In the last decade, Mozambique became one of the top coal producers in Africa. The coal mines are in Moatize district in the center of the country, inside the lower Zambezi river basin. Pollution due to mining activities has negative impacts in agriculture, fishing, water supply, and presents risks to public health. Mozambique is one of the largest cassava producers in the world. More than 1.5 million tons of cassava peel are generated in the country and this agro-waste is discarded. The aim of this study is to discuss the possibility of using cassava peel to treat mine water. Cassava peel can be used as carbon sources for sulfate reducing bacteria in bioremediation, as an adsorbent and as a filter mediu m. An integrated method is proposed for treatment of mine water in Moatize. It was suggested that applying cassava peel generated in developing countries for treatment of mine water could be a good solution to protect the environment against mining pollution.
Cassava peel is an agro-industrial waste that is discarded in Mozambique; yet it could be used in the treatment of mine water to remove metals and sulfate. Cassava peel could be used as carbon source in bioremediation of mine water, as an adsorbent in adsorption and as a filter medium for filtration of mine water. Implementing this method, metals, sulfates and cassava peel (agricultural waste) would be treated simultaneously, thus, avoiding pollution. Biological treatment of mine water using sulfate reducing bacteria (SRB) to remove sulfate and metals is a pro- mising alternative treatment due to low cost and better quality of sludge compared to conventional treatment methods [
Surface mining has been used in Moatize to exploit coal which results in the generation of waste rock pile, dust and mine water. The coal production in Mozambique increased significantly since 2011 due to start of operation of the first multinational coal mining company operating in Mozambique (
Acid mine drainage (AMD) is one of the most serious environmental pollution problems in mining industry around the world [
and it also represents a risk for human health [
Active treatment is conventionally used for treatment of mine water and its aim is to remove pollutants by chemical and physical process (filters or membranes) [
Bioremediation of mine water using SRB seems to be an attractive technology to treat polluted mine water, because it is economically viable and environmentally friendly [
where: M2+ are metals: Cu2+, Zn2+, Fe2+, Pb2+, Ni2+, Co2+, Cd2+, or Ag+.
Mine water has very low content of organic matter which is why extra supply source of organic matter is needed. Low molecular weight compounds like orga- nic acids (lactate, formate, acetate, and propionate) and alcohols (ethanol, methanol, and glycerol) are preferable carbon source for SRB [
Cassava (Manihotesculentacrantz) is mostly used in tropical regions as a staple food because it is tolerant to drought and it is productive in poor soils. Africa is the continent with the highest cassava production (230 million tons in 2012 which correspond to 53% of world production). Mozambique was ranked 5th in Africa and 8th in the world regarding to cassava production [
In Mozambique, there is a very limited usage of cassava for industrial purposes, because the sector is oriented toward family consumption. Most of produced cassava in Mozambique is processed by individual households to produce traditional food (rale and ugali) or it is sold fresh in local markets for domestic consumption. The potential for industrialization of cassava that Mozambique has is not yet being exploited due to lack of investment in the sector. The main cassava products that can be produced at industrial level are: high quality cassava flour, cassava chips, starch and ethanol. To date, in Mozambique, there is only one beer company called “Cervejas de Mocambique (CdM)” that produces beer called “Impala” that derives from cassava. The company was launched in 2011 and currently, it uses 315 tons of fresh cassava weekly. The cassava is supplied by farmers located in Nampula and Inhambane province [
According to [
To avoid metal and sulfate pollution in Mortize’s water resources due to coal mining activities, it is necessary to treat the mine water that comes from the coal washing process, pits, waste pile drainage, tailing dams, and surface runoff. The
Hemicellulose | Cellulose | Proteins | Lignin | Ash | Reference |
---|---|---|---|---|---|
32.36 | 9.71 | 3.7 | 16.89 | 11.38 | [ |
27.0 | 14.0 | 3.5 | 11.0 | 7.0 | [ |
23.40 | 14.17 | 5.29 | 10.88 | 3.7 | [ |
only treatment method that is used in coal mines that are in Moatize is sedimentation. In sedimentation process, dissolved metals and sulfate are not removed and they end up in nearby water resources. It is urgent to find cost-ef- fective treatment methods for mine water in Moatize in order to convince the coal mining companies to apply those technologies to avoid water pollution. The aim of this paper is to describe the cassava’s potential as carbon source for SRB, adsorbent and filter medium in the treatment of mine water, and suggest a cost- effective treatment method for mine water in Moatize.
In this section, the potential application of cassava peel for treatment of mine water will be described. In the coming section, SRB, adsorption and filtration will be discussed for mine water treatment.
There are no published studies where cassava peel was applied as carbon source for treatment of mine water. The choice of carbon source is vital for the economy of the process. In active bioreactor, liquid or gaseous carbon source is preferred, because continuous operation is needed [
The ethanol production from cassava peel has different steps and the first one is to prepare the cassava peel for the process. In order to increase the availability of sugar that can be hydrolyzed biologically for ethanol production using cassava peel, it is necessary to make a pretreatment in cassava peel biomass to reduce re- calcitrance biomass [
The procedures for ethanol production and subsequent treatment of mine water are described in the coming section. Cassava in Mozambique can be bought in local markets and from local farmers. The cassava must be washed with clean water and the peel must be removed. Cassava peel contain cyanide and thus, it is necessary to soak it in water for 5 hours to remove cyanide [
Chemical precipitation, ion exchange, adsorption, ultrafiltration, reverse osmosis, nan-filtration, electro-dialysis, coagulation, flocculation, flotation and electrochemical process are quite used to remove metals and metalloid from wastewater. According to, [
To date, activated carbon is the adsorbent that is most used in the world due to its high efficiency in removing metals and nonmetal pollutants from polluted water. Activated carbon has high surface area, good microporous structure and
porosity. For that reason, it can be used in batch and continuous operation [
Cassava peel can be used as filter medium in passive treatment of mine water. In this low cost treatment process, flow of mine water goes through filter medium (e.g cassava peel) and pollutants are removed. Organic filter material such as lignite [
SSF would be used in the enzymatic hydrolysis to obtain high efficiency in ethanol production. After fermentation, the mixture should go to two distillation co- lumns organized in series. In the first column, carbon dioxide that remained would be released and unreacted substrate (including unfermented sugar) would be collected and sent back to SSF. The ethanol produced with an estimated concentration of 37 wt% [
The coal mines in Moatize are inside of one of the most important river basins in Southern Africa, the Zambezi river basin. Due to climate change, the temperature is expected to rise by 0.3˚C to 0.6˚C per decade. This rise in temperature will have impact in the rainfall which is expected to decrease by 10% to 15% and the evaporation also is expected to decrease by 10% to 25% by 2050. It means that Zambezi river basin will become more dry and less water will be available for mining activities, agriculture and drinking water supply for the local people [
According to FAO, Mozambique produced 10 million tonnes of cassava in 2012, which means that 1.5 million tonnes of cassava peel were generated. This agro-waste was just discarded. According to Vale, coal mine in Mozambique, 5 billion liters of mine water were generated inside Vale coal mine in Moatize in 2012. Only 47% of the generated mine water was recycled and the other 53% was discharged to the local environment [
Mine water that is treated using SRB eliminates toxic metals that are able to form metal sulfide as shown in Equation (2). The metal sulfides have low solubility and thus, they can be removed through sedimentation. The disadvantage of SRB is that it cannot remove some metals (Ca2+, Mg2+) because they are not able to form metal sulfide. To overcome this problem, adsorption can be coupled with SRB bioreactor to increase the treatment of mine water. Cassava peel would be used as carbon source for SRB and as adsorbent for adsorption process. The treated mine water could be used for the irrigation of native plants and crops that are produced in the Zambezi river basin. Some of the coal mines in Moatize have plans to restore the environment impacted by mining through planting native species. For that, mine water treated with combination of SRB and adsorption can be used as irrigation water.
Ethanol produced from cassava peel can be used as fuel and as a carbon source for SRB. The use of ethanol as a carbon source for SRB in the treatment of mine water appears to be a good solution for polluted mine water in Moatize. There is a need to couple bioremediation with an adsorption column where the cassava peel would be used as adsorbent to remove metals that do not form metal sulfide in the bioremediation process. The treated mine water could be used for various purposes inside of the Zambezi Basin. In conclusion, applying agro-waste generated in developing countries in the treatment of mine water could be a good solution to protect the environment against pollution and also a source of water for local farmers to do agriculture.
The authors would like to thanks SIDA for providing financial support for this research and also the Lund University for providing technical support.
Pondja Jr., E.A., Persson, K.M. and Matsinhe, N.P. (2017) The Potential Use of Cassava Peel for Treat- ment of Mine Water in Mozambique. Jour- nal of Environmental Protection, 8, 277- 289. https://doi.org/10.4236/jep.2017.83021