Distribution and Sequential Extraction of Heavy Metals in Solidwaste from the Industrial Belt of Delhi, India

Moturi M. C. Z.*, M. Rawat and V. Subramanian**

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India

* Kenya Industrial Research and Development Institute (KIRDI), P. O. Box 30650, Nairobi, Kenya

** Corresponding author, e-mail: subra@jnuniv.ernet.in

Abstract:

The National Capital Territory (NCT) of Delhi is situated along the Yamuna River and hosts the largest cluster of small-scale industries in India, generating 6500 tonnes of solidwaste per day. This waste is often disposed-off by the roadsides, low lying areas, or in landfill sites which are often not scientifically engineered, thus posing a potential risk to millions of people downstream. In this study, solidwaste samples were obtained from five selected industrial zones representing the regional spread of the industrial belt in the NCT of Delhi. Bulk estimation of heavy metals (Hg, Pb, Cd, Co, Mn, Fe, Ni, Cu and Zn) was carried out using GBC model 902 atomic absorption spectrophotometer. In addition, sequential extraction was used to fractionate five heavy metals (Pb, Ni, Cd, Cu and Zn) into six operationally defined phases.

INTRODUCTION

The NCT of Delhi is situated along the Yamuna River, a major tributary of the river Ganges. It hosts one of the largest clusters of small-scale industries in India. The industrial establishments are situated in 28 industrial zones spread across the city. The nature of the industrial activities varies from fabrication of garments, consumer electronics, etc, to electroplating and steel processes. These processes generate waste of varying characteristics and quantities. It is estimated that the total municipal solidwaste generated in the NCT of Delhi is of the order of 6500 tonnes per day. The solidwaste is often dumped on the roadsides within the industrial areas from where it is collected and tipped-off in low lying areas and abandoned quarries, etc which act as dump sites.

It is recognized that heavy metals are omnipresent in the environment, occurring in varying concentrations in air, bedrock, soil, water and all biological matter. In pretechnological times, the cycling of each metal was at a steady state, and a tight control was maintained on its distribution in any ecosystem. Anthropogenic inputs have now overwhelmed the natural biogeochemical cycles of heavy metals in many ecosystems, resulting in greatly increased circulation of toxic metals in soil, air and water and the inevitable build-up of such toxins in the human food chain. It has been estimated that the toxicity of all metals released annually into the environment far exceeds the combined total toxicity of all radioactive and organic wastes as measured by the quantity of water needed to dilute such wastes to drinking water standards (Nriagu and Pacyna 1988). It is also recognized that pollutant heavy metals are non-degradable, and their continued build-up in mankind’s life support systems constitutes a serious threat to human health (Purves 1985, Lottermoser 1995).

Since many biological systems exist on the margin of metal toxicity, the physical and geochemical redistribution of toxic metals in the environments by human activities has a strong potential to disrupt ecosystems. However, such disruptions are not determined merely by the quantity of metals distributed. A number of environmental, chemical and biological processes may influence the accessibility of metals to organisms. In general therefore, availability of heavy metals depends upon the properties of particle surface, bond strength and external conditions such as pH, Eh, salinity and the concentration of organic and inorganic complexation agents. Thus the determination of the total content of heavy metals alone is insufficient in assessing their environmental impact, since it is the chemical forms that determine metal behaviour in the environment and their mobilization ability (Chakrapani and Subramanian 1996, Ma and Rao 1997). It is thus necessary to assess both total contents of the hazardous substances, as well as the chemical forms in which they may be present.

In this study attempts were made to evaluate the potential mobility and bioavailability of heavy metals in solidwaste from selected industrial sites representing the regional spread of the industrial belt of the NCT of Delhi.

MATERIALS AND METHODS

Solidwaste samples used in this study were obtained from five industrial sites (Jhilmil (JL), Naraina (NAR), Mayapuri (MAY), Wazirpur (WAZ) and Badli (BAD)) within the industrial belt of the NCT of Delhi. Samples were collected between November 1997 and December 1998, representing the three major seasons in Delhi, i.e. monsoon, winter and summer. The samples were collected from the roadsides within the industrial areas and placed in polypropylene bags, processed and stored at 4⁰C until required for analysis.

Bulk estimation of heavy metals was carried out according to procedure of Loring and Rantala (1992). Sequential extraction of heavy metals was carried out according to the procedure of Tessier et al (1979), as described by Ma and Rao (1997). Six operationally defined phases were separated, viz. water-soluble, exchangeable, carbonate-bound, Fe-Mn oxides, organic-bound and residual fractions.

RESULTS AND DISCUSSION

Bulk Distribution of Heavy Metals

The release of solidwaste to the environment may lead to contamination of water supplies, endemic diseases, offensive smell and eutrophication of waterways. In addition, solidwaste is known for its potentially elevated organic pollutants (e.g. phenols, polychlorinated biphenyls) and heavy metals and metalloids such as As, Pb, Cr, Cu, Ni, Zn and Hg (Lottermoser 1995).

The results obtained in this study (Table 1) show a wide range of pH values viz. from highly acidic (pH 2.3) to highly basic (pH 10). Wazirpur, Badli and to some extent Jhilmil exhibited lower pH ranges as compared with the other industrial sites. It is acknowledged that in aqueous systems availability of heavy metals is often dependent upon pH. As well, the solidwaste from Mayapuri, Wazirpur and Badli consisted of significantly higher levels of Mn and Fe. This seemed to suggest that a significantly higher percentage of ferrous-based industries were located in these industrial sites. Also, Wazirpur and Badli had higher amounts of Hg (23090 and 31000 mg g^-1) respectively. Similarly, Jhilmil, Naraina and Mayapuri had relatively higher amounts of Zn, Pb and Cd compared with the other industrial sites.

Fractionation of Heavy Metals

The results of the sequential extraction of heavy metals were tabulated in Table 2. Discussions will be carried out on the basis of distribution of the individual heavy metal. It should however be noted that evaluation of the results for each metal alone does not take into account possible synergistic or antagonistic effects due to influences of other metals (e.g. Ca, Fe etc.) that may be present in the samples.

Table 1: Bulk Distribution (mg Kg^-1) of Heavy Metals in Solidwaste

Serial Site No. of pH Cu Ni Zn Pb Cd Co Mn Fe Hg

No: Ref: Samples Range (µg/Kg)

1 JL 10 3.0-9.1 10144 192 3836 391 52 29 763 35684 1755

2 NAR 6 6.1-10 3065 415 23321 329 224 27 494 66071 504

3 MAY 10 6.7-8.6 3147 1534 675 530 50 48 11170 154096 417

4 WAZ 30 2.4-7.9 3770 962 116 23 0.78 19 15475 184338 23090

5 BAD 19 2.3-6.3 4096 154 122 37 0.14 11 19964 233119 31000

Table 2: Mean Distribution (mg Kg^-1) of Heavy Metals in Solidwaste

Cu Zn

Fraction JL NAR MAY WAZ BAD Fraction JL NAR MAY WAZ BAD

Water-soluble 30 3 3 145 113 Water-soluble 25 4 4 0.7 0.5

Exchangeable 22 8 12 2 112 Exchangeable 26 30 14 0.4 0.4

Carbonate-bound 698 41 165 11 75 Carbonate-bound 38 44 35 0.3 0.3

Fe-Mn oxides 180 36 44 26 369 Fe-Mn oxides 47 44 37 0.8 0.8

Organic-bound 645 200 282 338 658 Organic-bound 99 92 69 0.9 1.1

Residual 5863 2779 3642 875 1956 Residual 3601 23107 522 97 131

% Non-Residual 31 42 32 37 45 % Non-Residual 25 11 62 3 3

% Residual 69 58 68 63 55 % Residual 75 89 38 97 97

Pb Ni

Fraction JL NAR MAY WAZ BAD Fraction JL NAR MAY WAZ BAD

Water-soluble 1 0.9 0.8 0.7 0.5 Water-soluble 8 6 4 4 9

Exchangeable 10 16 19 1 1 Exchangeable 18 29 26 1 7

Carbonate-bound 12 17 11 1 6 Carbonate-bound 47 69 49 2 4

Fe-Mn oxides 20 30 47 2 4 Fe-Mn oxides 62 42 55 2 8

Organic-bound 54 61 57 4 2 Organic-bound 62 78 52 26 28

Residual 294 203 377 28 33 Residual 1188 191 180 113 70

% Non-Residual 39 40 36 24 29 % Non-Residual 57 53 52 24 44

% Residual 61 60 64 76 71 % Residual 43 47 48 76 56

Cd ND = Not Detected

Fraction JL NAR MAY WAZ BAD

Water-soluble 0.7 2 0.9 ND ND

Exchangeable 5 27 4 ND ND

Carbonate-bound 8 24 4 ND ND

Fe-Mn oxides 7 23 3 ND ND

Organic-bound 7 28 5 ND ND

Residual 24 118 26 ND ND

% Non-Residual 57 56 45 - -

% Residual 43 44 55 - -

Copper: The results obtained showed that Cu was present in varying proportions in the solidwaste. Jhilmil, Mayapuri and Badli sites had higher mean levels of heavy metals in all phases. While most of Cu (55-69%) was in the residual form, there were significant portions (31-45%) in the non-residual phases. Often, metals in these phases signify anthropogenic inputs from varying activities. It should be noted that these phases constitute potentially more mobile and bioavailable fractions, and are often responsible for heavy metal toxicity in plants and animals. As well, apart from the residual phase, the organic fraction had the highest amount of Cu (200-658 mg Kg^-1). This was consistent with observations made by other workers (Harrison et al 1981, Ma and Rao 1997).

Zinc: While Zn was present in all the chemical fractions of the solidwaste, Jhilmil, Naraina and Mayapuri had higher levels of Zn in the various non-residual phases compared with Wazirpur and Badli sites. The majority of Zn was mostly (except for Mayapuri) concentrated (75-97%) in the residual phases, while the non-residual phases constituted 3-25% of the heavy metals. It has been suggested that the greater proportion of Zn in the residual phase reflects on the tendency to become unavailable once in the soils (Ma and Rao 1997). In general, the composition of Zn in the more mobile phases was low in the solidwaste from Wazirpur and Badli sites

Lead: The bulk estimation of Pb was higher in the solidwaste from Jhilmil, Naraina and Mayapuri than from the other sites, and this was reflected in the composition of the various fractions. As in previous cases, the residual phase constituted the major portion (60-76%) of Pb, while the non-residual fractions had somewhat lower (24-40%) but never-the-less substantial levels. This significant concentrations of Pb in the more labile and bioavailable fractions suggest that they may be from anthropogenic sources. In the urban environment, the major sources of Pb include industrial processes (e.g. battery manufacturing) and petrol driven automobiles.

Cadmium: Elevated Cd concentrations are often identified with industrial uses, particularly in electroplating processes and from exhaust systems of petrol propelled automobiles. The fractionation results of this study show that the Cd levels in the solidwaste from Wazirpur and Badli were below the detection limit (<0.01mg Kg^-1). The distribution of Cd in the other sites was spread out and had a general trend of: residual > carbonates ³ organics ³ exchangeable > Fe-Mn oxides > water-soluble. Overall, the levels of Cd in the labile and bioavailable fractions (45-57%) were relatively high in view of its toxicity.

Nickel: The Ni levels in the solidwaste from Wazirpur and Badli were generally lower than those from the other industrial sites. There were relatively higher levels of Ni in the non-residual phases (24-57%). This was consistent with the findings of Tessier et al (1979), Belzunce-Segarra et al (1997) and, Ma and Rao 1997).

Conclusion:

Contamination of heavy metals in the environment is of high concern because of their toxicity and threat to human life and the environment. The extent to which a metal becomes toxic depends upon the amounts present as well as the forms in which they occur. The results obtained in this study show evidence of substantial anthropogenic fluxes indicated by the relatively high mean levels and proportions of the potentially mobile forms of heavy metals associated with the non-residual phases.

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