Assessment of Pulverised Fly Ash (PFA) as an ameliorant of lead contaminated soils.

Author:Gatima, Edmond
 
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Abstract: Fly Ash (FA) is obtained by electrostatic or mechanical precipitation of dust-like particles from the flue gases of furnace fired with coal or lignite at 1100 to 1400[degrees]C. About 95-99% of Fly Ash consists of oxides of Si, Al, Fe and Ca, about 0.5 to 3.5% consists of Na, P, K and S and the remainder is composed of trace elements. PFA has also been used as an adsorbing material when applied in treatment effluents. The use of Fly Ash as a chemical conditioner has previously been investigated with results indicating that Fly Ash does facilitate the filtering process since it decreases both specific resistance and capillary suction time. Therefore, the aim of this paper was to assess the potential of PFA as an ameliorant for soil artificially spiked with various Lead compounds (PbS[O.sub.4], PbC[O.sub.3], PbN[O.sub.3] and PbS). Additions of quicklime and Fly Ash to the contaminated soils effectively reduced heavy metal leachability well below the regulatory limits for hazardous wastes. The results showed the effect of PFA on leaching of lead was significant for all the samples. A high interaction value depicting sampling effect over the use of the PFA as an ameliorant was observed. The order of the difference between samples treated with PFA and without PFA was PbN[O.sub.3] > PbS[O.sub.4] > PbC[O.sub.3] > PbS (17 mg [L.sup.-1]) when compared to that of the control. The results also demonstrated that, effect of filtration and PFA as an ameliorant had a significant effect in reducing toxicity. However, it is important to consider the source of PFA, as certain sources could in essence impart certain toxic elements, defeating the primary purpose of amelioration.

Key words: Pulverised fly ash, toxicity, ameliorant, lead contamination

INTRODUCTION

Coal Combustion Products (CCP) are produced in coal-fired power stations, which burn either hard or brown coal. Due to the mineral component of coal and combustion technique, fly ash (FA), bottom ash (BA), boiler slag (BS) and fluidized bed combustion (FBC) ash as combustion products, as well as the products from dry or wet flue gas desulphurization, especially semi dry absorption (SDA), products and flue gas desulphurization (FGD) gypsum are produced.

The volume of CCP generated worldwide is reported to be on the increase due to the reliance on coal as a major source of energy [1]. For example, approximately 55% of U.S. energy is generated from coal and it is expected that coal will continue to be the leading electrical power source at least until 2010 [2]. About 105 million tons of coal combustion products were produced by American power generating utilities in 1997 [3]. In China, coal mining waste management is a major problem because of the huge quantities of coal produced for energy production, 75% of which comes from coal [4].

Fly Ash (FA) is obtained by electrostatic or mechanical precipitation of dust-like particles from the flue gases of furnace fired with coal or lignite at 1100 to 1400[degrees]C. By mid 1950's, fly ash from coal combustion became known as Pulverised Fuel Ash (PFA) within the UK. This was to differentiate it from Fly Ashes delivered from other processes, since PFA is delivered from firing boilers with pulverised coal [5]. PFA is still produced in large quantities during the day-to-day operations of coal-fired power plants.

Economically viable ways of using CCP rather than having to dispose of, it have to be investigated. There is already a vast body of information on utilisation of Fly ash (FA) in building/construction, production of aggregates and more recently for agriculture[1]. Within the UK, PFA has been used for over 50 years for a wide range of applications [5]. Knowledge of the physical and chemical properties of Fly Ashe is essential for understanding and in the future, predicting the behaviour of PFA in soil. The physical structure of Fly Ash often consists of "hollow spheres" and these particles show an increased surface area, capillary action and nutrient-holding capacity compared with sands [6]. Fly Ash also has been reported to improve the nutritional status of soils via increases in cation exchange capacity (CEC) and by provision of some essential nutrients [7-9]. However, since almost all naturally existing elements are present in PFA [10,11], the potential release of trace elements may also be an issue in determining the suitability of some sources for use as a soil amendment [10,12,13].

Chemically, the composition of Fly Ash varies depending on the quality of coal used and the operating conditions of the Thermal Power Stations. About 95-99% of Fly Ash consists of oxides of Si, Al, Fe and Ca, about 0.5 to 3.5% consists of Na, P, K and S and the remainder is composed of trace elements [14]. In fact, Fly Ash consists of practically all the elements present in soil except organic carbon and nitrogen (Table 1). Thus, it was found that this material could be used as an additive/amendment material in agriculture applications [14]. The chemistry of PFA reflects the mineral origins of the coals when formed millions of years ago. The combustion process concentrates these minerals. However, most elements are held in the glassy particles that are formed in the furnace [5]. While the trace elements composition may indicate potential for environmental effects, the available leachable elements are minimal. With proper design, unbound PFA can be used as a fill material posing only negligible risk, even to sensitive aquifers [15].

PFA has also been used as an adsorbing material when applied as an ameliorant in treatment effluents [16]. The idea of using coal fly ash to synthesize artificial zeolite for water treatment is based on the fact that both materials have a similar chemical composition, namely a high content of aluminisilicate glass and high surface areas [17]. Another example for using PFA, is to remove metal ions [18] and phenols from water [19]. In addition PFA has always been used for sludge conditioning for sludge originating from wastewater treatment containing high amounts of water (up to wt. 95%) and that needed to be dewatered in order to reduce its volume and disposal costs [16]. The use of Fly Ash as a chemical conditioner has been investigated [20] with results indicating that Fly Ash does facilitate the filtering process since it decreases both specific resistance and capillary suction time. Fly Ash has been found to have great potential for agriculture. For example, some of the advantages relate to modification of soil texture and bulk density, improvement of water holding capacity of soil, optimisation of soil pH, increase crop yield, as a micronutrient supplement to soil, creation of conducive conditions for better plant growth and reduction of soil crusting [21]. The addition of appropriate quantities of Fly Ash can alter the soil texture. Fly Ash addition at 70 t [ha.sup.-1] for example, was reported to alter the texture of sandy and clayey soil to loamy [22].

However, despite positive uses of PFA, the rate of its production clearly far outweighs utility as a by product. This is because the remaining PFA material and its disposal practices involve holding ponds, lagoons, landfills and slag heaps, all of which can be regarded as unsightly, environmentally undesirable and/or a non-productive use of land resources, as well as posing an on-going financial burden through their long-term maintenance [23]. Although as mentioned earlier, Fly Ash has immense potential in agriculture, there are some possible concerns, which need to be attended to or kept under control, such as the release of toxic elements into ground water, decreased germination rates of some crops due to high levels of Fly Ash application including uptake of heavy metals toxic elements by the plants [16]. The uptake of heavy metals, toxic elements by plants was demonstrated when Fly Ash was applied to the soil [24]. These elements were found to be absorbed by plants grown on such soils (treated with Fly Ash) and could enter into the food chain [16]. However, the data on trace element uptake and accumulation by plants are limiting. Boron in Fly Ash is reportedly readily available to plants and investigators consider it to be a limiting factor in utilisation of unweathered Fly Ash. Regional Research (RRL), Bhopal conducted a study regarding the uptake of heavy and trace metals from Fly Ash by some vegetable crops and it was observed that the uptake was quite low and remained within the normal range.

Some studies have shown possible negative effects of Fly Ash application [14]. For example, Fly Ashes contain...

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