Introduction:
This study is motivated by a real problem at the Alang ship-dismantling facility near Bhavnagar in Gujarat . The focus of this study is the paint chip waste which is generated in shipbreaking activity. Alang, located 56 Km away from Bhavnagar city in Gujarat , came up as shipbreaking site during 1982-83. Hundreds of old ships, after their useful life, are broken at the Alang shipbreaking yards without any precaution to protect the environment. Shipbreaking and its harmful consequences have increased rapidly in past few years, creating a big challenge to the environment.
Most of the materials from shipbreaking are already defined as hazardous wastes under Basal Convention. Mass environmental pollution, disposal of toxic materials in the sea or on nearby agricultural land are some of the incidents observed in shipbreaking yard and in its vicinity. In light of the scientific data available, there should be no doubt that ships from the 70’s contain maximum levels of hazardous substances and when dismantled these compounds are released into environment.
Paint waste is one of the major hazardous wastes materials from the shipbreaking industry. These paints were mostly made up of lead. Paint waste, which is toxic due to its chemical composition such as lead oxide, pesticides, antifouling agents, TBT etc., occurs on the inside and outside surface of the ship. In general, metal-based paints, some containing as much as 30% heavy metals, were intended to protect ship surface from corrosion. The soil in and around the ship breaking yards are highly polluted with heavy metals. It is known that paint chips are main polluting agents, which contaminates the soil.
There are two issues related to paint chip waste in shipbreaking industry, namely:
a) protection of soil from further contamination by proper collection, treatment, and disposal of paint chips and
b) cleanup of soil that has already been contaminated due to paint chips.
The present research work emphasizes on both the above issues related to paint chip wastes. Aiming at evaluation of extent of soils pollution due to heavy metals site survey was carried out at Alang, in the state of Gujarat and Sewri, Mumbai in the state of Maharashtra . Soil as well as paint samples were collected and analyzed for heavy metal content using atomic adsorption spectrophotometer (AAS). The results of soil analysis showed that soils from Alang as well as Sewri were highly contaminated with heavy metals. The reason behind such high contamination could be attributed to the previous shipbreaking practice of last 20-25 years. It is observed from paint chips analysis that the concentrations of lead, chromium, and zinc were much higher as compared to soil samples. The reason for such high 2 concentrations could be the fact that, these metals are in their purer form in paints as compared to soil.
Various methods are in practice to remove paints from ship surface. One of the methods used is sand blasting. Around 1,000 to 2,000 tonnes/ship (ship of 10,000 tonnes) of sand-paint waste matrix is generated in the sand blasting process assuming 5% of paint chip. Disposal of such sand-paint waste matrix leads to contamination of water and land.
In order to address the first issue i.e. treatment and disposal of paint chips, the potential of cement based solidification / stabilisation (S/S) system for immobilization of Cr and Pb has been explored using ordinary Portland cement and waste blasting sand. The focus of present work was to study the effectiveness of S/S on leachability of heavy metals using a) synthetically contaminated blasting sand and b) paint chips – blasting sand mixture. Although, immobilization of heavy metals was the primary objective of the study it was felt necessary that the blocks created by solidification should have minimum compressive strength to withstand the dead load when disposed in landfill and abrasion during transportation. The blasting sand collected from a local sand blasting unit was mixed with cement and water in mortar mixer. Various combinations of sand to cement used were 100:15, 100:20, and 100:30 (volume ratio). A constant water to cement ratio of 0.5 was maintained for all the experiments to achieve workability as well as strength. Dry and damp curing was adapted to find the effect of curing. It was observed from the results of dry and damp curing that damp curing exhibited very good results within 14 days of curing where as dry curing did not produce sufficient strength even after 28 days of curing. While carrying out experiments to study the leachability of heavy metals from S/S blocks, two metal oxides (lead and chromium) with concentration of 0.5, 1, and 1.5 g/kg of sand were added to water, separately. This water was used for preparing cement – blasting sand S/S blocks. Sand to cement ratio used was same as used in the compressive test experiments. Water to cement ratio of 0.5 was maintained for all experiments. These S/S blocks were tested for concentration of heavy metals in leachate adopting batch leaching experiment (BLE) and TCLP. The extraction fluid for BLE was prepared by soaking the crushed S/S blocks prepared using cement and uncontaminated blasting sand in water for 72 h. This water was 3 then filtered and used as extraction fluid. Batch leaching experiments were carried out with three intact and one crushed block of each combination. Blocks after curing were immersed in 500 ml of extraction fluid in a 1 L capacity plastic jar. 20 mL sample of extraction fluid was taken out periodically from the jar and replaced by same quantity of fresh extraction fluid. These samples were analyzed for lead and chromium concentration using AAS. The BLE shows that lead concentration in all the combinations was below 1 mg/L, which is much below the regulatory levels (5 mg/L). Anhydrous calcium silicate phase, which comprises 70-80% of cement, dominates the composition of Portland cement. Therefore, it is sufficient to assume that the hydration products of these phases, dominated by calcium silicate hydrate (C-S-H) gel, play an important role in the retention of lead [Ouki and Hills, 2002].
However, chromium concentration in various combinations in both the leaching tests exceeds the prescribed limits, except for the combination with high cement content (30 parts of cement per 100 parts of blasting sand) and low chromium concentration (0.5 gm per kg of blasting sand). To study leaching behavior of chromium release in extraction fluid, complete analysis for one combination was carried out. The results show that chromium concentration increases with time up to 48 hrs, thereafter the concentration of Cr in extraction fluid decreases (Figure 1(a)). The mechanisms like reduction and adsorption are responsible for chromium release, which are pH dependent. Figure 1(b) shows mass of Cr release as function of pH. It can also be seen that the mass of chromium increases with decrease in pH from 11.50 to 9.00 for first 48 h and thereafter decreases as pH increases from 9.00 to 11.40. Actual paint chips (5% of sand by volume) were mixed with blasting sand before preparing S/S blocks using actual paint chips. It is assumed that the paint chip waste-blasting sand that is generated from blasting of ship surface could contain 5 % of paint chips. The results of TCLP for S/S blocks containing paint chips revealed that all the heavy metals were well within the regulatory limits indicating that S/S of paint chips – blasting sand mix is a superior alternative for disposal of these wastes. In order to address the issue related to decontamination of soil at ship breaking sites, soil washing studies were carried out. To design the soil washing units effectively, information on both sorption and desorption of individual contaminant is necessary. Sorption studies were carried out to study effect of surrogate organic matter (SOM) and ionic strength (IS) on sorption of lead and chromium onto soil.
Prepared by: Amit B. Mahindrakar
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