© by PSP Volume 13 – No 10. 2004 Fresenius Environmental Bulletin 995 STABILIZATION OF Cu AND Cd IN THE PRESENCE OF MONTMORILLONITE BY MEANS OF COAL FLY ASH Roberto Terzano 1 , Matteo Spagnuolo 1 , Luca Medici 2 and Pacifico Ruggiero 1 1 Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, Università degli Studi di Bari, Bari, Italy 2 I.M.A.A. - Istituto di Metodologie per l’Analisi Ambientale - C.N.R., Tito Scalo (PZ), Italy Presented at the 12 th International Symposium on Environmental Pollution and its Impact on Life in the Mediterranean Region (MESAEP & SECOTOX), Antalya, Turkey, 04 – 08 Oct. 2003 SUMMARY Coal fly ash pre-treated with NaOH at high tempera- ture added to montmorillonite artificially contaminated with high amounts of Cu or Cd (15 mg/g dry solid matter) was capable of drastically reducing the solubility of both metals. Cu and Cd solubility decreased permanently with in- creasing aging time and after 6 months their solubility, both at 30 and 60 °C, became lower than 1 ppm. Experiments by EDTA sequential extractions showed that Cu and Cd stability in/on the solid phase increased over time. After 6 months of aging at 30 and 60 °C, 16% - 31% of the metals are no longer extractable by EDTA. No stabilization was observed at identical pH values in the absence of coal fly ash. The effects observed support the general mechanism for the formation of amorphous geo- polymers originating from pre-treated coal fly ash, which immobilized Cu and Cd inside the solid phase. KEYWORDS: coal fly ash, montmorillonite, geo-polymers, heavy metals, soil remediation. INTRODUCTION Solidification/stabilization techniques are widely em- ployed for the remediation of soils heavily polluted by or- ganic xenobiotics or heavy metals, in particular under condi- tions where bioremediation or phytoremediation are not suitable because of toxicity to microorganisms or plants [1]. Many of these physico-chemical techniques adopt complex mixtures of stabilizing agents, such as cement, kiln dust, soluble silicate, coal fly ash, phosphates, etc. Alkalizing agents are also added to these mixtures in order to increase the stabilizing properties of treated soils towards heavy metals. Coal fly ash is a by-product of coal combustion in thermoelectric plants and widely employed in these mix- tures owing to its cheapness, efficiency and the opportu- nity of partly solving the problem of its disposal [2]. This waste material contains large amounts of amor- phous alumino-silicates, which can readily dissolve in alkali medium and promote the so called “geo-polymerisation” reactions [3]. The amount of this highly reactive amorphous alu- mino-silicates can be largely increased if coal fly ash is pre-treated with NaOH at high temperatures before its utilization [4]. The most reactive soil mineral constituents related to heavy metals are clay minerals and, in particular, smec- tites among which montmorillonite is largely widespread in Italian soils. Montmorillonite has peculiar properties (very high CEC, large surface area, etc.) and is very effective in catching heavy metals from soil solutions, thus reducing their solubility [5]. The addition of pre-treated coal fly ash to montmoril- lonite could further reduce the concentration of heavy metals in solution and enhance their stabilization in the new formed geo-polymeric matrix. In order to assess the importance of fly ash in the mixtures employed in many physico-chemical remedia- tion technologies, this research aimed to evaluate its role in reducing the solubility and stabilizing copper and cad- mium presence. MATERIALS AND METHODS The coal fly ash used in this study was obtained from ENEL thermoelectric power plant of Cerano (Brindisi, Italy).