Cleansing leaks from toxic tips

Researchers in the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE) are developing an underground wall that filters and cleanses the poisonous fluids which leach out of old rubbish dumps and enter the groundwater which many communities rely on for drinking or food production.

The 'permeable reactive barrier' filters and cleanses the toxic flow before it can enter the aquifers which are used by communities for drinking, domestic, stock or industrial water.

While modern tips are designed to prevent the flow of toxic leachate from reaching groundwater, older tips were not, and many are still feeding the pollution from decades ago back into the community.

"The contamination depends on what was in the garbage placed in the tip. It usually contains old industrial and household chemicals, solvents and oils, antibiotics and endocrine disruptors, medical drugs, personal care products, biological pollutants and heavy metals," said CRC CARE Project Leader, Professor 'Vigi' Vigneswaran of the University of Technology Sydney.

According to Professor Ravi Naidu, Managing Director of CRC CARE, Vigneswaran's barrier can be buried 20 m or more into the ground and be up to 2 m thick.

"It is a sophisticated sandwich of materials which adsorb, break down or otherwise render harmless the contaminants in the leaching tip water," Naidu said.

Traditionally, permeable reactive barriers have consisted of iron particles or slag which helped remediate the contaminated leachate. Vigneswaran's design tailors the barrier precisely to the particular pollutants in the leachate and subjects them to a series of processes which ensure they are destroyed or rendered harmless. 

"For example, first we use biosorption to remove and neutralise biodegradable hydrocarbon compounds. This uses a material on which biofilm forms – and these do the job of breaking down the pollutants," Vigneswaran said.

The second layer deals with much tougher pollutants – organic compounds that either do not degrade or only do so very slowly. These are known as POPs (persistent organic pollutants). The second layer of the barrier subjects these to fierce oxygen-free radicals which break down the poisons into harmless by-products such as water, carbon dioxide and nitrogen.

The third layer is designed to deal with toxic heavy metals, such as arsenic, or the lead and mercury left by the millions of discarded batteries and other poisonous metals from old consumer electronics. These metals are scavenged out of the leachate by an adsorption layer consisting of activated carbon, wood waste, or a special clay which either locks the metals up or oxidises them into less harmful compounds which can be removed by biosorption.

Depending on the type of contaminants in the leachate, these layers may be arranged in different sequences to obtain the best performance.

Vigneswaran said the laboratory research into ways of dealing with the problem showed strong promise that these relatively low-cost barriers can be designed to keep toxic leachate out of groundwater in the longer term.