Use of bauxite refining residue to reduce the mobility of heavy metals in municipal waste compost (1994)

Use of bauxite refining residue to reduce the mobility of heavy metals in municipal waste compost.&#13;\n&#13;\nDue to the adverse environmental impact of landfills and the lack of space for nearby population centres an unprecedented interest has developed in waste recycling with the objective to reduce the volume of waste disposed. Generally organic matter represents the largest fraction in the domestic waste stream including food, garden and paper waste.&#13;\n&#13;\nIn the past efforts have been made to process this organic fraction into muniapal compost.However contamination of the compost with heavy metals has in many cases resulted in the lack of suitable markets and subsequent closure of compost facilities. A study was undertaken and aimed at reducing the concentration and mobility of heavy metals in municipal compost.&#13;\n&#13;\nBauxite refining residue ha; a high capacity to immobilise metals in soils through precipitation, cation exchange and chemisorption (Fe- and Al- oxides).&#13;\n&#13;\nThe first step in the study was to assess the metal adsorption capacity of red mud by equilibrating red mud with metal solutions. Metal removal from solution proved very high.&#13;\n&#13;\nSubsequently, the effect of red mud addition to municipal compost was studied in relation to metal mobility. In particular the interaction of metals with the mobile soluble organic fraction, the humic and fulvic acids, was investigated. It was found that most metals remained in the insoluble humin fraction and the metals in solution remained complexed to the soluble organic fraction. Red mud appeared to be able to adsorp fulvic and humic acids and thus indirectly immobilise the metals in solution. Red mud did not appear to be able to transfer metals from the organic fraction to the mineral red mud fraction.&#13;\n&#13;\nft was kypothesised that, in order for the metals to be adsorped to red mud, the red mud needed to be present in the organic waste prior to composting. Metal adsorption to red mud is a rapid chemical reaction while metal complexation to the organic fraction takes a number of weeks, since it must be sufficiently oxidised to contain functional groups.&#13;\n&#13;\nIn order to verify this hypothesis, an installation with seven incubators was developed which allowed composting to be undertaken under computer controlled conditions and monitored for temperature, airflow and carbon dioxide respiration. The incubators were filled with 10 kg of a mixture of grass clippings and sawdust. The first incubator functioned as a control. The contents of the rest of the incubators were spiked with the following metals: Cd, Cr, Cu, Ni, Pb and Zn. In addition the contents of the 3rd - 7th incubators were amended with an increasing percentage of red mud, respectively 10,20,25,30 and 40 % by weight. After the mixture was composted, the metal mobility was reduced by approx. 80 - 99 %, depending on the metal. The mobility was assessed as leachable and plant available in relation to the total metal content. Red mud did not affect the composting process if &lt; 40% red mud was added.&#13;\n&#13;\nThe next step was to compost municipal organic waste in a pilot plant (capacity 5 m3) using batches with and without red mud. Mixed waste was collected from 150 households and the glass, metal and plastic fractions were removed bx- manual sorting. The rest was composted.&#13;\n&#13;\nThe red mud - compost had a relatively lower metal content and a reduced metal mobility compared to compost without red mud. The red mud - compost also had a 300 times higher pH buffer capacity and 90%less soluble organic matter.&#13;\n&#13;\nThe addition of red mud prior to composting was found to reduce the concentration and mobility of metals in compost, improve the quality of compost as a soil conditioner, increased the resistance of the organic fraction against further breakdown and this process thus adds value to both red mud and organic waste.