (Epiloamic, Endosiltic, Aric, Humic). The systematic unit for profile P-10 was defined as Rendzina according to the Croatian Soil Classification or Calcic Chernozem (Siltic) according to the WRB. Based on the results of the pedological analysis of the soil profile horizons, a gradual degradation of the chernozem was observed as a result of anthropogenic influence, but also due to recent climate change. The degradation is particularly evident in the form of a reduction in organic matter and the relocation of carbonates from the surface to deeper zones. Due to the increasing degree of weathering caused by recent climate changes, some differences in the mineralogical composition of the studied soils were also observed. The progressive degradation of the chernozem due to the effects of recent weathering processes is indicated mainly by the presence of goethite in the fraction < 2 μm as a weathering product of iron minerals (magnetite, pyroxenes…). Although the parent material of all three profiles is loess sediments, the reason why the soil material of profile P-10 has not developed a chernic horizon is the constant contribution of aeolian material and a short period of exposure to pedogenetic processes.

Effect of initial solution pH on 4-nitrophenol oxidation through homogeneous/heterogeneous photo-Fenton process using goethite/H2O2 system

The photodegradation of 4-nitrophenol (4-NP) through homogeneous/heterogeneous photo-Fenton process was examined in the presence of goethite (α-FeOOH)/H2O2/UV system in solution at different pH values. The degradation of 4-NP was achieved with excellent performance. Conversion reached 100%, 92% and 83% in the first 120 min of irradiation for acidic, point of zero charge (pzc) and weakly alkaline pH, respectively. Meanwhile, 69% of degradation was reached under strong alkaline pH conditions and this resulted in the weakest removal percentage under comparable experimental conditions. The rate of photocatalytic degradation followed pseudo-first order kinetics with kinetic constants of 6.30 × 10−2 min−1, 2.37 × 10−2 min−1, 1.73 × 10−2 min−1 and 0.70 × 10−2 min−1 at pH = 3.0, 5.5, 9.0 and 11.0, respectively. The iron(II) species dissolved in the solution enhanced the photocatalytic degradation and the hydroxyl radicals (OH) production. The involvement of free radicals was evidenced by using scavengers and the results revealed that OH is the main active species. The stability of goethite to repeated uses was investigated too and, after five runs, the removal efficiency of 4-NP was still complete, with values attaining 100%.

Release of chromium from Cr(III)- and Ni(II)-substituted goethite in presence of organic acids: Role of pH in the formation of colloids and complexes

High levels of Cr(III) are hosted in Fe (oxyhydr)oxides in soils derived on (ultra)mafic rocks, which can pose potential risks to the environment. Organic acids can cause the solubilization of Fe (oxyhydr)oxides and the release of Cr(III). However, the release behaviors of Cr(III) from Fe (oxyhydr)oxides by organic acids and its main factors remain unclear. This study investigates the speciation of Cr released from Cr(III)-substituted goethite in the presence of citrate and oxalate and the effects of pH (3–7). Batch experiments showed that Fe(III) and Cr(III) dissolution were significantly enhanced by citrate and oxalate, and the extent of dissolution was negatively correlated with pH. When at relatively high pH (5–7), AF4-ICP-MS results revealed that large proportions of dissolved Fe (>58 %) and Cr (18 %–73 %) were presented in the form of Cr(III)-citrate colloids in the sizes of 1–125 nm and 125–350 nm. Further, FTIR and cryogenic XPS characterization demonstrated that the formation of·Cr(III)-citrate colloids was attributed to the adsorption and complexation of citrate on the substituted goethite surface. However, Cr was mainly released as soluble Cr(III)-organic complexes when presented at pH 3. While low pH inhibited the formation of Cr(III)-organic colloids, it promoted the release of Cr by facilitating the dissociation of surface Cr(III)-organic complexes. In addition, the incorporation of Ni(II) in Cr(III)-substituted goethite weakened the adsorption of organic acid by shortening the crystal size of goethite, thus significantly inhibiting the formation of Cr(III)-organic complexes and colloids. This study confirms the formation of Cr(III)-organic acid colloids and highlights the importance of pH on Cr release behavior, which is essential for evaluating Cr transport and fate in soils with high background values.

Unveiling the crucial role of iron mineral phase transformation in antimony(V) elimination from natural water

Antimony (Sb) in natural water has long-term effects on both the ecological environment and human health. Iron mineral phase transformation (IMPT) is a prominent process for removing Sb(V) from natural water. However, the importance of IMPT in eliminating Sb remains uncertain. This study examined the various Sb–Fe binding mechanisms found in different IMPT pathways in natural water, shedding light on the underlying mechanisms. The study revealed that the presence of goethite (Goe), hematite (Hem), and magnetite (Mag) significantly affected the concentration of Sb(V) in natural water. Elevated pH levels facilitated higher Fe content in iron solids but impeded the process of removing Sb(V). To further our understanding, polluted natural water samples were collected from various locations surrounding Sb smelter sites. Results confirmed that converting ferrihydrite (Fhy) to Goe significantly reduced Sb levels (<5 μg/L) in natural water. The emergence of secondary iron phases resulted in greater electrostatic attraction and stabilized surface complexes, which was the most likely cause of the decline of Sb concentration in natural water. The comprehensive findings offer new insights into the factors governing IMPT as well as the Sb(V) behavior control.

Closing the knowledge gap on the composition of the asbestos bodies

Asbestos bodies (AB) form in the lungs as a result of a biomineralization process initiated by the alveolar macrophages in the attempt to remove asbestos. During this process, organic and inorganic material deposit on the foreign fibers forming a Fe-rich coating. The AB start to form in months, thus quickly becoming the actual interface between asbestos and the lung tissue. Therefore, revealing their composition, and, in particular, the chemical form of Fe, which is the major component of the AB, is essential to assess their possible role in the pathogenesis of asbestos-related diseases. In this work we report the result of the first x-ray diffraction measurements performed on single AB embedded in the lung tissue samples of former asbestos plant workers. The combination with x-ray absorption spectroscopy data allowed to unambiguously reveal that Fe is present in the AB in the form of two Fe-oxy(hydroxides): ferrihydrite and goethite. The presence of goethite, which can be explained in terms of the transformation of ferrihydrite (a metastable phase) due to the acidic conditions induced by the alveolar macrophages in their attempt to phagocytose the fibers, has toxicological implications that are discussed in the paper.

New insights into persulfate decomposition by soil minerals: radical and non-radical pathways

Persulfate (PS)-based in situ chemical oxidation (ISCO) has been widely used for pollutant remediation in soil and groundwater. However, the underlying mechanism of interactions between mineral and PS was not fully explored. In this study, several soil model minerals including goethite, hematite, magnetite, pyrolusite, kaolin, montmorillonite, and nontronite were selected to investigate their potential effects on PS decomposition and free radical evolution. It was found the decomposition efficiency of PS by these minerals varied significantly, and both the radical and non-radical decomposition processes were included. Pyrolusite has the highest reactivity for PS decomposition. However, PS decomposition is prone to form SO42- through non-radical pathway, and thus, the amounts of free radicals (e.g., •OH and SO4•-) produced are relatively limited. However, PS mainly decomposed to produce free radicals in the presence of goethite and hematite. In the presence of magnetite, kaolin, montmorillonite, and nontronite, PS both decomposed to produce SO42- and free radicals. Furthermore, the radical process exhibited the high degradation performance for model pollutant such as phenol with relatively high utilization efficiency of PS, while non-radical decomposition has limited contribution to phenol degradation with extremely low utilization efficiency of PS. This study deepened the understanding of interactions between PS and minerals during the PS-based ISCO in soil remediation.

Evidence of Copper and Iron Deposits of the Protohistoric City of Temesa

With the name ‘Temesa’ (Latin Tempsa), the ancients identified a settlement located along the Tyrrhenian coast of Calabria, cited by sources as an international metal exchange emporium. The town is mentioned by Homer as being famous in the ancient world for the production of bronze, and in the I century A.D. Strabo wrote that there were rich copper mines near the city. Many years of study led to the recognition of Temesa as a complex urban system located between the Oliva and Savuto rivers, near Amantea. To confirm this hypothesis, we searched, in the surrounding rocky outcrops, for the presence of minerals useful for the extraction of iron and copper. Samples of 3 different rock stratifications were taken near the protohistoric settlement of Serra Aiello. The observation under an polarized reflected light microscope and the X-ray diffraction patterns revealed the presence of many minerals useful for the extraction of iron and copper in every sample. The heating of samples under both oxidizing and reducing conditions helped us to better quantify copper and iron minerals content causing, at the same time, the appearance of a marked paramagnetic behavior that could be associated with the presence of goethite. X ray fluorescence analysis showed a high concentration of iron and a low copper content.

Study of Mineralogy and Metallurgical Properties of Lump Ores

The ironmaking process in blast furnaces using iron lumps is an energy-efficient and low-carbon initiative that helps lower the cost of the ironmaking process. The physical and chemical properties of raw materials, process mineralogy, and metallurgical properties of three Malaysian iron ore lumps were researched in this paper, with the goal of showing the relationship between metallurgical qualities and mineralogy. The results show that lump ore A has a better decrepitation index, with a DI−6.3mm of only 0.2%; lump ore J has better reducibility and low-temperature reduction disintegration index, with RI and RDI+6.3mm values of 88.39% and 87.55%, respectively; and the three lump ores have excellent softening and melting properties and a high permeability index. The metallurgical properties of lump ore and mineralogical characterizations are generally correlated, the decrepitation performance of lump ore is mostly determined by the presence of goethite, and the original porosity is unrelated. The higher the content, the worse the decrepitation performance; lump ore’s reducibility is mostly determined by the open porosity and the content of newly generating hematite at high temperatures, which has no relationship to its original porosity. The higher the open porosity, the higher the goethite content, the higher the newly generated porous hematite content, and the better the lump ore’s reducibility; the higher the original hematite content in the lump ores, the lower the open porosity at high temperatures and the worse the reduction degradation characteristics.

Coexisting Goethite Promotes Fe(II)-Catalyzed Transformation of Ferrihydrite to Goethite.

In redox-affected soil environments, electron transfer between aqueous Fe(II) and solid-phase Fe(III) catalyzes mineral transformation and recrystallization processes. While these processes have been studied extensively as independent systems, the coexistence of iron minerals is common in nature. Yet it remains unclear how coexisting goethite influences ferrihydrite transformation. Here, we reacted ferrihydrite and goethite mixtures with Fe(II) for 24 h. Our results demonstrate that with more goethite initially present in the mixture more ferrihydrite turned into goethite. We further used stable Fe isotopes to label different Fe pools and probed ferrihydrite transformation in the presence of goethite using 57Fe Mössbauer spectroscopy and changes in the isotopic composition of solid and aqueous phases. When ferrihydrite alone underwent Fe(II)-catalyzed transformation, Fe atoms initially in the aqueous phase mostly formed lepidocrocite, while those from ferrihydrite mostly formed goethite. When goethite was initially present, more goethite was formed from atoms initially in the aqueous phase, and nanogoethite formed from atoms initially in ferrihydrite. Our results suggest that coexisting goethite promotes formation of more goethite via Fe(II)–goethite electron transfer and template-directed nucleation and growth. We further hypothesize that electron transfer onto goethite followed by electron hopping onto ferrihydrite is another possible pathway to goethite formation. Our findings demonstrate that mineral transformation is strongly influenced by the composition of soil solid phases.

The effect of iron oxide types on the photochemical transformation of organic phosphorus in water

Iron oxides play an important role in the transport and transformation of organic phosphorus in aquatic environments. However, the effect of different types of iron oxide on the environmental fate of organic phosphorus has remained unclear. In this study, the photodegradation of the organic phosphorus compound adenosine triphosphate (ATP) via the activity of crystalline (goethite) and amorphous (ferrihydrite) iron oxides was investigated. It was found that ATP was photodegraded by goethite, resulting in the release of dissolved inorganic phosphate under simulated sunlight irradiation. The concentration of ATP on goethite decreased by 75% after 6 h of simulated sunlight irradiation, while the concentration of ATP on ferrihydrite decreased by only 22%. ATR-FTIR spectroscopy revealed that the intensity of the peaks for the P–O and PO stretching vibrations in the goethite−ATP complex decreased significantly more after simulated sunlight irradiation than did those for the ferrihydrite treatment. Combined with the higher TOC/TOC0 values for the goethite treatment, the results indicate that a more vigorous photochemical reaction took place in the presence of goethite than with ferrihydrite. Reactive oxygen species analysis also showed that hydroxyl and superoxide anion radicals were generated when goethite was exposed to simulated sunlight irradiation, while ferrihydrite did not exhibit this ability. Overall, this study highlights that the type of iron oxide is an important factor in the transformation of organic phosphorus in aquatic environments.

Bentonite Powder XRD Quantitative Analysis Using Rietveld Refinement: Revisiting and Updating Bulk Semiquantitative Mineralogical Compositions

Bentonite is a claystone formed by a complex mineralogical mixture, composed of montmorillonite, illite, and accessory minerals like quartz, cristobalite, feldspars, carbonates, and minor amounts of iron oxy-hydroxides. Bentonite presents complexity at various scales: (1): a single mineral may present different chemical composition within the same quarry (e.g., feldspars solid solutions); (2): montmorillonite presents variability in the cation-exchange distribution while illite may be presented as mixed-layer with smectite sheets; and (3): hardness and crystal size are larger in accessory minerals than in clay minerals, preventing uniform grinding of bentonite. The FEBEX bentonite used is originally from Almería (Spain), and it is a predominantly calcium, magnesium, and sodium bentonite. This Spanish FEBEX bentonite has been hydrothermally altered at laboratory scale for 7–14 years. A thermal gradient was generated by heating a disk of pressed iron powder, simulating the metal waste canister, in contact with the compacted bentonite sample. Hydration was forced from the opposite direction. XRD recorded patterns were very similar. In order to minimize the bias of XRD semi-quantitative determination methods, Rietveld refinement was performed using BGMN software and different structural models. Confidence in the quantification of the main phases allows us to convincingly detect other subtle changes such as the presence of calcite in the hydration front, right at the interface between the saturated and unsaturated bentonite, or the presence of goethite, and not hematite, in the saturated bentonite, near the source of hydration. Smectite component was 72 ± 3% and the refinement was consistent with the presence of ~10% illite, comparable with previous characterizations.

Evaluation of the Ability of Rhizobacterial Isolates to Solubilize Sparingly Soluble Iron Under In-vitro Conditions

One of the problems of calcareous soils is the lack of absorbable iron for plants grown in these areas. Research by various researchers has shown that some rhizosphere-dwelling microorganisms have the ability to solubilize resources containing low-soluble iron and conversion it to soluble and absorbable for the plants. Different mechanisms such as proton secretion, production of organic acids, and production of siderophore can be effective in solubilizing low-soluble iron. Accordingly, in this study, the potential of twenty rhizobacteria for solubilizing low-soluble iron sources (eg, iron phosphate, hematite, and goethite) has been evaluated. The experiment was performed under in-vitro condition in a completely randomized design with three replications. According to the results, three isolates, namely, ZP15, ZC10, and ZP13 were characterized as promising isolates. In the presence of iron phosphate, the concentration of solubilized iron by isolates ZP15, ZP13, and ZC10 was 5.19, 4.35, and 4.27 mg/l, respectively. These values for the isolates ZC10 and ZP13 in the presence of hematite was recorded 6.49 and 5.35 mg/l, respectively, and in the presence of goethite was 7.02 and 6.70 mg/l, respectively. The amount of siderophore production for the three isolates ZP15, ZC10, and ZP13 was 16.47, 17.1, and 31.55 µM, respectively. The high-performance liquid chromatography analysis revealed that the most important acids produced by these isolates were 2-ketogluconic acid (303.8 µg/ml) for ZP15, oxalic acid (246.9 µg/ml) for ZC10, and gluconic acid (100.2 µg/ml) for ZP13, respectively. The molecular and biochemical identification of these three isolates ZP15, ZC10, and ZP13 showed that they belong to the genera Stenotrophomonas, Pantoea, and Agromyces, respectively.

A comparative study on the microstructure, hardness and corrosion resistance of epoxy coated and plain rebars

Corrosion of steel rebars and susceptibility of reinforcement steel to chloride ion attacks are the two major problems for the construction industries and thereby a huge amount of money is spent to repair it. Epoxy coating on the steel rebars can be one cost-effective solution to alleviate the detrimental effects of corrosion in concrete structures. In the present research, plain and epoxy coated rebar (ECR) samples were chosen to study the correlation between microstructure, hardness and corrosion performance. The microstructures of the investigated thermomechanically treated (TMT) rebars primarily reveal tempered martensitic rings at the outer surface followed by a narrow bainitic transition zone in between along with a ferrite-pearlite microstructure at the inner core. The corrosion resistance of plain and epoxy-coated rebars in naturally aerated 3.5% NaCl and 1% HCl solutions were studied using gravimetric test, open circuit potential (OCP) test, and linear polarization monitoring techniques. It has been witnessed that the corrosion current (icorr) has been shifted towards lower values and polarization resistance (Rp) values are higher for ECR samples which is a clear indication of higher corrosion resistance of the ECRs than the plain rebars. Energy dispersive spectroscopy (EDS) analysis reveals the presence of iron hydroxides and iron oxides. However, x-Ray diffraction (XRD) analysis indicates the existence of various types of oxides, hydroxides, and oxy-hydroxides like iron chloride hydroxide [Fe2(OH)3Cl], goethite (α-FeO(OH)), lepidocrocite (γ-FeO(OH)), magnetite (Fe3O4) and bernalite [Fe(OH)3(H2O)0.25] in the epoxy coated rebar samples whereas, plain rebars indicate the presence of goethite (α-FeO(OH)), maghemite (γ-Fe2O3), magnetite (Fe3O4), hydrogoethite (Fe2O3.H2O), lepidocrocite (γ-FeO(OH)) and iron oxide (Fe21.34O32). All the experimental results confirm that ECR samples are more corrosion resistant under both acidic and saline environments.

Rhodochrosite Oxidation by Dissolved Oxygen and the Formation of Mn Oxide Product: The Impact of Goethite as a Foreign Solid Substrate.

Rhodochrosite conversion to Mn (oxyhydr)oxides significantly affects the fate and transport of various substances in the environment. We examined rhodochrosite oxidation by dissolved oxygen and the oxidation product formation with an emphasis on the effects of goethite substrate. Without goethite, rhodochrosite oxidation was slow as no detectable change was observed for 28 d with microscopic and spectroscopic analyses, except a minor change in X-ray diffraction. Interestingly, by contrast, it was greatly accelerated and completed in 7 d in the presence of goethite, resulting in the heteroepitaxial growth of groutite (α-MnOOH)-like Mn oxides on the goethite (α-FeOOH) tip surfaces. The formation of this secondary Mn oxide likely induced the acidification of the microenvironment in the vicinity of rhodochrosite particles and thereby promoted their dissolution. Subsequently, their oxidative conversion to the Mn oxide was expedited by the surface catalyzed Mn(II) oxidation on the goethite tip. Our results revealed that goethite as a foreign substrate imparts a decisive control on not only the rate but also the type of the reaction product of rhodochrosite oxidation. This study presents a new insight into the geochemical roles of foreign particles on the dynamics of redox-sensitive solid phases in the environment.

Characteristics of Kaolinitic Raw Materials from the Lokoundje River (Kribi, Cameroon) for Ceramic Applications

Eight kaolinitic materials from the Lokoundje River at Kribi were sampled and investigated for their physical, chemical, mineralogical and thermal characteristics in order to evaluate their potential suitability as raw materials in ceramics. The Lokoundje kaolinitic materials are clayey to silty clayey and are predominantly composed of kaolinite and quartz. The alkali (Na2O + K2O) content ranges between 1 and 2.5 wt.%; these low values do not favor vitrification of the ceramics but may be improved through flux amendment. The presence of goethite in some samples limits their utilization in white ceramics. The minerals content, color, metallic sound, cohesion, linear shrinkage, flexural strength, bulk density, water absorption and microstructure were determined. The XRD data reveal that kaolinite and goethite were transformed, respectively, into mullite and hematite. The colors of the fired products are characteristic of their mineral assemblage. The metallic sound is indicative of low vitrification which is confirmed by the presence of cracks due to low flux contents. The cohesion is good to very good, due to the abundance of kaolinite. The physicomechanical properties increase with temperature as well as densification. The geochemical data show that the Lokoundje alluvial clays are suitable for the manufacture of white stoneware tiles.