STUDIES ON THE SPECIATION OF METALS IN SLUDGES

Abstract

In third world countries like India, a part of the wastewater is treated and the major portion of it flows through the drain to larger water bodies or is discharged on land. The sludges from the treatment plants are mainly disposed of on land. Through these disposal practices the metals enter the environment. The determination of the total metal concentration in sewage sludge is often inadequate for understanding its impact on environment. In order to gain an insight in the metal pollution caused by the sludges it is necessary to know the different metal species present. The speciation invariably helps in evaluating the metal bioavailability, mobility and potential for contamination of ground and surface water. Metal species in sludges have generally been investigated by selective sequential extraction techniques which in turn reflect on exchangeable, adsorbed, organic, carbonate and sulphide forms. A survey of literature reveals that most of the speciation studies have been carried on digested sludges. Scanty references are available wherein the fractionation pattern of primary, secondary and digested sludges have been compared. More investigations are still required to study change in metal form with the sludge type. Therefore, in the first phase of the work speciation studies in sludges from a treatment plant and sludge deposits from a drain flowing through a industrialised metropolitan city were undertaken. The variable nature of wastewater entering the plant yields a complex sludge matrix. The metals already present in the sludge may influence the metal uptake and its distribution pattern. In the next phase of the work activated sludge has been synthesized under controlled conditions and digested with Ill added metal. In all these studies the speciation profiles of mainly copper and zinc have been investigated using the sequential extraction procedure proposed by Stover et al. The sequence of reagents, extraction times and solutiomsolids ratios are as given below: 1M KN03, 16 hr, 50:1; 0.5M KF, 16 hr, 80:1; 0.1M Na4P207, 16 hr, 80:1; 0.1M EDTA, 16hr, 80:1 and 1MHN03, 16 hr 50:1. The forms of metal extracted by the sequence of reagents are exchangeable, adsorbed, organic, carbonate and sulphide respectively. For convenience and clarity of presentation the subject matterofthe thesis has been divided into the following chapters. "L GENERAL INTRODUCTION II. MATERIALS AND EQUIPMENT III. SPECIATION OF TREATMENT PLANT SLUDGES V/TV. SPECIATION OF OPEN DRAIN SLUDGE SEDIMENT V. SPECIATION OF SYNTHETIC ACTIVATED SLUDGE VI. SPECIATION OF SYNTHETIC DIGESTED SLUDGE Chapter I presents a brief background of environmental pollution, different types of sludges, methods of their disposal and their environmental impact. The mechanism of concentration of metals in sludges and the need for speciation have also been discussed. Based on the available information the objectives of the work embodied in the thesis have been identified. 9 IV Chapter II gives the details ofdifferent materials and instruments used during the experiments. Metal analyses were carried out using Atomic Absorption Spectrophotometer (IL 751) and Inductively Coupled Plasma Atomic Emission Spectrometer (PLASMALAB 8440). Chapter III contains the speciation profiles of copper and zinc in primary, secondary and digested sludges collected from the same treatment plant. Sorption capacities of the various sludges have been determined. Sludges were found to have greater affinity for copper than zinc. However, activated sludge exhibited greater capacity for zinc. The sudden increase observed in the bound metal concentration at higher metalloading is probably due to the precipitation of basic carbonates, which is reflected in the speciation profile of the loaded sludge. Significant variations in the copper and zinc forms with the sludge type and hundred fold zinc loading have not been observed. Chapter IV gives the results of metal concentrations in the sludge sediments collected from an open drain flowing through a large metropolitan city. This sludge sediment has significant concentrations of copper, zinc and lead. The speciation profiles indicate that zinc is largely associated with the organic fraction, lead with the carbonate and most of the copper remains in the residual form. None of these metals are present in exchangeable and adsorbed forms which can either be due to the fact that drain water flushes them downstream or the ageing process renders them insoluble. The details of preparation and the characteristics of the synthetic activated sludge are given in Chapter V. The sludge was loaded with metal solutions of different pH and the metal uptake and its distribution at different pH was investigated. The studies conducted herein reveal that the metal uptake by activated sltfjge initially is afast process and during the process, pH of the system changes due !0fJudge-metal interaction. The equilibrium pH affects the metal uptake as well as -tiespeciation profile. The results ofsynthetic sludge when compounded with those Ofrtyeatment plant sludge indicate that the speciation profiles of copper are more switive to the sludge matrix and metal ion concentration than those of zinc. No worked difference in the speciation profiles were observed when chloride nitrate and acetate were the associated anions of Cu(II) and Zn(II). Synthetic activated sludge was digested in the presence of different concentrations ofcopper and zinc. Chapter VI contains the results ofgas production 'and speciation profiles of the digested sludges. The association of copper with the &ftj&»C matter seems to be responsible for the deactivation of the digesters. However, the reverse is true for zinc. The speciation profiles of synthetic activated 5lvd9fi and synthetic digested sludge are different thereby indicating that the TnicTOenvironment might play a role in determining the speciation pattern.