Investigating metal/nanocolloid interactions in landfill leachates using AF4-HR-ICP-MS

Abstract

Landfill leachates contain a wide range of pollutants including potentially toxic metal(loids) e.g. arsenic. Current landfill risk assessment models predict the fate and transport of these pollutants in the environment, however they consider all species below 0.45 ?m to be dissolved, thus the presence of these pollutants in colloidal form is not considered.
In order to investigate the presence and distribution of metal(loids) within the nanocolloidal fraction (<100 nm), AF4 coupled with HR-ICP-MS was selected (alongside AFM and DLS) and optimised for use with landfill leachates. UV254 and Fluorescence spectroscopy were also used as detectors for AF4 to detect organic colloids.
AF4-HR-ICP-MS analysis was carried out both offline (fraction collecting and subsequent HR-ICP-MS analysis) and online (interfacing the AF4 output directly with the HR-ICP-MS) with parameters optimised for lower MW particles. Online coupling provided a higher resolution analysis than the offline method. The concentration of elements within the AF4 system was found to be in flux and therefore baseline concentrations were established for each sample injection. Method repeatability and a recovery mass balance of each element were also established.
The method was validated by fractionation of a MSW, an aged MSW and a MBT leachate. All three leachates were found to show the same nanocolloidal distribution with two distinct nanocolloid populations present: a low MW organic rich fraction; and a larger, less organic rich fraction consisting of a mixture of organic and inorganic particles. Metals predominated in the lower MW fraction associated with humic or fulvic-like particles. The similarities between the leachate metal distributions showed that treatment of leachate prior to landfill did not alter the colloidal characteristics.
Preliminary results examining the effects of pH and ionic strength of metal distribution showed that pH had no effect; however the lowering of ionic strength appeared to cause aggregation of colloidal Fe particles, presumably due to the lower organic content, which appears to control the distribution of metals in this size fraction.
This research highlights the importance for landfill risk assessments to be updated to include the presence of colloidal facilitated transport and the necessity for further particle transport studies to be conducted.

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