System and method for decommissioning a pipeline. The system comprises a mechanical assembly and a chemical assembly. The mechanical assembly includes a main body and a contact assembly. When the mechanical assembly is provided in the pipeline, the contact assembly is configurable to contact with the interior wall of the pipeline. The chemical assembly is arranged serially in line with the mechanical assembly. The chemical assembly includes a front section having a cross-sectional portion configurable to resemble the cross-section of the pipeline and a rear section having a cross-sectional portion. The front and the rear section are arranged in such a way that, when the chemical assembly is provided in the pipeline, the cross-sectional portions of the front and the rear section cooperate with the interior wall of the pipeline to form a chamber operable to receive and house a removal medium. The system and the method can effectively and efficiently remove most or all of such residual mercury and/or other toxic substance from the interior wall of the pipeline before abandonment.