A composition containing titanium and iron compounds, in particular decomposition residue that accumulates as a production residue after the sulphate process during the production of titanium dioxide, is used as an adsorbing agent for inorganic and organic compounds, for example compounds containing phosphorus, arsenic, antimony, sulphur, selenium, tellurium, the cyano group or heavy metal. Toxic substances and pollutants may be removed from a fluid using the adsorbing agent.