Daniel Njopwouo

Functionalization of natural smectite-type clays by grafting with organosilanes: physico-chemical characterization and application to mercury(II) uptake

The surface properties of organosilane-modified smectite-type clays were examined by various physico-chemical techniques, including thermal gravimetric analysis (TGA), surface area measurements from N2 adsorption/desorption experiments (BET method), Fourier transform infrared spectroscopy (FTIR), chemical stability measurements in aqueous medium, and elemental analysis. The organically modified clays derived from the reaction of two natural Cameroonian smectites and one standard bentonite sample (for comparison purposes) with γ-aminopropyltriethoxysilane and 3-mercaptopropyltrimethoxysilane. Efficiency of the grafting process was demonstrated by comparing the physico-chemical characteristics of the clays before and after reaction with the organosilanes, which resulted in decreased porosity and decreased amounts of surface hydroxyl groups concomitantly to the presence of aminopropyl or mercaptopropyl groups attached to the aluminosilicate structure. The amine-bearing materials were subject to rather low chemical stability in neutral aqueous medium, which was improved with decreasing pH, while the thiol-containing ones were found to be stable over a wide pH range (2–10). Possible use of these materials as adsorbents for mercury(II) species was discussed and some preliminary experiments based on carbon paste electrodes modified with such grafted clays highlighted their interest for electroanalytical applications.

Synthesis of geopolymer composites from a mixture of volcanic scoria and metakaolin

Abstract The aim of this work is to valorize volcanic scoria by using them as starting material for geopolymers production. Nevertheless, volcanic scoria possesses low reactivity. Various amounts of metakaolin (5%, 10%, 15%, 20% and 25%) were added into two volcanic scoria (ZD and ZG) in order to improve their reactivity. Two alkaline solutions were used to activate the aluminosilicate materials. The starting materials were characterized by particle size distribution, specific surface area, chemical and mineralogical composition. The geopolymers were characterized by the setting time, XRD, FTIR, SEM and compressive strength. The results indicated that volcanic scoria have low specific surface area (2.3 m2/g for ZD, 15.7 m2/g for ZG), high average particle size (d50 = 13.08 μm and 10.68 μm for ZD and for ZG respectively) and low glass phase contents. Metakaolin have a smaller average particle size (d50 = 9.95 μm) and high specific surface (20.5 m2/g). The compressive strength of geopolymers increased in the ranges of 23–68 MPa and 39–64 MPa for geopolymers from ZD–MK and ZG–MK respectively. This study shows that despite the low reactivity of volcanic scoria it can still be used to synthesize geopolymers with good physical and mechanical properties.

A fixed-bed column for phosphate removal from aqueous solutions using an andosol-bagasse mixture

It is difficult to eliminate phosphate from large volumes of water in batch mode using an adsorbent such as andosol. In a fixed-bed column, andosol has a very low permeability. In this study, andosol was mixed with bagasse to increase permeability. The mixture was then applied for the adsorption of phosphate in a fixed-bed column. Optimum and stable permeability was obtained with a 50/50 mixture of andosol and bagasse. The maximum adsorption capacity obtained was 4.18 mg/g for a column with a bed depth of 1.8 cm and a flow rate of 4 mL/min. The experimental data fit best to Thomas and Adam-Bohart models. These experimental results were applied in the treatment of natural phosphate-containing water from Yaoundé Municipal Lake in Cameroon. Column performance increased by 60% due to the presence of Ca(2+) and Mg(2+) in the natural water. These cations form complexes with phosphate at the andosol surface. The standard enthalpy 15.964 kj/mol indicated that phosphate adsorption on andosol-bagasse mixture was an endothermic process. Kinetic experiments demonstrated that phosphate adsorption fitted better with a pseudo-second-order model.

Surface Modification of Smectite Clay Induced by Non-thermal Gliding Arc Plasma at Atmospheric Pressure

Smectite clay from Sabga (west-Cameroon) was treated in aqueous suspension by gliding arc plasma to modify its surface properties. The evolution of the modifications was followed with the exposure time and post-discharge duration using Fourier transformed infra red spectroscopy and scanning electron microscopy. X-ray diffraction and nitrogen physisorption analyses were also performed to evaluate if both crystalline and textural properties of the material are affected by the treatment. The results obtained show that the plasma treatment causes the breakdown of structural bounds at the clay surface and induces the formation of new hydroxyl groups (Si–OH and Al–OH) on the clay edges. Crystallinity, sheet structure and textural properties are not significantly affected by the plasma treatment. However, it should be noted that an intensive treatment of the clay lowers the pH of the suspension, which subsequently induces an acid attack of the clay. In such case, the specific surface area of the clay increases. This study demonstrates that gliding arc plasma treatments can be used to activate clay minerals for environmental application.

Physicochemical properties of talc ore from Pout-Kelle and Memel deposits (central Cameroon)

Abstract Physicochemical properties of representative samples from talc deposits discovered at Pout-Kelle and Memel in Cameroon have been investigated using a variety of techniques. The data allow the study of the mineral crystal-chemistry, chemical and modal compositions, grain size distribution, textures and surface heterogeneity, and morphometric characteristics. As a result, we found that talc (platy or round, Fe-rich) prevails (65−90%) over Cr-chlorite (7−26%), halloysite, chromite, rutile, brucite and magnesite. Chemically, talc ores are made up of SiO2, MgO, Fe2O3 and minor Al2O3. Mode values range from 50 to 55 μm at Memel and from 30 to 90 μm at Pout-Kelle. Specific surface areas measured by BET and t-plot methods range from 1.3 to 2.5 m2/g. The large values of morphometric characteristics are indicative of high crystallinity and platiness. Potential uses of these talc ores in the rubber, plastics and paper industries require beneficiation processes. Interestingly, the Memel deposit and most zones of the Pout-Kelle deposit are free of needle-shaped crystals, an advantage for environmental safety.

Characterization for industrial applications of clays from Lembo deposit, Mount Bana (Cameroon)

Abstract The Lembo clay deposit occurs on orthogneiss, but it also comprises clays with lithorelicts of volcanic rocks. In this study, ten samples from two sites were investigated. The mineralogical compositions consisted of kaolinite + halloysite + illite + quartz + hematite + anatase ± rutile ± orthoclase ± sanidine ± magnetite ± maghemite ± goethite ± Ba-Al-Fe-phosphates ± carbonates ± sulphates. Kaolinite-halloysite and quartz are the prevailing minerals. Some volcanic-derived clays contain Fe-rich kaolinite-halloysite (9.6-14.1 wt.% Fe2O3). The chemical compositions, colours and specific surface area (SSA) measurements reveal two groups of clays: one with a positive whiteness index (IB), small SSA and small Fe content, and the other showing a large SSA, negative IB and relatively large Fe contents. The compositions of the first group are close to those of clays from the Mayouom deposit (Cameroon), and from some European commercial kaolins used in ceramics. Relatively Fe-rich clay materials may conform to most formulations of earthen bricks. As a whole, the Lembo clay deposit comprises various compositions of kaolinitic clays, which may yield the opportunity for extensive application in ceramics.

Petrological, Physico-Chemical and Mechanical Characterization of the Topomorphic Vertisols from the Sudano-Sahelian Region of North Cameroon

In Cameroon, topomorphic vertisols are widespread in the Sudano-sahelian zone above latitude 9oN. With a depth of about 2 to 2.5 m above the water table, these soils show four main horizons from bottom to top: a dark grey horizon with hydromorphic patches (B3g), dark grey horizon (B21), dark grey horizon with slickensides (B1) and a surficial grey humiferous horizon (A1) with desiccation cracks. Also, they show a heavy clayey texture, very massive structure, high bulk density, very low porosity and a high compacity. The microfabric of the soils is marked by abundant plasmas, isotic at the surface but birefringent at depth, with numerous stress cutans. The microscopic analysis of heavy minerals reveals the presence of magmatic and metamorphic minerals from the upper part of the watershed. Quartz grains are mainly angular to sub-angular revealing short fluvial transport distance. The vertisols are thus formed from recent alluvial deposits from weathering, erosion and deposition of rocks from upper parts of the watershed. Physico-chemically, the studied soils are characterized by high cation exchange capacity (26-42 meq/100g), high sum of bases (74.30-94.23 meq/100g), high base saturation, low organic carbon and a very high C/N ratio. Geochemically, Si and Al are the dominant elements, characterised by a Si/Al ratio range of 2.27 and 2.94. According to this rate, 2/1 clay minerals, namely smectite, are predominant and their presence confirms the shrink-swell behaviour of the soils. Total base concentrations are high and could be related to the contrasted climate and the low landscape position. Mechanical analyses revealed very high liquid limits (36-46%), low plastic limits (10-17%), high plasticity indices and high shrinkage limits that permit to classify those soils as medium to high plasticity inorganic clays. Those soils could be potentially useful in many domains like the chemical industry, pharmaceutics, petroleum industry, agronomy or environmental protection.

Talc as raw material for cementitious products formulation

Abstract This study reports the characterization of a talc from Cameroon as a possible source material for cement formulation. To that end, the talc sample was characterized and mixed with a solution of sodium polyphosphate to formulate the cementitious products. Addition of magnesia (MgO) was done to analyze the effect of available MgO on the products. Fourier transform infrared, X-rays diffraction, linear shrinkage, compressive strength and scanning electron microscopy were used to analyze the products. The compressive strength increased with addition of MgO and the linear shrinkage decreased. All the analyses indicate that talc is a raw material of interest in cementitious products formulation; however, the inclusion of the MgO is a key factor for a better performance of the products.

Talc-based cementitious products: Effect of talc calcination

Abstract This study reports the use of calcined talc for cementitious products making. The calcination is used to enhance the availability of magnesium from talc to react with phosphate for cement phase formation. It is shown that previous calcination of talc leads to products having enhanced mechanical performance due to the formation of more cement phase than in products based on raw talc. Talc fired at 900 °C was found to be the one in which magnesium release was maximal. Firing at temperature higher than 900 °C leads to the stabilization of enstatite, which decreased the magnesium availability. The cement phase is struvite, which was better detected on the X-ray patterns of the products involving fired talc. All the products have very rapid setting time and low shrinkage.

Efficiency and chemical recycling capability of magnetite-rich clay towards Eriochrome Black T remediation in the fixed-bed system

The present study investigates a fixed-bed column by using magnetite-rich clay from Cameroon as an adsorbent for the removal of the Eriochrome Black T (EBT) from aqueous solution. The effect of various parameters such as bed depths and EBT concentrations was investigated. The Adams–Bohart, Thomas and Yoon–Nelson models were applied to adsorption under varying experimental conditions to predict the breakthrough curves and to evaluate the model parameters of the fixed-bed column that can be useful for the process design. The column regeneration studies were carried out for three different sorption–desorption cycles using HCl or NaOH solution at 1u2005M. The 1u2005M HCl solution was found to have the best bed regeneration capacity and the fixed-bed could be reused for several sorption–desorption cycles. The elution efficiency of EBT, 75u2005mgu2005L−1 was greater than 99.1% for all three cycles. GRAPHICAL ABSTRACT