Mika Sillanpää | University of the Witwatersrand (original) (raw)

Papers by Mika Sillanpää

• Polyethylenimine-modified chitosan materials were prepared via one-step synthesis. • The materi... more • Polyethylenimine-modified chitosan materials were prepared via one-step synthesis. • The materials exhibit excellent buf-fering capacity at low pH. • The materials showed fast adsorption of La(III) with high capacity. • This adsorbents afford remarkable performance for capture of La(III) from Al(III). A B S T R A C T The separation and recovery of rare earth elements (REEs) from leachates of bauxite residue has attracted increasing attention. Yet, the characteristics of bauxite residue leachates (low pH, low concentration of REEs, and coexistence of other trivalent ions) results in a longstanding challenge in the recovery of REEs. Here, we reported on the development of polyethylenimine (PEI) modified chitosan materials as efficient adsorbents for REE, La(III). The introduction of PEI brought abundant protonatable amino nitrogen atoms, which endows materials with excellent buffering capacity at extremely acidic pH. The PEI-chitosan materials can easily separate La(III) from Al(III), a major co-existing ion, with a separation factor of 3.1. The single-metal adsorption behavior showed fast and efficient adsorption capacity of 2.015 mmol/g for La(III). In binary systems, La(III) was preferentially adsorbed over Al(III) due to the higher degree of association with PEI. The FT-IR, XPS and EDS mapping results revealed that in the binding mechanism the N atoms form coordination bonds with La(III) by sharing an electron pair, resulting in eight-membered chelate rings. The PEI-chitosan materials also exhibited an excellent reusability with regeneration efficiency of 90% after 4 recycles. Overall, PEI-chitosan demonstrates that it is a viable and economical material for the separation and preconcentration of REEs from leachates of bauxite residue.

Microplastics have recently been detected in the atmosphere of urban, suburban, and even remote a... more Microplastics have recently been detected in the atmosphere of urban, suburban, and even remote areas far away from source regions of microplastics, suggesting the potential long-distance atmospheric transport for micro-plastics. There still exist questions regarding the occurrence, fate, transport, and effect of atmospheric micro-plastics. These questions arise due to limited physical analysis and understanding of atmospheric microplastic pollution in conjunction with a lack of standardized sampling and identification methods. This paper reviews the current status of knowledge on atmospheric microplastics, the methods for sample collection, analysis and detection. We review and compare the methods used in the previous studies and provide recommendations for atmospheric microplastic sampling and measurement. Furthermore, we summarize the findings related to atmospheric microplastic characteristics, including abundance, size, shapes, colours, and polymer types. Microplastics occur in the atmosphere from urban to remote areas, with an abundance/deposition spanning 1-3 orders of magnitude across different sites. Fibres and fragments are the most frequently reported shapes and the types of plastic which generally aligns with world plastic demand. We conclude that atmospheric microplastics require further research and greater understanding to identify its global distributions and potential exposure to human health through further field sampling and implementation of standardized analytical protocols.

Hydroxyl radical (• OH) can hydroxylate or dehydrogenate organics without forming extra products ... more Hydroxyl radical (• OH) can hydroxylate or dehydrogenate organics without forming extra products and is thereby expediently applied in extensive domains. Although it can be efficiently produced through single-electron transfer from transition-metal-containing activators to hydrogen peroxide (H 2 O 2), narrow applicable pH range, strict activator/H 2 O 2 ratio requirement, and byproducts that are formed in the mixture with the background matrix necessitate the need for additional energy-intensive up/ downstream treatments. Here, we show a green Fenton process in an electrochemical cell, where the electro-generated atomic H* on a Pd/graphite cathode enables the efficient conversion of H 2 O 2 into • OH and subsequent degradation of organic pollutants (80% efficiency). Operando liquid time-of-fight secondary ion mass spectrometry verified that H 2 O 2 activation takes place through a transition state of the Pd−H*−H 2 O 2 adduct with a low reaction energy barrier of 0.92 eV, whereby the lone electron in atomic H* can readily cleave the peroxide bridge, with • OH and H 2 O as products (ΔG r = −1.344 eV). Using H + or H 2 O as the resource, we demonstrate that the well-directed output of H* determines the pH-independent production of • OH for stable conversion of organic contaminants in wider pH ranges (3−12). The research pioneers a novel path for eliminating the restrictions that are historically challenging in the traditional Fenton process.

The photocatalytic degradation of carbamazepine (CBZ) in ultra-pure water was investigated by usi... more The photocatalytic degradation of carbamazepine (CBZ) in ultra-pure water was investigated by using neodymium (Nd)-doped antimony trioxide (Sb 2 O 3)/titanium dioxide (TiO 2) photocatalyst under the UVC irradiations of 254 nm wavelength. The hydrothermal method was used for the fabrication catalyst samples with different ratios of Nd (0%-2%) dopant, and characterised by X-ray diffraction pattern (XRD) to investigate the crystallinity. Scanning electron microscopy (SEM) provided the surface morphologies, Bruanuer-Emmer-Teller (BET) analysis gave the textural properties, and UV-Vis diffuse reflectance absorption spectroscopy (DRS) was used for the investigation of the optical properties of synthesized catalysts. TEM images of Sb 2 O 3 showed a nanorod-like structure while, in the Nd-doped Sb 2 O 3 /TiO 2 , a small dot-like structure was observed along with the nanorods. The surface area and band gap of 1% Nd-doped Sb 2 O 3 /TiO 2 were found to be 9.56 m 2 g À1 and 3.0 eV respectively. It was observed that the CBZ cannot be degraded in the absence of catalyst under UV light, while photocatalyst 1% Nd-doped Sb 2 O 3 /TiO 2 at 0.5 g/ L of catalyst dose showed the best photocatalytic activity towards CBZ degradation. The main degradation products were identified with high-resolution mass spectrometry. Moreover, the degradation of CBZ followed pseudo first-order kinetics and the rate constant was 0.017 min À1. Quenching tests by the addition of methanol from 100 to 500 mM were carried out to determine the major reactive oxygen species, which showed that Å OH radicals was involved in the CBZ degradation. Active species-trapping experiments revealed that • O 2 À is also responsible for the degradation of CBZ.

The photocatalytic degradation of an artificial sweetener, acesulfame-K (ACE) was investigated us... more The photocatalytic degradation of an artificial sweetener, acesulfame-K (ACE) was investigated using an ultraviolet light emitting diode (UV-LED) based irradiation in presence of titanium dioxide (TiO 2), zinc oxide (ZnO), hydrogen peroxide (H 2 O 2), peroxomonosulfate (PMS, HSO 5 −) and peroxodisulfate (PDS, S 2 O 8 2−). The pH of ACE solution showed significant effect on the degradation of ACE. It was observed that low pH value enhanced the ACE degradation rate. After 120 min by UV-LED/TiO 2 treatment, the degradation of ACE reached up to 90%, while in UV-LED/ZnO, UV-LED/H 2 O 2 , UV-LED/PMS, and UV-LED/PDS degradation rate were found to be 63%, 87%, 76% and 86% respectively. PMS and PDS both oxidant showed good results without the generation of any secondary sludge like other heterogeneous catalysts. The present study showed that in presence of catalysts and oxidants, UV-LED illumination significantly enhanced the degradation rate of ACE in comparison to direct photolysis by UV-LED. Possible degradation pathway of ACE was determined by gas chromatography-mass spectrometry (GC-MS) which confirmed the ACE degradation by generation of three byproducts.

Benzophenone-3 (BP-3) is a widely used organic UV filter in sunscreen which has been detected in ... more Benzophenone-3 (BP-3) is a widely used organic UV filter in sunscreen which has been detected in surface and groundwater. BP-3 can affect the aquatic environment and human health. In this study, PbO/TiO 2 and Sb 2 O 3 /TiO 2 photocatalyst were synthesized for the photocatalytic degradation of Benzophenone-3 (BP-3) and various degradation parameters such as initial pH value, initial concentration, and the dose of catalysts were optimized. Two different TiO 2 based catalysts PbO/TiO 2 and Sb 2 O 3 /TiO 2 were synthesized by hydrothermal method. Synthesized photocatalysts were characterized by X-ray diffraction pattern (XRD), scanning electron microscope (SEM), Energy Dispersive Spectroscopy (EDS), BET and UV-Vis DRS techniques. Molar ratio variation of PbO and Sb 2 O 3 with respect to TiO 2 significantly affected the surface area, structure, and bandgap of photocatalyst and hence the variation in degradation efficiency of the photocatalyst was observed. The BP-3 can be completely degraded by using PbO/TiO 2 within 120 min under UV-C irradiation. The highest degradation of BP-3 was obtained for the 20 µM concentration at pH 7 when the dose was adjusted to be 0.75 g/L. However, negligible degradation of BP-3 was demonstrated in the absence of a catalyst. Moreover, with the catalysts PbO/TiO 2 and Sb 2 O 3 /TiO 2 , BP-3 followed the pseudo-first-order kinetics with a rate constant of 3.58 × 10 −2 min −1 and 0.92 × 10 −2 min −1 respectively. Electron paramagnetic resonance (EPR) spectrum with three distinct peaks with an intensity of 1:1:1 showed the presence of TEMP-1 O 2 adduct which suggested the generation of 1 O 2 (singlet oxygen) in both catalysts. The plausible mechanism of BP-3 degradation was proposed by the Gas chromatography-mass spectrometry (GC-MS) analysis which showed the formation of pentamethyl-and 5-Hydroxy-7-methoxy-2-methyl-3-phenyl-4-chromenone byproducts on BP-3 photocatalytic degradation by the synthesized catalyst.

Archives of environmental contamination and toxicology, 2003

Acute toxicity of four relatively new chelating agents and their equimolar manganese and cadmium ... more Acute toxicity of four relatively new chelating agents and their equimolar manganese and cadmium complexes was studied. The chelating agents studied were gluconic acid (GA), beta-alaninediacetic acid (ADA), diethylenetriaminepentakismethylenephosphonic acid (DTPMP), and nitrilotriacetic acid (NTA). Three common bioassays, namely Daphnia magna, Raphidocelis subcapitata, and Photobacterium phosphoreum (Microtox bioassay) were applied. R. subcapitata proved the most sensitive to these compounds. With D. magna bioassay the LC(50) values were 600-900 mg/L with all other studied chelates and their Mn complexes, except Mn-GA, which yielded LC(50) value of 240 mg/L. The Cd-chelate complexes proved highly more toxic compared to Mn-chelate complexes or uncomplexed chelates exhibiting LC(50) values of 130-200 microg/L. However, Cd-DTPMP was an exception exhibiting LC(50) value of 2170 microg/L. That is to say, DTPMP proved the strongest chelating agent to reduce the Cd toxicity in the present study. The results from these bioassays were well in agreement to each other as well as with the results published elsewhere.

This article reports on recent developments in heterogeneous AOP processes such as photocatalysis... more This article reports on recent developments in heterogeneous AOP processes such as photocatalysis, Fenton-like process and catalytic ozonation. The principle, mechanism, and influence of experimental conditions on the degradation of pollutants in heterogeneous catalytic ozonation and the photocatalytic process were discussed. Introducing solid catalysts substantially increased the efficiency of the ozonation process by producing hydroxyl radicals in the degradation process. The different types of catalyst, catalyst dosage, solution pH, ozone flow rate, water matrix and catalytic reusability and stability are reported on here. The list of various semiconductor materials used as photocatalysts, their light absorption properties, various light sources and surface properties such as surface area, pore size and pore volume as a factor in the photocatalytic degradation of various pollutants are discussed. The review article also discussed the pollutants degraded using these three processes.

Archives of environmental contamination and toxicology, 2003