Jonas Addai-Mensah

Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions

Abstract The influence of an anionic polyacrylamide–acrylate copolymer (PAM) and a nonionic polyethylene oxide (PEO) polymer on the surface chemistry, shear yield stress, settling rates and consolidation behaviour of kaolinite dispersions has been investigated at pH 7.5. The magnitude of the particle zeta potential decreased with increasing flocculant concentration for both flocculants but was much greater for PEO, due to an increased adsorbed layer thickness compared with PAM. Furthermore, the adsorption studies showed a higher affinity for PEO than PAM. The optimum flocculation for improved clarification rate occurred at less than half the plateau adsorbed amount for both polymers. At similar polymer concentrations, the kaolinite floc sizes were larger and the settling rates greater in the presence of PEO than PAM. Upon shear, the consolidation of pre-sedimented kaolinite slurries significantly improved under flocculation with PEO, while PAM produced no such effect. The magnitude of yield stress was strongly dependent upon the polymer structure, with greater yield stress (stronger interparticle bridging forces) observed for PEO than for PAM flocs. The difference in shear sensitivity of the flocculated slurries may be attributed to polymer structure-related adsorption and interfacial conformation behaviour. PEO polymer chains adsorb via hydrogen bonding interactions between the ether oxygen (Lewis base) and OH groups (Bronsted acid) associated with aluminol and silanol groups, which led to flexible and compressible floc structures. For PAM, however, although the adsorption is still via hydrogen bonding between the silanol and aluminol OH groups at the particle surface and the polymers primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the polymers COO− pendant groups and the negatively charged kaolinite surface. This, in conjunction with possible steric hindrance due to PAM polymer chain branching, led to the formation of loose, open and fragile flocs. A strong correlation between the polymer structure type and flocculation, shear sensitivity and the dewatering behaviour of kaolinite dispersion was established.

Synthesis of self-supporting gold microstructures with three-dimensional morphologies by direct replication of diatom templates.

Diatoms (unicellular algae) form porous silica walls (frustule) with intricate, hierarchically organized three-dimensional (3D) structures with micro- to nanoscale dimensions. This paper presents the fabrication of self-supporting gold microstructures with complex 3D morphologies by using electroless gold deposition onto a diatom silica substrate, followed by the substrate removal by acid dissolution. It was demonstrated that gold diatom replicas with distinct micro- to nanoscale structures can be created by a simple and scalable process based on electroless gold deposition. Excellent catalytic properties (catalytic rate constant k = 23.5 +/- 1 x 10(-2) min(-1)) of prepared gold replica catalysts were confirmed for the reduction process of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of NaBH(4) as the reductant. This synthetic approach is general and flexible, and it is envisaged that it can be applied for the preparation of a wide range of different metals (Pt, Pd, Ag, Ni, etc.) offering more efficient catalytic, optical, or magnetic properties.

The mechanism of the sodalite-to-cancrinite phase transformation in synthetic spent Bayer liquor

Abstract The precipitation of zeolite, sodalite and cancrinite and subsequent phase transformations were investigated under a variety of conditions in sodium aluminate liquor. At sufficiently high SiO 2 relative supersaturation, amorphous sodium aluminosilicate and zeolite precipitated at temperatures as high as 160°C. They subsequently transformed to sodalite and finally to cancrinite. Thus the sequence of the transformation of phases is: [Aluminosilicate species]→Amorphous phase→Zeolite (Linde A)→Sodalite→Cancrinite. It was found that sodalite did not transform to cancrinite in the absence of a liquid medium. The transformation of sodalite to cancrinite was demonstrated to involve a solution-mediated mechanism with sodalite dissolution and subsequent cancrinite precipitation. Neither the amorphous phase nor the zeolite phase precipitates at typical spent Bayer liquor SiO 2 supersaturation.

Functionalized diatom silica microparticles for removal of mercury ions

Abstract Diatom silica microparticles were chemically modified with self-assembled monolayers of 3-mercaptopropyl-trimethoxysilane (MPTMS), 3-aminopropyl-trimethoxysilane (APTES) and n-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AEAPTMS), and their application for the adsorption of mercury ions (Hg(II)) is demonstrated. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy analyses revealed that the functional groups (–SH or –NH2) were successfully grafted onto the diatom silica surface. The kinetics and efficiency of Hg(II) adsorption were markedly improved by the chemical functionalization of diatom microparticles. The relationship among the type of functional groups, pH and adsorption efficiency of mercury ions was established. The Hg(II) adsorption reached equilibrium within 60 min with maximum adsorption capacities of 185.2, 131.7 and 169.5 mg g−1 for particles functionalized with MPTMS, APTES and AEAPTMS, respectively. The adsorption behavior followed a pseudo-second-order reaction model and Langmuirian isotherm. These results show that mercapto- or amino-functionalized diatom microparticles are promising natural, cost-effective and environmentally benign adsorbents suitable for the removal of mercury ions from aqueous solutions.

A multi-drug delivery system with sequential release using titania nanotube arrays

A multi-drug delivery system with sequential release based on titania nanotube arrays and polymer micelles as drug carriers is presented. Delivery of multiple water insoluble and soluble drugs required for combined local therapy is demonstrated.

The influence of sodium carbonate on sodium aluminosilicate crystallisation and solubility in sodium aluminate solutions

Abstract Isothermal batch precipitation experiments have been carried out in synthetic Bayer liquors to investigate the effects of sodium carbonate concentration on both silica solubility and the crystallisation of sodium aluminosilicates. At both 90 and 160°C cancrinite (generically defined as a sodium aluminosilicate of space group P6 3 ) is the stable solid phase. Sodalite (generically defined as a sodium aluminosilicate with space group P43n seed transforms to cancrinite at both these temperatures. A high concentration of sodium carbonate in the synthetic liquor causes a decrease in the rate of conversion of sodalite to cancrinite. The solubility of both cancrinite and sodalite decreases as the concentration of sodium carbonate in the synthetic liquor is increased. For instance at 90°C and with 40.0 g dm −3 sodium carbonate in the synthetic liquor after 13 days the sodium aluminosilicate concentration is 0.52 g dm −3 compared to 0.85 g dm −3 with 4.6 g dm −3 of sodium carbonate in solution. At 160°C the sodium aluminosilicate concentration is 0.47 g dm −3 with 40.0 g dm −3 sodium carbonate in solution after 13 days and 0.79 g dm −3 with 4.6 g dm −3 sodium carbonate in solution. Throughout all these experiments a progressive loss of carbonate from the sodium aluminosilicate crystallisation products was observed as a function of time.

Silica microcapsules from diatoms as new carrier for delivery of therapeutics

AIM This study explores the use of natural silica-based porous material from diatoms, known as diatomaceous earth, as a drug carrier of therapeutics for implant- and oral-delivery applications. MATERIALS & METHODS To prove this concept, two drugs models were used and investigated: a hydrophobic (indomethacin) and hydrophilic (gentamicin). RESULTS & DISCUSSION Results show the effectiveness of diatom microcapsules for drug-delivery application, showing 14-22 wt% drug loading capacity and sustained drug release over 2 weeks. Two steps in the drug release from diatom structures were observed: the first, rapid release (over 6 h is attributed to the surface deposited drug) and the second, slow and sustained release over 2 weeks with zero order kinetics. CONCLUSION These results confirm that natural material based on diatom silica can be successfully applied as a drug carrier for both oral and implant drug-delivery applications, offering considerable potential to replace existing synthetic nanomaterials.

The solubility of sodalite and cancrinite in synthetic spent Bayer liquor

The equilibrium SiO2 solubility of sodalite and cancrinite crystals in sodium aluminate solutions containing low concentrations of aluminium (synthetic spent Bayer liquor) has been determined over a range of temperatures (90–220°C) and as a function of solution NaOH and Al(OH)3 concentration for cancrinite. The same SiO2 solubilities, within experimental error, were measured when equilibrium was approached from both above and below SiO2 solubility for spent Bayer liquors in contact with either sodalite or cancrinite crystals. The solubility of sodalite and cancrinite (expressed in terms of SiO2 concentration) increased linearly with increasing temperature. The equilibrium SiO2 solubility of sodalite was higher than that of cancrinite at all temperatures. The solubility of cancrinite increased with increasing concentration of NaOH (3.87–5.42 M) and Al(OH)3 (1.39–2.23 M) in solution. The SiO2 solubilities of four types of dimorphic sodalite/cancrinite mixed phase seed crystals synthesised from Bayer plant spent liquor and synthetic Na2CO3, Na2SO4 or NaNO3 rich NaOH solutions containing dissolved kaolinite were investigated. The solubility of the four types of mixed phase seed crystals was found to be substantially the same as the solubility for pure cancrinite.

A methodology for quantifying sodalite and cancrinite phase mixtures and the kinetics of the sodalite to cancrinite phase transformation

A simple methodology for determining the cancrinite proportion of a sodalite and cancrinite phase mixture was developed using powder X-ray diffraction. Using this methodology to quantify phase mixtures, the kinetics of the sodalite to cancrinite phase transformation were determined. The transformation reaction leading to cancrinite formation was found to be first order with respect to the relative concentration of sodalite. Over the temperature range 160–240°C, an activation energy for this process was estimated to be 133 kJ mol−1. The techniques of 29Si MAS NMR and FTIR were both found to be unsuitable for quantifying dimorphic phase mixtures due to the ambiguity of the results they produced.

The kinetics of desilication of synthetic spent Bayer liquor seeded with cancrinite and cancrinite/sodalite mixed-phase crystals

Isothermal, batch desilication kinetics of synthetic, sodium aluminate solution (spent Bayer liquor) via cancrinite and cancrinite/sodalite mixed-phase crystal growth, have been studied under conditions at which sodium aluminosilicate scale forms at the surfaces of steel heat exchangers of alumina plant. Seeding with the pure cancrinite and mixed-phase crystals results in the suppression of scale formation and a faster rate of liquor desilication in comparison with its sodalite dimorph. Cancrinite seed crystals prepared from NO−3-rich solutions exhibited crystal growth mechanism and kinetic behaviour different from dimorphic mixed-phase crystals prepared from CO2−3-rich solutions, when both were used to desilicate CO2−3-rich spent Bayer liquor. The rate of desilication due to crystal growth on CO2−3-cancrinite/sodalite mixed phase crystals followed a second-order dependence on the relative supersaturation of SiO2. An activation energy of 52 kJ mol−1 was estimated for the crystal growth process. For desilication kinetics involving NO−3-cancrinite seed crystal growth, a third-order dependence on relative supersaturation of SiO2 and an activation energy of 63 kJ mol−1 were obtained.