MOBILITY OF HEAVY METALS FROM THE UNIVERSITY OF NIGERIA SEWAGE- SLUDGE DISPOSAL SITE TO THE SURROUNDING SOILS AND PLANTS

ABSTRACT

This study was carried out at the University of Nigeria, Nsukka sewage disposable site. The study was to quantify the mineral contents of sewage sludge and assess their mobility in the soil and plants around UNN disposal site. The experimental layout was a 4×3 factorial in RCBD in which distance from the sewage disposal site and soil depth were the two factors under consideration. The distances from sewage pond were 0 (edge), 100, 300 and 2000 m away from the sewage pond with 2000 m serving as control while soil depths were 0 – 40, 40 – 80, and 80 – 20cm.  Soil and plant sampling was carried out from three distances in both sewage site and at the control site. Physico-chemical properties of the soil were determined. Four heavy metals (Cd, Cu, Hg and Pb) were determined from the soil, plant samples as well as the sewage sludge. Results showed that the effects of distance and soil depth were significant (P < 0.05) on pH, exchangeable bases, cation  exchange  capacity,  organic  matter,  total  nitrogen,  exchangeable acidity and  available phosphorous. At the control site there were no differences among soil properties except organic matter that decreased significantly (P < 0.05) with increase in depth. All the chemical properties determined decreased with increase in depth, except exchangeable acidity which increased with increase in depth. Cadmium, Cu, Hg, and Pb, significantly (P < 0.05) decreased as both distance and depth increased. There was an obvious high heavy metal content in the soil of disposal site when compared with the control soil. In the tissues of the three plants (Manihot spp, Zea mays, Pannicum maximum) studied, Cd, Cu, Hg, Pb concentrations were significantly different (P <

0.05)  at 0, 100, 300 and 2000 m distances.  Heavy metal content observed in plant tissues at the disposal site was relatively high when compared to that of the control site. Therefore, it follows that the studies at the University of Nigeria Nsukka sewage disposal site results in an increased concentration of soil property, such as organic matter, macro and micro nutrients. The mobility of heavy metals (Cd, Cu, Hg, and Pb) from soil to leaves suggest that all these metals were mobile from soil to plant components. Heavy metal concentrations varied among the tested crop plant, which reflects their differences in their uptake capabilities and their further translocation to the shoot portion of the plant. The bioconcentration factor at both root and leaf tissues of crop plant did not show any abnormal accumulation of heavy metals at sewage site. From the study the heavy metal contents of the sludge, soil and plant  species were  below toxicity levels when compared to WHO/FAO standards.

CHAPTER ONE

1.0  INTRODUCTION

Sewage sludge is a solid, semisolid, or liquid muddy looking residue that results after human and other waste from households, industries and run-off from road is treated at  a sewage plant. Quantitative and qualitative composition of the sewage sludge is very complicated. It is rich in organic  matter,  nitrogen, phosphorus, calcium,  magnesium, sulphur  and  other  microelements necessary for the survival of plants and soil fauna. Of the microelement commonly found in sewage sludge heavy metals such as, cadmium, lead, mercury and arsenic are toxic to humans whereas zinc, copper, and nickel are harmful to plants (Jane-Hope, 1986). There release to soil liquid phase may cause leaching through the soil profile to the ground water table, and also facilitate plant uptake of heavy metals thus posing a risk to environment.

Heavy metals have a density greater than 5.0 g cm-3 (Seaward and Richardson, 1990) and metal transfer  from sewage  sludge  to  soil and  subsequently to  groundwater and  plants  represents potential health and environmental risks (McBride et al., 1997; Bhogal et al., 2003). The greater the mobility the higher the toxicity risk of these metals. Heavy metal mobility mainly depends on soil properties (Rowell, 1994).

One important process affecting heavy metal mobility in soil is sorption. The importance of pH in metal solubility is well-known as it influences heavy metal adsorption, retention and movement (Sauve et al., 1997).Organic matter is another important soil component that influences metal mobility. It has a nutritional function by serving as a source of N, P and S, and has a high binding capacity for cations and organic contaminants (Oste et al., 2002). It has been shown that heavy metals are accumulated in surface organic layers in agricultural and urban soils. It therefore appears that soil properties such as sorptivity, organic matter and soil pH may have a major effect on metal mobility. The addition of organic matter may also affect metal mobility, by increasing soluble organic matter and Cu mobility may be enhanced, especially in sandy soils of high pH (Udom et al., 2000, McBride et al., 1999).

There are several indices of heavy metal mobility in soils, the retardation factor, Rd, being a straight forward, unitless one (Kookana et al., 1994). The meaning of Rd is that it is the ratio of the rate of movement of the heavy metal to the rate of movement of water in which the metal is dissolved.

The  fate  of  heavy  metals  in  sewage  sludge  is  mainly  related  to  their  mobility  in  soil.  A traditionally held view on the mobility of heavy metals in sewage sludge-amended soils is that the metals’ migration is insignificant and that the metals tend to remain in the site of input, i.e. in the topsoil (Chang et al., 1984; Schirado et al., 1986). Heavy metals are dangerous because they are non-biodegradable,  bioavailable  and  toxic  to  different  crops  (Mahler,  2003;  Garcia  and Dorronsoro, 2005).

Distribution of heavy metals in plant body depends upon availability and concentration of the heavy metal as well as upon the particular plant species and its population ( Punz and Seighardt, 1973). Plants have a natural ability to extract elements from the soil and to distribute them between roots and shoot depending on the biological processes in which the element is involved (Ximénez-Embun et al., 2002). In addition to the uptake of nutrients, toxic compounds such as heavy metal can also be taken up by the plants. The plants store high levels of toxic heavy metals within their roots, shoots, and leaves. The growth and metabolism of many plant species were reported to be affected adversely by excess supply of heavy metals (Foy et al, 1978, Wong and Lau 1985).

Potentially toxic metal concentrations are not the same in all portions of the plant. A survey of a number of food crops revealed that the tuber, seed, or fruit tissue had a lower amount of Cd than plant leaf tissue. Comparisons between crop species show wide variations in their ability to absorb potentially toxic trace elements from the sludge-soil system. Cereals and legumes accumulated less Cd in shoots than leafy vegetables, like curly cress (LepidiumsativamL.), lettuce (Lactuca sativa L.), and spinach (Spinaciaoleracea L.) (Bingham et al., 1975).  An earlier study in the site of present study showed that the build-up of heavy metals to critical level could be phyto-toxic and might result in reduced plant growth (Udom, 2000, and Asadu et al, 2008). There is a concern that heavy metals in the composted product may transfer from soil and accumulate in edible plants. Some of these heavy metals can be detrimental to human, plant or animal life if they are present above certain limits.

At the University of Nigeria Nsukka (UNN) the sewage sludge discharged from the administrative blocks, hostels and surrounding staff quarters are continuously disposed at the sewage sludge site. Heavy metal content of the sewage sludge is likely to be high due to the usage of cosmetics, detergents, fats, oils, chemicals and cigarette ends flushed down the toilets. There are vegetation and farmlands with different kinds of crops surrounding the sewage disposal site. Heavy metals in sewage sludge and the toxicological implications are of concern in agricultural production due to the adverse effects on food quality (safety and marketability) and crop growth (phytotoxicity). Metals such as lead, mercury, cadmium and copper are cumulative poisons. These metals cause environmental hazards and are exceptionally toxic. Crops and vegetations around take up metals by absorbing them from contaminated soils, animal graze upon this vegetation and this heavy metal enter into the food chain. This exposes the consumers of these crops and animals to bioaccumulation of heavy metals with time. It has been reported that nearly half of the mean ingestion of lead, cadmium and mercury through food is due to plant origin (Mohsen and Mohsen 2008).

Therefore, the major objective of this work was to assess the mobility of some heavy metals from sewage sludge in soils and plants around UNN disposal site. The specific objectives were to:

1           determine the chemical composition of sewage sludge;

2           determine the chemical compositions of the soils around the pond;

3           quantify the accumulation of the heavy metals laterally away from the pond to the soil around; and

4           determine the accumulation of heavy minerals in the roots, and leafs systems of selected crops/plants so as to compare with safety standards.

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