Fate of Heavy Metals in Selective Vegetable Plants Irrigated with Primary Treated Sewage Water at Slightly Alkaline Medium

IT is well known that pH is the controlling factor for solubility of metals, which is affected by the "law of chemical solubility product constant" (ChSP). The present investigation focuses on the fate of heavy metals resulted from the use of primary treated sewage water for irrigating edible plant in a local sewage farm. The studied plants are maize, green beans, and alfalfa. Extensive sampling program was designed to collect both irrigated water and plant samples every two weeks. The plant samples were partitioned into roots, stems, and leaves, as well as the grains of maize and been plants. The physical and chemical characteristics of irrigated water were determined. Level of heavy metals in sewage water and all plant samples was determined. It was found that the availability of metals to plant is, generally, governed by the law of ChSP constant. In present study, the pH of both irrigated water and the soil were within the slight alkaline medium which restricted the solubility of metals. Thus, negative impact on the accumulation of metals by the irrigated plants was detected. The level of metals in all the studied plant parts was within the permissible limits according to the WHO, FAO and the Egyptian regulations. This is mainly attributed to the limited solubility of metals at the slight alkaline medium according to the law ChSP constant. It was thus concluded that the availability of metals from both irrigated waters and soils were greatly restricted and controlled by the pH value. This explains the limited accumulation of metals by all studied plants. However, log-term irrigation with sewage will certainly result in a dramatic accumulation of metals by both soil and plants. It was recommended to restrict the reuse of disinfected tertiary treated water for irrigating vegetable plants according to the local regulations.