Effect of wastewater and sewage sludge on transfer heavy metals to depth in soil column under Mint coltivation

Due to its especial geographic condition, Iran, as a semi-arid country, suffers from water shortage for domestic, industrial and agricultural usages. As limited water supplies, the use of non-conventional water resources, such as industrial or municipal wastewater, is a common practice in many parts of the world including Iran. It is estimated that at least 20 million hectares in 50 countries are irrigated with raw or partially treated wastewater. Also, it is estimated that the potential volume of wastewater in urban and rural areas of Iran will be 4369 and 823 million m3, respectively. On the other hand, the use of sewage sludge in agricultural soils has been widespread in many countries around the world, also in Iran. The use of sewage sludge in agricultural has been shown to improve the soil’s characteristics such as organic matter, nutrients, porosity, aggregate stability, bulk density and plant productivity. Despite the growing interest in wastewater and sewage sludge usage, excessive use of them may have some harmful effects such as human health problems, runoff and leaching of nutrients to surface and groundwater, undesirable chemical constituents, pathogenesis, accumulations of heavy metals in plants and soils, negative environmental and health impacts. So, wastewater and sewage sludge use should be under controlled conditions to minimize the health risks of pollution to agricultural products, soil. Heavy metals are extremely persistent in the environment; as they are non-degradable and thus, readily accumulated at toxic levels. Metals can also accumulate in the soil at toxic levels due to long term use of wastewater. There is potential for inorganic nutrients and heavy metals present in wastewater and sewage sludge that transfer in the soil column and pollutant surface and groundwater. Today, medicinal plants are cultivated commercially in the polluted environments where soil, water and air contain rather high levels of pollutants. These plants appear to be a good choice for phytoremediation since these species are mainly grown for secondary products (essential oil) thus, the contamination of the food chain with heavy metals is eliminated. Aromatic and medicinal plants also have a demonstrated ability to accumulate heavy metals. The Mint (Mentha spicata) is a medicinal plant which has received considerable economic importance due to the large demand for its essential oil in food, pharmaceutical, cosmetic and hygiene industries. The purpose of this study was to determine the leach of heavy metals (Zn, Pb, Niand Cd) from the wastewater and sewage sludge to the soil column under Mint cultivation. This study was conducted at the Bu-Ali Sina University greenhouse located in Hamedan, western Iran. The experiment was run in May 2015- February 2016), including 5 months for soil preparation (in lysimeters) and 5 months for crop cultivation and harvesting. To evaluate the use of wastewater and its sewage sludge on on transferring heavy metals in soil column under Mint cultivation, a factorial experiment based on completely randomized was designed with three replications in 27 lysimeters. The factors included three types of watering: tap water (W1), raw wastewater (W2) and treated wastewater (W3), and three sewage sludge levels: 0 (S1), 50 (S2) and 100 tha-1 (S3). Therefore, 9 treatments (W1S1 to W3S3) were considered for investigation. 27 volumetric lysimeters were applied as cultivation beds (26 × 30 × 30 cm). The soil had two layers: the upper layer (0 to 50 cm) with sandy loam texture, and the bottom layer (50 to 110 cm) with sandy clay loam texture. After beds preparation, Mint (Mentha spicata) was planted in them. The raw and treated wastewaters as well as sewage sludge were prepared from wastewater treatment plant of Hamedan. Overall, 13 irrigation programs with 10-day intervals were applied. Zn, Pb, Ni and Cd concentrations in leachate were measured after each watering. The results showed the effect of watering and sewage sludge on heavy metals concentration of drainage water was statistically significant. Also, the result showed that wastewater and sewage sludge increase all (except Cd) leached heavy metals comparing to the control. In all treatments, concentrated Zn, Pb, Ni and Cd in the soil profile ranged from 0 to 1.66, 0 to 0.05, 0 to 0.05 and 0.0 to 0.0 mg/l, respectively. The concentrations of all heavy metals in drained water, was very lower than the FAO standard. By performing continuous irrigations, the trends of heavy metals (zinc, lead and nickel) were irregular schema (especially the use of wastewater and sewage) at different times. The first five irrigations showed an upward and the others showed a downward path.