**1. Introduction**

Potentially toxic elements (PTEs) pollution is one of the most important environmental problems in many developed and developing countries [1–8]. PTEs are naturally occurring elements throughout the earth's crust and they exert key

biochemical and physiological functions in plants and animals. At low concentrations as trace elements, they are important constituents of several enzymatic and biological functions and play important roles in various oxidation-reduction reactions in the human body.

At higher concentrations, however, PTEs become environmental contaminants, causing many ailments in humans through exposure to anthropogenic activities such as agriculture, mining and smelting operations [9–12]. Significant contribution to PTEs contamination may also come from natural phenomena i.e. metal corrosion, atmospheric deposition, soil erosion of metal ions and leaching of potentially toxic elements. Other natural phenomena include sediment re-suspension and metal evaporation from water resources to soil and groundwater volcanic eruptions and weathering [4, 13–15]. Industrial sources of PTEs include: metal processing in refineries, coal burning in power plants, petroleum combustion, nuclear power stations and high tension lines, plastics, textiles, microelectronics, wood preservation, leather tanning and paper manufacture plants [4, 14, 16–19].

Metal occurrence in the environment can increase significantly from industrial effluent leachates and accumulate near the surface layer of soil and at water column. Metal chemistry, biotic species or specific differences in water resource can affect patterns of metal exposure, uptake and metabolism. Analyzing sediments best assesses metal pollution in river, as they are less susceptible to flow conditions than water column and act as sink for metal pollutants. The metals attach to any of these phases (water, soil and sediments) in proportions that depend on the physicochemical conditions such as conductance, pH and temperature prevailing in these phases.

The pulp and paper industry, in particular, has historically been a major contributor of pollutants into the environment, including PTEs [18, 20, 21]. There are five steps in the production of pulp and paper, namely, 1) preparation of raw materials, 2) pulping, 3) bleaching, 4) pulp handling and/or paper production, and 5) recovery of chemicals through the combustion of spent or black liquor [22, 23]. The paper manufacturing process consumes large volumes of fresh water, most of which becomes contaminated with recalcitrant compounds before effluent discharge.

The composition of each mill's effluent depends on the pulping process used; the grade of paper manufactured; the volume of fresh water consumed; the wastewater cleaning technique; and wastewater reuse programs. Paper mills effluents are complex mixtures with more than 200–300 different organic chemicals [24, 25] and approximately 400–500 inorganic compounds [26, 27]. Raw effluent from paper mills has characteristics of high BOD, COD, high levels of chlorinated ligno-sulphonic acids, total dissolved and suspended solids, chlorinated resin acids, chlorinated phenols and hydrocarbon. In addition, the effluent may contain PTEs such as Fe, Zn, Cu, Cd, Mn, Ni, Cr and Pb, that can be present in high concentrations well above permissible limits [20].

### **1.1 Source of PTEs in a Kraft pulp mill**

The main chemical ingredients in a Kraft pulp mill are sodium hydroxide (NaOH) and sodium sulphide (Na2S). The primary PTEs found in wood and wood products are Ca, Mg, K, Mn, Ba, Al, Ni, Cu, Zn, Cd, Pb. In addition, PTEs are generated through other make-up chemicals, such as sodium sulfate (Na2SO4); bleaching agents; water; the chemical recovery system where the cooking liquor is regenerated; Green Liquor Dregs (GLD); Lime Mud (LM); Slaker Grits (SG); Boiler Fly Ash (BFA) and waste

*Distribution of Potentially Toxic Elements in Water, Sediment and Soils in the Riparian… DOI: http://dx.doi.org/10.5772/intechopen.102440*


#### **Table 1.**

*Content of potential toxic element [23].*

paper recycling [28–30]. **Table 1** shows the chemicals composition of GLD, LM, SG, BFA determined in a few mills throughout the pulp and paper industry. These elements leave the mill system through wastewater, fugitive uncontrolled air emissions or flue gases from the recovery boiler [38, 39]. Apart from the fugitive emissions, the mill effluents (liquid and gaseous) undergo treatment in the wastewater purification plant or the air pollution control system depending on the local effluent discharge standards before final disposal into the environment.

There is increasing awareness of potentially toxic elements pollution from industrial discharges, especially their potential effects in a rural setting such as Western Kenya. The area houses several agro-industrial factories; including a Kraft pulp and paper mill, several sugar milling plants, tea factories and fish processing plants. Because of the various amounts of chemicals used in the manufacturing processes, the roles of these industrial discharges could be significant in metal concentration, fate and transport in the adjacent water bodies and soils. An extensive assessment of PTEs contamination in such a setting is therefore required. The objective of this study was to estimate PTEs (Pb, Cd, Zn and Cu) distribution in water, sediments and soils collected near a Kraft pulp and paper mill in Western Kenya.
