Charcoal contains some residual hydrogen and oxygen, and it is the charred product from pyrolyzing wood. ScienceDirect explains that pyrolysis is the process of thermal conversion of organic matter using a catalyst in the absence of oxygen. Carbon is a naturally occurring pure element found in various allotropes. Allotropes are described as one or more forms of a chemical element that occur in the same physical state. For example, graphite and diamond are both allotropes of carbon that occur in the solid state. Graphite is soft, while diamond is hard. Pollutants can be absorbed by charcoal, a porous material. Both filters are viable options for your composting crock.