Faculty of Science (Research Outputs)

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Start date: 2020

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Now showing 1 - 10 of 25
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    An inertial-type method for solving image restoration problems
    (Springer, 2023-07) Izuchukwu, Chinedu; Shehu, Yekini; Reich, Simeon
    We first establish weak convergence results regarding an inertial Krasnosel’skiĭ-Mann iterative method for approximating common fixed points of countable families of nonexpansive mappings in real Hilbert spaces with no extra assumptions on the considered countable families of nonexpansive mappings. The method of proof and the imposed conditions on the iterative parameters are different from those already available in the literature. We then present some applications to the Douglas–Rachford splitting method and image restoration problems, and compare the performance of our method with that of other popular inertial Krasnosel’skiĭ-Mann methods which can be found in the literature.
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    Numerical simulation of nanofluid flow due to a stretchable rotating disk
    (Elsevier, 2023-05) Ayano, Mekonnen S.; Otegbeye, Olumuyiwa; Mathunjwa, Jochonia S.
    In this study, a steady magnetohydrodynamic (MHD) flow due to stretchable rotating disk in the presence of gyrotactic microorganisms is investigated. The governing equations modeling the flow are solved numerically using the newly introduced simple iteration method (SIM) that seeks to linearize a system using relaxation technique that effectively decouples the system. To verify the convergence and accuracy of the method, solution error and residual error analysis are carried out, respectively. The obtained results suggest that the SIM is a highly efficient method that produces convergent and highly accurate solutions. The effects of various parameters as well as combined parameter effects on the solution profiles are also investigated. An increase in the Hall and permeability parameters leads to a corresponding rise in the microorganism’s density and nanoparticle volume fraction.
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    Projecting Podocarpaceae response to climate change: we are not out of the woods yet
    (Oxford University Press, 2023-06) Twala, Thando C.; Fisher, Jolene T.; Glennon, Kelsey L.
    Under the changing climate, the persistence of Afrotemperate taxa may be threatened as suitable habitat availability decreases. The unique disjunct ranges of podocarps in southern Africa raise questions about the persistence of these species under climate change. Here, we identified likely environmental drivers of these distributions, characterized the current and future (2070) environmental niches, and projected distributions of four podocarp species in South Africa. Species distribution models were conducted using species locality data for Afrocarpus falcatus, Podocarpus latifolius, Pseudotropheus elongatus and Podocarpus henkelii and both historical climate data (1970–2000) and future climate scenarios (Representative Concentration Pathway [RCP] 4.5 and 8.5, 2061–2080) to estimate the current and future distributions. We also used this opportunity to identify the most important climatic variables that likely govern each species’ distribution. Using niche overlap estimates, a similarity test, and indices of niche expansion, stability and unfilling, we explored how niches change under different climate scenarios. The distribution of the study species was governed by the maximum temperature of the warmest month, temperature annual range, mean temperature of the wettest quarter, and precipitation of the wettest, driest and warmest quarters. The current distribution of A. falcatus was predicted to expand to higher elevations under RCP 4.5 and RCP 8.5. Podocarpus henkelii was predicted to lose most of its suitable habitat under RCP 4.5 and expand under RCP 8.5; however, this was the opposite for P. elongatus and P. latifolius. Interestingly, P. elongatus, which had the smallest geographic distribution, showed the most vulnerability to climate change in comparison to the other podocarps. Mapping the distribution of podocarps and understanding the differences in their current and future climate niches provide insight into potential climate drivers of podocarp persistence and the potential for adaptation of these species. Overall, these results suggest that P. elongatus and P. henkelii may expand to novel environmental niches.
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    Stakeholder perceptions reveal obstacles and opportunities to change lethal methods of protecting bathers from sharks
    (2023-09) Atkins, Shanan; Mann-Lang, Judy; Cliff, Geremy; Pillay, Neville; Cantor, Mauricio
    Bather protection gear—shark nets and baited hooks—is set to catch and kill sharks to protect bathers at popular swimming beaches. This lethal practice contributes to human well-being and safeguards beach tourism, a valuable income-generator. However, it is costly—financially and environmentally. Here we identify obstacles and opportunities to change this lethal method of bather protection in KwaZulu-Natal, South Africa, by assessing the knowledge of people closely involved in this bather-shark conflict. We conducted semi-structured interviews with 29 stakeholders from various organisations—KwaZulu-Natal Sharks Board which manages the gear, three government levels (local, provincial, national), and tourism and conservation organisations—to identify their perceptions about how the gear works, its advantages and disadvantages, and the possibility of changing this method of protecting bathers. Half the interviewees were unaware that the gear intentionally kills sharks. Barriers to changing the 70-year status quo include: mindsets regarding sharks and bather protection in KwaZuluNatal (compared to other places); government officials’ fear of negative consequences of change; politicians prioritising constituents’ short-term well-being; lack of proven alternatives; high cost of potential alternatives; challenging surf conditions; and slow progress of innovation. Opportunities included promising technologies, research and education to rectify misconceptions. We recommend assessing the obstacles and opportunities for change to governance structures (institutions, policies, systems) to implement a programme that accelerates the development and testing of alternatives in KwaZulu-Natal, coupled with a well-designed communication campaign. It is crucial to revisit this long-standing bather protection programme and make it safe for both humans and sharks.
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    Extensions and variations of Andrews–Merca identities
    (Springer, 2023-07) Nyirenda, Darlison; Mugwangwavari, Beaullah
    Recently, Andrews and Merca have given a new combinatorial interpretation of the total number of even parts in all partitions of n into distinct parts. We generalise this result and consider many more variations of their work. We also highlight some connections with the work of Fu and Tang.
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    Precipitation gradients drive high tree species turnover in the woodlands of eastern and southern Africa
    (Wiley, 2023-07) Davies, Robert W.; Ryan, Casey M.; Harrison, Rhett D.; Dexte, Kyle G.; Ahrends, Antje; te Beest, Mariska; Benitez, Lorena; Brade, Thom K.; Carreiras, Joao M. B.; Druce, Dave J.; Fayolle, Adeline; Finckh, Manfred; Godlee, John L.; Gonclaves, Francisco M.; Grundy, Isla M.; Hoche, T.; Holdo, Ricardo M.; Makungwa, Steve; McNicol, Iain M.; Mograbi, Penelope J.; Muchawona, Anderson; Muhate, Aristidies; Muledi, Jonathan; Pritchard, Rose; Revermann, Rasmus; Ribeiro, Natasha S.; Siampale, Abel; Staver, A. Carla; Syampungani, Stephen; Williams, Mathew; Swemmer, Anthony M.; Edwards, David P.
    Savannas cover one-fifth of the Earth’s surface, harbour substantial biodiversity, and provide a broad range of ecosystem ser-vices to hundreds of millions of people. The community composition of trees in tropical moist forests varies with climate, butwhether the same processes structure communities in disturbance-driven savannas remains relatively unknown. We investigatehow biodiversity is structured over large environmental and disturbance gradients in woodlands of eastern and southern Africa.We use tree inventory data from the Socio-Ecological Observatory for Studying African Woodlands (SEOSAW) network, cov-ering 755 ha in a total of 6780 plots across nine countries of eastern and southern Africa, to investigate how alpha, beta, andphylogenetic diversity varies across environmental and disturbance gradients. We find strong climate-richness patterns, withprecipitation playing a primary role in determining patterns of tree richness and high turnover across these savannas. Savannaswith greater rainfall contain more tree species, suggesting that low water availability places distributional limits on species,creating the observed climate-richness patterns. Both fire and herbivory have minimal effects on tree diversity, despite theirrole in determining savanna distribution and structure. High turnover of tree species, genera, and families is similar to turnoverin seasonally dry tropical forests of the Americas, suggesting this is a feature of semiarid tree floras. The greater richness andphylogenetic diversity of wetter plots shows that broad-scale ecological patterns apply to disturbance-driven savanna systems.High taxonomic turnover suggests that savannas from across the regional rainfall gradient should be protected if we are tomaximise the conservation of unique tree communities.
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    Testing the suitability of portable X-Ray Fluorescence (pXRF) analysis of dried herbarium specimens to detect Ni hyperaccumulators in South Africa
    (Elsevier, 2023-07) Balkwill, Kevin; Samojedny Jr. Thomas J.; Rajakaruna, Nishanta; Siebert, Stefan J.
    Metal hyperaccumulators accumulate particular metals or metalloids in their leaves to concentrations hundreds or thousands of times greater than is normal for most plants. Globally, Ni is the most often hyperaccumulated metal, with 532 hyperaccumulator species documented to date. Hyperaccumulators have attracted much attention as potential candidates for green technologies, including phytoremediation and agromining. In South Africa, six serpentinite-associated plants in the genera Berkheya and Senecio hyperaccumulate Ni (to > 0.1% of leaf tissue dry weight). It is surprising that only six of about 70 Berkheya and 290 Senecio species native to South Africa hyperaccumulate Ni, given about ∼10–20% of taxa from each genus occur on serpentinite. While it is costly and time consuming to field collect and chemically analyze leaves of all species in these genera, a novel method (portable X-Ray Fluorescence or pXRF analysis of herbarium specimens) allows for rapid (100 s of specimens/day) and non-destructive measurement of Ni in dry herbarium specimens. We tested the accuracy of this approach on known Ni hyperaccumulators vouchered at two South African herbaria (C.E. Moss Herbarium (J) of the University of the Witwatersrand and A.P. Goossens Herbarium (PUC) from North-West University). While the absolute concentrations of Ni determined by ICP-MS and pXRF were not always directly in agreement, we had 100% success in confirming those that were known to hyperaccumulate Ni with those that did not. We propose pXRF as a cheap, effective, and efficient approach to rapidly screen herbarium specimens across South Africa to discover additional metal hyperaccumulators for much-needed remediation purposes.
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    Seasonal Pollution Levels and Heavy Metal Contamination in the Jukskei River, South Africa
    (MDPI, 2025-03) Mukwevho, Nehemiah; Ntsasa, Napo; Chimuka, Luke; Tshilongo, James; Mothepane H. Mabowa; Mkhohlakali, Andile; Letsoalo, Mokgehle R.
    Monitoring river systems is crucial for understanding and managing water resources, predicting natural disasters, and maintaining ecological balance. Assessment of heavy metal pollution derived valuable data which are critical for the environmental management and regulatory compliance of the Jukskei River. Heavy elements were evaluated in the Jukskei River for seasonal impact, potential health risks, and contamination level with concentration levels ranging from 6900 mg/kg iron (Fe) to 0.85 mg/kg cadmium (Cd) in the dry sampling season and 6900 mg/kg Fe to 0.26 mg/kg Cd in the wet season. Enrichment factor analysis indicated high contamination levels of Fe and Pb in both dry and wet seasons. Moreover, pollution indicators revealed extremely high contamination of geo-accumulation and enrichment factors in the downstream to upstream in both seasons with a mild contamination factor for mercury (Hg). Principal Component Analysis revealed anthropogenic sources of arsenic (As), Cd, and Pb due to wastewater and agricultural pesticide application while Thorium (Th), uranium (U) and Hg were attributed as a results of gold mining activities. ANOVA and Pearson correlation analysis showed a high and moderate link between As–Pb, Cd–Pd, and As–Hg, which are significantly correlated. The potential ecological risk index assessment revealed a significant impact of heavy metals on the freshwater ecosystem.
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    Chemical analysis of low grade gold from mine tailings after size fractionation and acid digestion using reverse aqua regia
    (Nature Research, 2025-03) Chimuka, Luke; Tshilongo, James; Mashale, Kedibone Nicholine; Sehata, James; Ntsasa, Napo Godwill
    The growing interest in reprocessing mine tailings for gold recovery requires a suitable quantification method that is accurate, rapid, and not harsh to the environment. Acid digestion is often used to determination of gold; however, it often faces the challenge of incomplete digestion due to the presence of minerals such as quartz, and homogeneity is compromised due to small sample masses, which can result in low bias. This study investigated a shorter acid digestion method employing reverse aqua regia, both in the presence and absence of hydrofluoric acid. Before digestion, the sample was subjected to gold depot analysis, which showed that 78% was free-milling gold and that only 0.8% was associated with pyrite, increasing the chances of accurate quantifications. Furthermore, the size screening test showed that most of the gold could be recovered on the −38 μm screen. This proposed method provided good linearity (5–100 µg. L−1) and low detection limits (0.139–0.183 µg.kg−1). The concentrations obtained by the acid digestion was 0.258 g.t−1 with the recoveries ranging between 80% and 82%, which fit the criteria set. The method also worked well for the certified reference materials (CRM), AMIS 610 (accurate value=0.068 g.t−1) and AMIS 646 (accurate value=0.166 g.t−1), which are of a similar matrix and are also lower in grade compared to the sample. The method was also evaluated for uncertainty (±value) using the bottom-up approach, and the expanded uncertainty (k=2) was reported to be 0.258±0.092 g.t−1, which was comparable to that offered by the fire assay with the ICP‒OES finish, which was 0.28±0.10 g.t−1. This implies that the acid digestion method is suitable for quantifying gold from mine tailings without large uncertainties.
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    Topological rejection of noise by quantum skyrmions
    (Nature Research, 2025-03) Ornelas, Pedro; Forbes, Andrew; de Mello Koch, Robert
    An open challenge in the context of quantum information processing and communication is improving the robustness of quantum information to environmental contributions of noise, a severe hindrance in real-world scenarios. Here, we show that quantum skyrmions and their nonlocal topological observables remain resilient to noise even as typical entanglement witnesses and measures of the state decay. This allows us to introduce the notion of digitization of quantum information based on our discrete topological quantum observables, foregoing the need for robustness of entanglement. We compliment our experiments with a full theoretical treatment that unlocks the quantum mechanisms behind the topological behavior, explaining why the topology leads to robustness. Our approach holds exciting promise for intrinsic quantum information resilience through topology, highly applicable to real-world systems such as global quantum networks and noisy quantum computers.