Valeria Bona

Introduction

Soil contanmination on brownfield land is a widespread problem and in recent years it has become an area of increasing interest and a topic of extreme relevance to sustainable development. Typical brownfield land contamination includes heavy metals, inorganics and total petroleum hydrocarbons (TPH). Rain and snow melt can carry these contaminants through the soil contaminating groundwater, accumulating in the food chain and therefore harming the ecosystem and threatening human and animal health. Clean up technologies are currently either expensive or can only be partially successful and often owners abandoncontanminated properties rather than try to decontaminate them. These contaminated brownfields represent lost opportunities for productive re-use. One technique which is widely used is the containment of the contaminants by immobilisation within a matrix by the addition of binders, which, although do not remove the contaminants, do prevent their further spreading. Current research is looking at innovative and also cost-effective binders; this includes the use of waste materials and minerals such as zeolites. The aim of this dissertation project is to investigate the effect and possible use for some innovative materials and some waste materials in the immobilisation of contaminated soil. The project uses model soils representing the ground conditions at the Ministry of Defence former chemical works site at West Drayton, near Heathrow Airport. This site has already been the subject of an immobilisation treatment using conventional binders such as cement. The project also
addresses future reuse of the site for various soft end uses.                                                                                                                                                                                                                                                                                                                                                                              Soils and Binders

The ground conditions at the site consisted of 1.7m of variable made ground overlying 3-4m of natural sand and gravel. Those two soils were reproduced in the laboratory limiting the contanUnants to five heavy metals (Pb, Cu, Cd, Zn, Ni) and paraffin oil. The binders used were three organic waste materials (Coir Compost, Sawdust & Chicken Manure Pellets), one inorganic waste (Pulverized Fuel Ash) and two inorganic novel binders (powder & granular zeolite). They were added to the soil in different ratios (10%, 20% and 30% by weight) and different combinations (up to 3 binders in a given mix).                                                                                                                                                                                                                                                Testing and Analytical Procedure

Batch leaching tests were carried out on 59 samples using the UK National Rivers Authority (NRA) leaching test. In addition, 7 selected samples were tested using the US Environmental Protection Agency Toxicity Characteristics Leaching Procedure which is considered a "worst case scenario". Column flow-through leaching tests were carried out on 20 sand & gravel selected samples in which different flow rates, tests durations and eluents were used. The
leachate solutions were then filtered and analysed using an atomic absorption spectrophotometer for the heavy metals and an ion chromatograph for the anions. The analysis of TPH was carried out commercially by low detection level GCIFID. The pH of the leachate solutions was also measured.

Results and Conclusions
The addition of the binders used did not have a significant effect on the leachate pH. It also reduced the heavy metal concentrations compared to the untreated soils to varying degrees, with the best results being obtained with powder zeolite and pfa and the worst with sawdust and compost. TPH leachate concentrations showed a very good immobilisation capacity of all the binders with compost and granular zeolite performing best and chicken manure and
sawdust performing the worst. Anion immobilisation was far less successful. TCLP leachate concentrations were, as expected, higher than those in the NRA leachates. The success of the
remediation will depend on the acceptable concentration levels for remediation and soft end use.