Accumulation of few heavy metals in sewage sludges, soils and plants of Coimbatore, Tamil Nadu (India).

A study was carried out in Coimbatore district of Tamil Nadu (India) to assess the distribution pattern of heavy metals in the soils and plants irrigated with sewage effluent/sludge. About 69 soil samples (surface and subsurface), 65 plant samples as well as 34-sewage sludge samples were collected from various tehsils of Coimbatore. Six tehsils in Coimbatore have been identified and categorized into two groups--Class I City (densely populated tehsils) and Class II city (thinly populated tehsils). The available micronutrients like Fe, Mn, Zn and Cu; heavy metals: Cr, Cd, Ni, and Pb were within the safe limits. However, the total Cr and Cd concentrations were relatively higher in the sludge samples collected from Coimbatore and Tiruppur tehsils compared to other tehsils, while for Ni, the sequence was in the order Coimbatore > Tiruppur > Palladam > Pollachi > Avinashi > Mettupalayam and for Pb, Coimbatore > Mettupalayam > Palladam > Tiruppur > Avinashi > Pollachi. Soil analysis results indicated that heavy metal concentration recorded higher level in soils of Class I city (densely populated tehsils) compared to Class II city (thinly populated tehsils). The plant samples analyzed had also registered higher concentration of total Cd, Ni and Pb, which were classified under toxic, excessive and below excessive level, respectively. Correlation analysis revealed that iron (Fe), zinc (Zn), copper (Cu), cadmium (Cd) and lead (Pb) were significantly negatively correlated with pH of soil. EC had a significant positive correlation with available iron (Fe), zinc (Zn), cadmium (Cd) and lead (Pb). A significant positive correlation of Fe, Mn, Zn, Cu, Cd and Pb was also registered with OC. Among the plant samples collected, it was evident that heavy metal concentrations were recorded higher in grass spp followed by Amaranthus spp. It was inferred from the study that soils samples had higher levels of heavy metals even though the values recorded were below the critical value/toxic limit. However, long-term and indiscriminate application of untreated (raw) sewage sludge and/or letting of sewage effluent directly to agricultural field without prior treatment may result in accumulation of toxic metals in surface and subsurface soils and subsequent biotransfer (bioaccumlation) into the food chain, it may further lead to toxicity not only to plants and animals but also to consumers of the harvested crops.

Dynamics of lead (Pb) in different soil conditions.

Four soil samples were collected from various locations having wider range of pH and EC for incubation experiment. These soil samples were added with Pb (NO3)2 salts, and samples were taken on 1st, 35th, 45thand 60th day. Soils samples were fractioned by sequential extraction to estimate the concentration of lead in different fractions viz, water soluble, exchangeable +adsorbed, organic, carbonate and residual fraction. The result reveals that concentration of organic and carbonate bound-Pb was high in waterlogged condition and soil pH has been comprehensively identified as the single most important soil factor controlling the availability of lead (Pb) in soil. Low content of Pb in exchangeable + adsorbed (KNO3) and water soluble (H2O) fraction in all soils (except in S1) could signify low availability of Pb to plants. Bioavailable fractions, viz. water soluble and exchangeable + adsorbed, were low in all soils (except S1) well below critical limits, which may not pose any toxicity in the food chain.

Use of low-cost biological wastes and vermiculite for removal of chromium from tannery effluent.

Biological wastes (sawdust, rice husk, coirpith and charcoal) and a naturally occurring mineral (vermiculite) have been tested for their effectiveness in removing Cr from tannery effluent through batch and column experiments. The adsorption capacities of the substrates were also evaluated using isotherm tests and computing distribution co-efficient. The sawdust exhibited a higher adsorption capacity (k = 1482 mgkg(-1)), followed by coirpith (k = 159 mgkg(-1)). The biosorbent and mineral vermiculite in columns were found very effective in removing Cr from tannery effluent. About 94% removal of Cr was achieved by a column of coirpith, and equally (93%) by a column containing a mixture of coirpith and vermiculite. This study showed that biological wastes are potential adsorbents of Cr, which could be successfully used to reduce the Cr concentrations in tannery effluent.