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Democritus University of Thrace

University of Algarve

UniLaSalle

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. This research topic explores the potential risks of soil contamination with heavy metals and trace elements in peri-urban farmlands in Nairobi city catchment in Kenya. It also looks into bioremediation, phytostabilization, and tree plantation trials to mitigate contaminant dispersal. <ul> <li>Findings call for proper zoning of suitable agricultural areas and sound waste management protocols in urban and peri-urban landscapes.</li> <li>Bioremediation is a biological method of reclaiming polluted soils which is less expensive and more sustainable than conventional methods.</li> <li>Tree species were able to survive on the tailings dams, with Acacia polyacantha and Psidium guajava showing the highest quantity of roots in the unamended tailing.</li> <li>Phytostabilization is an effective way to rehabilitate metal-polluted environments in the Copperbelt region, as demonstrated by a 15-year tree planting trial.</li> </ul>

Frontiers in Soil Science

Soil Pollution & Remediation

Global Excellence in Soil Pollution and Remediation: Europe 2022

IntroductionWith urbanization and industrialization in the developing world, urban and peri-urban agriculture is increasingly contributing to urban food systems, employment, and income generation opportunities. However, urbanization and industrialization may release harmful pollutants, including heavy metals and trace elements into agricultural soils, posing ecological, environmental and public health concerns. This paper assessed the potential risks of soil contamination with heavy metals and trace elements in peri-urban farmlands in Nairobi city catchment in Kenya. </sec>MethodsA total of 60 soil samples were collected from ten vegetable farming zones (S1-S10) and processed following standard protocols. The concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn were analyzed in the samples using inductively coupled plasma-mass spectrometry (ICP-MS). </sec>Results and discussionResults revealed that the soil samples have elevated concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn, with some elements including As (1.7%), Cd (13.3%), Mn (80%), Pb (1.7%) and Zn (11.7%) exceeding the permissible thresholds for agricultural soils. The spatial distribution of the elements exhibited three similar distribution patterns with slight variations between the hotspot sites of the different elements: (i) Co and Mn, (ii) Cu and Cr and (iii) As, Cd, Fe, Hg, Pb and Zn, while Ni and V exhibited singular spatial distributions compared to other elements. Elements such as As, Cd, Co, Fe, Hg, Mn, Pb and Zn appeared to be of anthropogenic enrichment, while Cr, Cu, Ni and V appeared to originate mainly from lithogenic and natural sources. The elements As, Cd, Hg and Pb exhibited high ecological risks, with Cd contributing about 46-66% of the overall ecological risk. The sampling sites S1, S5, S7, S9 and S10 depicted the highest ecological risks of 145, 103, 146, 121 and 146, respectively. The findings call for proper zoning of suitable agricultural areas and sound waste management protocols in urban and peri-urban landscapes. Further, remediation of contaminated soils and farmers’ sensitization are recommended for ecological and public health risk alleviation.</sec>

Introduction: With urbanization and industrialization in the developing world, urban and peri-urban agriculture is increasingly contributing to urban food systems, employment, and income generation opportunities. However, urbanization and industrialization may release harmful pollutants, including heavy metals and trace elements into agricultural soils, posing ecological, environmental and public health concerns. This paper assessed the potential risks of soil contamination with heavy metals and trace elements in peri-urban farmlands in Nairobi city catchment in Kenya. Methods: A total of 60 soil samples were collected from ten vegetable farming zones (S1-S10) and processed following standard protocols. The concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn were analyzed in the samples using inductively coupled plasma-mass spectrometry (ICP-MS). Results and discussion: Results revealed that the soil samples have elevated concentrations of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn, with some elements including As (1.7%), Cd (13.3%), Mn (80%), Pb (1.7%) and Zn (11.7%) exceeding the permissible thresholds for agricultural soils. The spatial distribution of the elements exhibited three similar distribution patterns with slight variations between the hotspot sites of the different elements: (i) Co and Mn, (ii) Cu and Cr and (iii) As, Cd, Fe, Hg, Pb and Zn, while Ni and V exhibited singular spatial distributions compared to other elements. Elements such as As, Cd, Co, Fe, Hg, Mn, Pb and Zn appeared to be of anthropogenic enrichment, while Cr, Cu, Ni and V appeared to originate mainly from lithogenic and natural sources. The elements As, Cd, Hg and Pb exhibited high ecological risks, with Cd contributing about 46-66% of the overall ecological risk. The sampling sites S1, S5, S7, S9 and S10 depicted the highest ecological risks of 145, 103, 146, 121 and 146, respectively. The findings call for proper zoning of suitable agricultural areas and sound waste management protocols in urban and peri-urban landscapes. Further, remediation of contaminated soils and farmers’ sensitization are recommended for ecological and public health risk alleviation.

Heavy metals and trace elements contamination risks in peri-urban agricultural soils in Nairobi city catchment, Kenya

Soils are polluted by both organic and inorganic substances. Plants growing in polluted soils suffer damages such as leaf rolls, chlorosis, growth inhibition, root tips browning, and death of plant. Soil pollutants such as hydrocarbon and heavy metals are absorbed by crops and such ends up being consumed by human posing health risk like cancer and respiratory abnormally. Conventional methods of remediation such as chemical and physical methods are very expensive and not sustainable. Excavation, which is a type of physical method, merely shifts the pollutant from one site to another. Bioremediation is a biological method of reclaiming polluted soils. Bioremediation is less expensive and more sustainable and safer when compared to the conventional methods of reclamation of polluted environment. This biological method of remediation is an extremely attractive, important, and productive alternative for cleaning, debugging, managing, and rehabilitating and consequently ameliorating contaminated environments via judicious utilization of microbial activities. The rate, at which the waste substances are degraded, is usually dictated by competitiveness among biological agents, sub-optimal supply of essential nutrients, unconducive abiotic conditions (in forms of temperature, aeration, pH, and moisture), and constrained pollutant’s bioavailability. Bioremediation is often effective only under conducive environmental conditions favorable for microbial growth and development. It has been successfully used at various parts of the world. Based on the significance of bioremediation in enhancing the reclamation of polluted environments by decontaminating and degrading heavy metals and xenobiotics, more focused researches would be needed so as to improve contaminated environments in much safer ways and conditions through bioremediation techniques. This research discussed the various types and methods of bioremediation. The mechanisms of actions and strategies of microorganisms in bioremediation were well expatiated. The interaction between bioremediators and the mineral particles in the soil environment was explained.

Soils are polluted by both organic and inorganic substances. Plants growing in polluted soils suffer damages such as leaf rolls, chlorosis, growth inhibition, root tips browning, and death of plant. Soil pollutants such as hydrocarbon and heavy metals are absorbed by crops and such ends up being consumed by human posing health risk like cancer and respiratory abnormally. Conventional methods of remediation such as chemical and physical methods are very expensive and not sustainable. Excavation, which is a type of physical method, merely shifts the pollutant from one site to another. Bioremediation is a biological method of reclaiming polluted soils. Bioremediation is less expensive and more sustainable and safer when compared to the conventional methods of reclamation of polluted environment. This biological method of remediation is an extremely attractive, important, and productive alternative for cleaning, debugging, managing, and rehabilitating and consequently ameliorating contaminated environments via judicious utilization of microbial activities. The rate, at which the waste substances are degraded, is usually dictated by competitiveness among biological agents, sub-optimal supply of essential nutrients, unconducive abiotic conditions (in forms of temperature, aeration, pH, and moisture), and constrained pollutant’s bioavailability. Bioremediation is often effective only under conducive environmental conditions favorable for microbial growth and development. It has been successfully used at various parts of the world. Based on the significance of bioremediation in enhancing the reclamation of polluted environments by decontaminating and degrading heavy metals and xenobiotics, more focused researches would be needed so as to improve contaminated environments in much safer ways and conditions through bioremediation techniques. This research discussed the various types and methods of bioremediation. The mechanisms of actions and strategies of microorganisms in bioremediation were well expatiated. The interaction between bioremediators and the mineral particles in the soil environment was explained.Keywords: bioremediation, biostimulant, soil environment, soil pollution, bioaugumentation,Citation: Alori ET, Gabasawa AI, Elenwo CE and Agbeyegbe OO (2022) Bioremediation techniques as affected by limiting factors in soil environment. Front. Soil Sci. 2:937186. doi: 10.3389/fsoil.2022.937186Received: 05 May 2022; Accepted: 14 July 2022; Published: 04 August 2022.Copyright © 2022 Alori, Gabasawa, Elenwo and Agbeyegbe. This is an open-access article distributed under the terms of the <a href="http://creativecommons.org/licenses/by/4.0/" rel="license">Creative Commons Attribution License (CC BY)</a>. The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*Correspondence: Elizabeth Temitope Alori, <a href="mailto:aloritope@yahoo.com">aloritope@yahoo.com</a>

Bioremediation techniques as affected by limiting factors in soil environment

Among mine wastes, tailings are known to have the largest environmental impact, as they have high concentrations of trace elements and are susceptible to wind dispersal and water erosion. A tree plantation trial was installed at Kipushi tailing (DR Congo) in order to mitigate the contaminant dispersal in the surrounding areas. Fifteen years later, the present study was conducted for the purpose of investigating the macronutrient and metal content in amended holes and assessing the performance of tree species through root behavior in the tailings dams. Results show elevated available P, K, Ca, and Mg concentration in the surface and amended layers, which is higher than the unpolluted soil of the miombo woodland. Trace metals were manifold higher compared to the pedo-geochemical background of the region, with Cu and Co concentration tending to increase in the organic matter-rich layers, while Zn, Cd, Pb, and As remained higher in tailings. Compared to the tailing layer, roots grew well in the amended layers, but few roots ranging from very fine to big from all the surviving species were able to grow beyond the amended layers, indicating the possibility of tree survival on the tailings dams over many years. Acacia polyacantha and Psidium guajava are species that showed a higher quantity of roots in the unamended tailing layers. Leaves should be avoided for human or animal consumption, but as the concentration of Cu, Zn, and Co in guava was lower, there is no indication of hazards in case of their consumption. Therefore, the use of well-adapted tree species on the mix-up of the organic amendments with the uncontaminated topsoil seemed to be a good technique for the reclamation of larger polluted areas.

Among mine wastes, tailings are known to have the largest environmental impact, as they have high concentrations of trace elements and are susceptible to wind dispersal and water erosion. A tree plantation trial was installed at Kipushi tailing (DR Congo) in order to mitigate the contaminant dispersal in the surrounding areas. Fifteen years later, the present study was conducted for the purpose of investigating the macronutrient and metal content in amended holes and assessing the performance of tree species through root behavior in the tailings dams. Results show elevated available P, K, Ca, and Mg concentration in the surface and amended layers, which is higher than the unpolluted soil of the miombo woodland. Trace metals were manifold higher compared to the pedo-geochemical background of the region, with Cu and Co concentration tending to increase in the organic matter-rich layers, while Zn, Cd, Pb, and As remained higher in tailings. Compared to the tailing layer, roots grew well in the amended layers, but few roots ranging from very fine to big from all the surviving species were able to grow beyond the amended layers, indicating the possibility of tree survival on the tailings dams over many years. Acacia polyacantha and Psidium guajava are species that showed a higher quantity of roots in the unamended tailing layers. Leaves should be avoided for human or animal consumption, but as the concentration of Cu, Zn, and Co in guava was lower, there is no indication of hazards in case of their consumption. Therefore, the use of well-adapted tree species on the mix-up of the organic amendments with the uncontaminated topsoil seemed to be a good technique for the reclamation of larger polluted areas.

Effect of Organic Amendment on the Physicochemical Characteristics of Tailings Dam Soil and Root Development of Tree Species, Fifteen Years After Planting

Phytostabilization is a more appropriate way for rehabilitation of metal-polluted environments in the Copperbelt region. A 1.5-hectare tree planting trial had been installed on the tailings dam (TD) with the help of organic matter (OM) mixed to topsoil. Then, 15 years later, this study aimed to evaluate the performance of the tree plantation, in regard to soils and spontaneous vegetation. The Cu and Co concentration was higher in the reclaimed soil compared with the unreclaimed. Acacia auriculiformis, Acacia polyacantha, Albizia lebbeck, Pinus sylvestris, Psidium guajava, Senna siamea, Senna spectabilis, and Syzygium guineense responded well to the criteria of phytostabilization, whereas Leucaena leucocephala was very invasive. Metal concentration was manifold higher in charcoals than in woods (Zn: 3.8–39.8 mg kg−1 in wood versus 272–523 mg kg−1 in charcoal) with no differences between charcoals regardless of the origin. The tree implantation aided with amendments can meet the criteria of phytostabilization through good survival and regeneration in the heavily contaminated TD.

Phytostabilization is a more appropriate way for rehabilitation of metal-polluted environments in the Copperbelt region. A 1.5-hectare tree planting trial had been installed on the tailings dam (TD) with the help of organic matter (OM) mixed to topsoil. Then, 15 years later, this study aimed to evaluate the performance of the tree plantation, in regard to soils and spontaneous vegetation. The Cu and Co concentration was higher in the reclaimed soil compared with the unreclaimed. Acacia auriculiformis, Acacia polyacantha, Albizia lebbeck, Pinus sylvestris, Psidium guajava, Senna siamea, Senna spectabilis, and Syzygium guineense responded well to the criteria of phytostabilization, whereas Leucaena leucocephala was very invasive. Metal concentration was manifold higher in charcoals than in woods (Zn: 3.8–39.8 mg kg−1 in wood versus 272–523 mg kg−1 in charcoal) with no differences between charcoals regardless of the origin. The tree implantation aided with amendments can meet the criteria of phytostabilization through good survival and regeneration in the heavily contaminated TD.