Comparative leaching behavior and immobilization of heavy metals in ceramic matrices derived from refinery oily sludge using TCLP and EN 12457-2.

Abstract

The increasing generation of refinery oily sludge (ROS) poses significant environmental challenges due to its high content of hazardous heavy metals, requiring effective stabilization methods. This study investigated the leaching behavior and immobilization efficiency of 31 elements in ceramic matrices incorporating ROS, comparing the toxicity characteristic leaching procedure and European standard (EN) 12457-2 protocols to evaluate environmental risk. Ceramic specimens were prepared with 0, 5, and 10 wt.% ROS and sintered at 950°C and 1050°C. Leachates were analyzed via inductively coupled plasma mass spectrometry to quantify elemental mobility. Results demonstrated high immobilization efficiency; Cu remained consistently below detection limits (<0.001 mg/kg) in all samples, while Zn concentrations remained stable across both firing temperatures and protocols. Firing temperature significantly influenced Cr mobility; in the EN 12457-2 protocol, increasing the temperature to 1050°C reduced Cr leaching from 0.529 mg/kg to undetectable levels for 5% wt. ROS content. However, specific variations were observed at higher waste loadings, with Cr reaching 4.590 mg/kg in the 10% wt. ROS samples at 1050°C, suggesting complex phase transformations. Ni was consistently immobilized below detection limits, and Pb showed minimal mobility in isolated cases. These findings confirm that ceramic processing is a viable stabilization technique for ROS, generally reducing metal mobility below regulatory limits. The study concludes that while higher sintering temperatures enhance the matrix stability for most elements through vitrification, the interaction between waste loading and temperature is non-linear, requiring precise optimization for large-scale applications. Refinery oily sludge (ROS) is a hazardous waste produced by the oil industry. It contains toxic heavy metals that can pollute soil and water if not properly treated. One promising solution is to reuse this waste in ceramic materials. This study examined whether heavy metals from ROS can be safely immobilized in ceramic materials. It also compared two common laboratory tests (toxicity characteristic leaching procedure and European standard 12457‐2) to understand how different environmental conditions affect the release of these metals. Our tests showed that turning the waste into ceramics is a very successful way to clean it up. Most toxic metals, such as Ni and Cu, were so tightly trapped that they could not be detected in the liquid surrounding the samples. The study demonstrates that ceramic processing can be an effective way to safely reuse ROS and reduce environmental risks. However, results also show that testing conditions greatly influence how safe a material appears.

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