Kinetic Modelling of the Adsorption Treatment of Waste Lubricating Oil using Activated Ukpor Clay (Bentonite)
Abstract
The re-refining of waste lubricating oil using activated Ukpor clay as an adsorbent was investigated. Clay activation was performed using sulphuric acid (H2SO4) with equivalent concentrations varying between 0.5 – 2 mol/L at a constant temperature of 90oC for 3.5 hours. The adsorption process was studied over a fixed amount of adsorbent at ambient conditions. Adsorbent performance was determined by its effectiveness in enhancing the properties of the waste oil and in removing trace metal ions contained in them. Furthermore, the effect of contact time and the adsorption performance of the adsorbent in the recycling treatment of the lubricating oil were evaluated. The kinetic data were modelled using the Pseudofirst order, the Pseudo-second order, the intra particle diffusion and the Elovich kinetic models. Analysis carried out on the lubricating oil showed the presence of many heavy metal ions and trace elements. Ca, Pb and Zn had the highest concentrations of 804, 398 and 222 mg/L respectively, while Ba had the least concentration of 0.1 mg/L. However, with treatment using this adsorbent, the concentrations of Ca, Pb and Zn were found to considerably reduce to 3.6, 5.6 and 0.01 mg/L respectively. The concentration of Ba was equally found to reduce to 0.01 mg/L, wherein the clay sample treated with 2 mol/L H2SO4 gave the best result in terms of improvement in the properties of the lubricating oil as well as on the percentage removal of these heavy metals and trace elements. For best describing the process within the adsorbent and with a high correlation coefficient (R2 value > 0.999), kinetic data were best fitted by the Pseudo-second order kinetic model and this result indicates that this adsorbent provides a good medium for the re-refining of waste lubricating oil.
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Introduction
One of the greatest challenges of environmental management is improper management of wastes such as waste lubricating oil. These forms of wastes are usually classified as hazardous because of the high content of heavy metals inherent in them. According to [1], lubricating oil, being a petroleum based product, originally contains little or no heavy metals. However, as a result of their activities through the engines, they pick up various heavy metals and trace elements such as Manganese, Cadmium, Arsenic, Copper, Lead, etc. High value content of ash, water, asphaltenic materials, and other dirty materials have also been reported to build up during the course of lubrication inside the engine [2].
The non-biodegradable nature of waste lubricating oil as well as its carcinogenic nature [3] makes its disposal a significant challenge. In developing countries such as Nigeria, it has been reported that over 20 million gallons of waste lubricating oil, which are generated annually, are not properly disposed [4]. Improper disposal of these waste lubricating oils therefore enriches the soil with some unwanted heavy metals, which could subsequently drain into groundwater and surface water ways. Heavy metal ions like Lead (Pb) and Cadmium (Cd) are pollutants associated with human health effects such as high blood levels in Children, anthralgia, osteomalacia, colic, constipation etc [5, 6] while Chromium (Cr), Iron (Fe) and Phosphorous (P) could have a significant negative correlation on freshwater plants such as microcrustacean species eg. Diaptomus leptopus and Daphnia Schodlert [7, 8]. Waste lubricating oil contamination of soil leads to a gradual build up of trace metals which when absorbed by plants could be potentially harmful, hence constituting health risk if consumed as food by man and any other animal [9]. Because of the threats they pose, maximum allowable contamination levels exist for these heavy metals/ trace elements by several regulating agencies such as World Health Organization (WHO), United States Environmental Protection Agency (USEPA) among others [10]. Indiscriminate disposal of waste lubricating oil would not only violate these standards, but equally give rise to attendant consequences both to man and the environment. Waste oil could have more debilitating effects on the environment than they do to automotive engines, in that they could poison plants and animals, destroy soil nutrients and equally reduce aeration of the soil.
Conclusion
The recycling of waste lubricating oil using activated Ukpor clay has been successfully executed. This study employed four different concentrations of H2SO4 ranging between 0.5 mol/L and 2mol/L. The effectiveness of this clay type as an adsorbent was determined by their ability to effectively remove the impurities in the waste oil, as well as to enhance properties such as: density, viscosity, flow point, flash point, and reduce moisture content, sulphur content, TAN and TBN; while equally remove the trace metal ions. The results from this study indicate that activation of the clay samples improved their physicochemical properties as adsorbent materials.
A general improvement in the properties of the recycled oil was observed on treatment with the activated clay. The best adsorption performance on the lubricating oil was obtained with the use of UK4 sample adsorbent, in that its use greatly enhanced the properties of the oil and also reduce the trace metal content in them. All the heavy metal ions found in the waste lubricating oil were significantly reduced, with Ca (804 mg/L), Pb (398 mg/L) Zn (222 mg/L) being the highest values of these metal ions obtained in the waste oil, reduced to 3.6 mg/L, 5.6 mg/L and 0.01 mg/L respectively. Pre-treatment condition that ensured the availability of larger surface area and more adsorptive sites is the possible reason for the high adsorptive capacity of the UK4 clay sample. The adsorption of impurities from the waste lubricating oil increased with increasing contact time with the adsorbent. Also, the decline in the adsorption performance of the UK4 sample after 30- minutes implies that this is the optimum contact time for all further studies involving the use of this adsorbent in the recycling of waste lubricating oil, bound by experimental set-up, conditions and samples used in this study. Kinetics for the adsorption treatment of waste lubricating oil using activated Ukpor clay were obtained and fitted to different kinetic models namely: Pseudo-first order, Pseudo-second order, intra particle diffusion and the Elovich kinetic models. Of all these models, kinetic data were best fitted by the Pseudo-second order model, being able to predict the adsorption equilibrium per time in as many adsorptive sites as possible. This also gave the best linear fit having an R2 value > 0.999. This result indicates that Ukpor clay (bentonite) is a good and effective adsorbent for the recycling (re-refining) of waste lubricating oil.