Question 2 of 10: Microencapsulation and Macroencapsulation

Question 2: Microencapsulation and Macroencapsulation (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Both mechanisms belong to the solidification/stabilization (S/S) family of hazardous-waste treatment technologies, and both work by physically isolating a contaminant from its surroundings rather than by chemically destroying it — the difference is entirely one of SCALE.

1. Microencapsulation. Fine waste particles — a metal-hydroxide sludge, a particulate residue — are dispersed directly into a binder that is mixed THROUGHOUT the waste, so that individual particles (or very small clusters of particles) each end up physically enclosed within the surrounding solidified matrix. Because the binder surrounds the waste at the particle scale, the diffusion path a leaching contaminant must travel is very short (micrometres to a few millimetres), so the technology depends on the matrix itself being chemically compatible with, and low-permeability around, every particle. Technology example: Portland-cement-based solidification/stabilization of a heavy-metal (e.g. chromium or cadmium) hydroxide sludge, in which the cement's hydration products (calcium-silicate-hydrate gel) grow around and physically enclose the precipitated metal-hydroxide particles as the mixture cures into a monolithic solid; pozzolanic (fly-ash/lime) and thermoplastic (bitumen, paraffin) binders achieve the same particle-scale encapsulation.
2. Macroencapsulation. A much larger, discrete mass of waste (often already-treated waste, debris, or containerized material) is enclosed within an outer coating, jacket, or vessel wall that forms the barrier — the technology does not attempt to bind or alter the waste's internal structure at all, only to isolate the whole bulk mass from infiltrating water and the surrounding environment. Because the barrier is external, it is inspected/engineered as a coating thickness and integrity problem (pinholes, cracks, seam welds) rather than a matrix-permeability problem. Technology example: encapsulating debris or treated waste in a polyethylene-resin or thermoplastic jacket — e.g. a rotationally-moulded high-density polyethylene overpack around a drummed hazardous waste, or a sprayed/cast polymeric resin coating over a large solidified waste block — before landfill disposal.