Stabilization of heavy metals in industrial sludge with concrete mix

Abstract

Leaching from industrial sludge is a growing problem worldwide, especially in developing countries like Bangladesh where the industrial sludge is usually highly polluted with persistent toxic heavy metals and bio-organics. Limitations in treatment facilities combined with the lack of environmentally fr.iendly disposal options is posing a potential threat of contaminating surface and ground water resources of the country with leachates from mismanaged sludge. The primary objective of this study was to assess the viability of stabilization of the heavy metals present in industrial sludge with concrete mix from environmental perspective, to assess the strength and usability of the resultant concrete and to propose a stable mix that can be used for m.inor construction. Dry sludge samples from two industries at the Dhaka EPZ were collected and analyzed for various pre-selected physical propert.ies and chemical constituents that are likely to affect the properties and behavior as wen as leaching from resultant concrete. The heavy metal contents of the sludge samples in terms of total Cadm.ium (Cd), Lead (Pb), Chromium (Cr) and Mercury (Rg) were determined following standard USEP A methods. The air-dried sludge samples were powdered prior to use in concrete mix. The sludge samples were used in different proportions in concrete mix replac.ing equal weight of finer portion of the fine aggregates. In the preliminary phase, sludge content varied from 3 to 10% by weight of cement in the concrete mix. Based on the cylinder compressive strength test results and slump results on concrete, a relatively narrow range of sludge content was selected for secondary analysis at different water-cement ratio. The durability of sludge-m.ixed concrete under sulfate laden environment at different water-cement ratio and sludge content were analyzed. The Toxicity Characteristic Leaching Procedure (EPA Method 1311) was performed on the raw sludge and crushed concrete samples for analyzing the long-term leaching potential of the sludges and concrete in terms of the heavy metals of concern for this study. A modified leaching test was performed on the same samples in an effort to simulate the scenario of open environment exposure expected for the stabilized sludge. The TCLP leachates from raw sludge showed significant concentration of Pb and Cr. The concentrations of heavy metals in leachates from crushed concrete samples were very low. In most cases, the concentrations were within the standard deviations of that observed in raw sludge leachates. If the background contamination of concrete is excluded, then the concentrations become negligible and can be attributed to standard experimental errors. The sludge-mixed concrete samples showed little difference in durability compared to the sludge free samples. However, noticeable change in durability was observed with change in water-cement ratio. The concrete based stabilization of heavy metals seemed successful. The watercement ratio seems to playa significant role in workability and strength and possibly the major role in durability in case of sludge-mixed concrete. Water-cement ratio should be kept in the range near 0.40. The test results of concrete suggest that the sludge amount is better to be limited within 5 to 6% by weight of cement from the conservative standpoint of possible presence of high amount organics in sludge in the fields. At 5% sludge content and water-cement ratio of 0.40, the 28-day average compressive strength of sludge-mixed concrete for the low organic sludge and the organic-rich sludge were observed to be 5050 psi and 4850 psi, respectively; which can be regarded as acceptable for structural purpose.