Toxicity of heavy metals in soils and crops and its phyto-remediation

Abstract

Soils, water and crops contaminated by heavy metals and other environmental toxins from different industrial wastes and wastewaters in and around Dhaka are producing unhealthy food through entering into the food chain, which is consumed by human beings. The present study has been undertaken to evaluate the amount and extent of pollution of the wastewaters, soils and crops and their phyto-remediation potentials. So, the study was conducted in two stages: field survey and green house study. In field study, ten industrial clusters at different locations in and around Dhaka were selected and their wastewaters, soils and plant samples were collected and analysed for the level of contamination. Parameters considered to study for wastewaters- pH, EC, Conductivity, TDS, Turbidity, DO, NO2- -N, NO3- -N, NH4+ -N, SO42-, total alkalinity, PO43-, Cl- , Na, K, Ca, Mg, Zn, Cu, Ni, Pb and Cd; for soils- pH, EC, OC, TN, TP, Na, K, Ca, Mg, Zn, Cu, Ni, Pb and Cd; for crops- N, P, K, Na, Ca, Mg, Zn, Cu, Ni, Pb and Cd. In green house study, for the phyto-remediation of heavy metals, pot experiments were conducted to evaluate the toxicity levels of Pb and Cd in soils and crops by using Brridhan- 28 rice as a test crop and to determine the tolerance potentials of five common high yielding rice varieties such as BR- 15, BR- 16, BR- 19, Brridhan- 29 and Brridhan- 45 by applying 200 mg/kg Pb and 7 mg/kg Cd separately, as well as to assess the phyto-extraction and tolerance potentials of Lal sak, Kalmi sak, Grass and Spinach by applying 200 mg/kg Pb and 15 mg/kg Cd alone. After analysing the wastewater samples of different industries in and around Dhaka, some polluting industries were identified. Among them the worse industries were tanneries at Hazaribagh. Because, EC, conductivity, TDS, NO2- -N, NO3- -N, NH4+-N, SO42-, PO43-, Cl-, Na, K, Ca, Mg and Cd concentration in the wastewaters at Hazaribagh tannery area was found very high. On the other hand, DO of the wastewaters at tannery industries were found the lowest, with the highest turbidity. But, pH of the wastewaters near I I Tubes Mills Ltd. at Demra- 3 was found extremely acidic with substantial amounts of NO3- -N, NH4+ -N and Zn. And lead-storage battery industries at Keranigonj- 3 contributed extremely acidic pH value, huge amounts of sulphates and toxic concentrations of Zn, Cu, Ni and Pb to the wastewaters. pH and total alkalinity in the wastewaters at Gazipur- 5 near Haq battery was found high. Pb, Ni and K concentration was also found above the tolerable limit. On the other hand, Ca, Mg, Zn, Cu and Cd concentrations near textile dyeing, spinning and tannery industries were found high as well. pH of the soils at Keranigonj- 3 near lead-storage battery industry and Demra- 3 near I I tube Mills Ltd. was found extremely acidic and near dyeing industries at Matuail- 2 and Shampur- 2, it was found alkaline. Organic carbon, Total N and Total P contents at Hazaribagh and Tejgaon areas were found higher as compared to other industrial sites. Na and K concentrations in soils at Hazaribagh tannery areas was found high, while at Gazipur- 5 near Haq battery, K concentration was found substantially high. Ca and Mg concentrations near textile dyeing industries and metal industries were found substantially high as well. Likewise, Zn, Cu, Ni and Pb concentration in the soils near electroplating, galvanizing, storage and dry cell batteries, metal processing products, steel and rerolling, tannery and textile dyeing industries were found high. An exceptionally high Pb concentration was found at Keranigonj- 3 near lead-storage battery industry, Cu was found at Demra- 4 near Alaksa Steel Mills Ltd., Ni near Metrocem Ispat Ltd. at Gazipur- 4 and Zn at Demra- 3 near I I Tube Mills Ltd. In contrast, textile dyeing and tannery industries contributed high Cd at different locations, and remarkably high concentration of Cd was found at Hotapara near dyeing and at Hazaribagh near tannery industries. In most crops, Zn and Cu concentrations were found above the phyto-toxic limit, while Ni, Pb and Cd contents were found below the phyto-toxic limit but above the tolerable range for human consumption. In green house study, the effect of Pb on the growth and yield of Brridhan- 28 rice had been initiated significantly at T2 treatment, where 125 mg/kg Pb was applied over control. And, the significant effect of Pb on the nutrient contents such as N, P, K, Ca and Mg in Brridhan- 28 rice was commenced at T1 and T2 treatments, where 100 mg/kg and 125 mg/kg Pb was applied as compared with control. On the other hand, the growth, yield and mineral nutrition of Brridhan- 28 rice had been significantly affected at 3 mg/kg Cd treatments and upwards over control and at 60 mg/kg and 90 mg/kg Cd treatments, Brridhan- 28 rice crops could not grow at all. A significant increment of Pb and Cd concentrations was found in grains, straw and roots of Brridhan- 28 with increasing metal treatments and simultaneously their growth and yields had been hampered. In phyto-remediation studies, among the five rice varieties BR- 16, BR- 19, Brridhan- 29 and Brridhan- 45 were found as 200 mg/kg Pb tolerant and BR- 15, BR- 19, Brridhan- 29 and Brridhan- 45 were identified as 7 mg/kg Cd stress tolerant as they showed better growth performance with minor quantities of Pb and Cd in grains. On the other hand, metal concentrations in the straw were found within the toxic range, while roots of those rice varieties sequestered maximum amounts of Pb and Cd and acted as metal sequester. In contrast, Brridhan- 15 was observed as Pb and Brridhan- 16 was found as Cd susceptible varieties as their growth and yield had been significantly affected and metal transportation in straw and grains from roots were increased. Among the leafy vegetables and grass, the growth and yield of spinach had been significantly affected by 200 mg/kg Pb and 15 mg/kg Cd application. The growth of Lal sak had been drastically affected by 15 mg/kg Cd as well, but remained unaffected by 200 mg/kg Pb treatment over control. Conversely, the growth of Kalmi sak and Grass had not been affected significantly by 200 mg/kg Pb and 15 mg/kg Cd treatments over control. But, Lal sak, Kalmi sak and Spinach accrued high Pb and Cd in their straw and roots as hyper accumulating plants, whereas grass accumulated the lowest quantities of Pb and Cd as metal excluder and/or metal phyto-stabilizer. Metal concentration in different parts of crops exhibited the following descending order: Roots> Straw> Grains. The transfer co-efficients of metals varied in a wide range among different crops and cultivars.