ADSORPTION OF DISSOLVED IONS IN MINE WASTEWATER WITH LATERITE AND IRON OXIDE NANOPARTICLES

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ABSTRACT


This work studies the adsorption capacity of laterite and iron oxide nanoparticles for dissolved ions in solution and therefore its ability to reduce conductivity of mine waste effluent. Three laterite samples were prepared and used as adsorbents. The first laterite adsorbent was used without any treatment, the second was heat treated and the third was mixed with 15 % iron oxide nanoparticles of particle size 100 nm. Laterite and nanoparticle characterization, pH and adsorption tests were conducted to ascertain the composition of laterite and as-prepared iron oxide nanoparticles, the adsorption capacity of the adsorbents and the optimum conditions for adsorption. The X-ray diffraction. (XRD) results of both pure and heat-treated laterite showed the main minerals present to be: Quartz (SiO2), Alumina (Al3O4), Berlinite (AlPO4) and Hematite (Fe2O3). The XRD results for synthesized iron oxide nanoparticles showed the mineral Magnetite (Fe3O4). The adsorption results of heat-treated laterite showed the highest adsorption capacity for total dissolved ions at a pH range of 6.5 - 6.8. Laterite and nanoparticles composite had the highest adsorption capacities for Ca and Mg ions.


CHAPTER ONE

BACKGROUND

1.1      Introduction

High quality water is a critical resource with invaluable socio-economic and environmental value and significance worldwide. The growing concern with water quality and increasing stringent environmental regulation has brought focus on water recycling, water treatment and minimization of water used in the mining and process industries. Conventional treatments to meet allowable concentrations of contaminants in water before discharge are being challenged due to economics and cost factors in technology and selection. Effective sustainable development must, therefore, ensure uncontaminated streams, rivers, lakes and oceans (Nkwonta and Ochieng 2013). Under current practice, water draining from process industries and base metal mines frequently contain organic, inorganic and heavy metals at high levels. When the contaminants in the effluents become higher than the set standards, disposal becomes a challenge.
As process water from mineral processing accumulates or when the water level overflows the depth of an open pit mine or an underground mine, the water is pumped out of the mine to ensure safety and stability or may be reused for process applications such as make-up water, dust suppression or mill operations, grinding, leaching, and steam generation depending on the water availability and quality. Nevertheless, it has been observed that more than 70% of all pollutants from the mining industry mostly contained in wastewaters are emitted into water bodies (Doll, 2012). With the fast development in industries, a huge quantity of wastewater is been produced and discharged into soils and water systems. The removal of these contaminants before discharge is receiving significant attention. There is, therefore, a growing necessity for finding versatile and low-cost treatment technologies to mitigate these contaminants. Currently, adsorption has emerged as a simple and effective technique for water and wastewater treatment even though its success largely depends on the development and improvement of materials for efficient adsorption. Activated carbon, clay minerals, zeolites, biomaterials and some industrial solid wastes materials have been used as adsorbents for adsorption of dissolved ions and organics in wastewater (Wang and Peng 2009).


In the past, membrane separation processes received much attention and are been applied in different industries especially in wastewater treatment (Mortazavi, 2008). These membrane processes which include: reverse osmosis, nanofiltration, ultrafiltration and microfiltration are effective (Dessouky and Ettouney, 2002) but considered to be very expensive.
1.2      Problem Definition

The contamination of water by dissolved ions is a significant worldwide problem (Nriagu and Pacyna, 1988) that warrants cost-effective methods for the removal of the undesirable species. This excessive pollution problem results from chemical substance and dissolved constituents from the ore added to the water at concentrations higher than established limits during processing. To make things worse, the regulated discharge limits of industrial wastewater effluents are getting more restrictive with time (Mortula and Shabani, 2012).
As inorganic components dissolve in water, one of the parameters that changes significantly is conductivity. Conductivity is a measure of the ability of water or an aqueous solution to carry an electric current. The current flow in water depends on the presence and concentration of ions in the water and therefore conductivity is often used as an indirect estimate for dissolved solids content of a solution (Coury, 1999). High total dissolved solids (TDS) discharged to rivers and streams can promote eutrophication, destroy sensitive ecosystems and endangers aquatic species (e.g., the cutthroat trout and cui-cui fish) in rivers and lakes (Mortensen et al., 2008). These water bodies can also be rendered unwholesome for both animals and plant usage, especially for people living in catchment areas, who do not only use these waters for drinking but also for other domestic purposes.
In recent years, most mining companies in the Western Region of Ghana have experienced relatively higher conductivity values of about 4000 µS/cm above Environmental Protection Agency (EPA) standard (≤ 1500µS/cm) (US EPA, 2011) for process waters. Since large volumes of wastewater are generated daily, high conductivity in the process water creates discharge problems. Existing wastewater treatment technologies such as oxidation, precipitation, alum coagulation/precipitation, reverse osmosis, nanofiltration, ion exchange, demand high capital investment, operation and maintenance cost (Lesmana et al., 2009).
The applicability of nano-materials and membrane separation processes for the reduction of dissolved ions in contaminated water has been demonstrated in the open literature (Mostafa and Hoinkis, 2012, Carlos et al., 2014, Rytwo, 2012). Reverse Osmosis (RO) and Nanofiltration (NF) are pressure-driven processes that are used to remove TDS and nutrients from water. Many researchers and full-scale applications have established RO as the most effective membrane process for removing dissolved solids (Mortazavi, 2008) and also for reducing the conductivity of water. RO is currently used by most mining companies in Ghana. However, it is considered to be very expensive and can render mining operations....


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