Extractive decontamination of heavy metals from CCA contaminated soil using organic acids

Abstract

In this paper, the mobilization and extraction of As, Cr and Cu from chromated copper arsenate (CCA) contaminated soil obtained from a wood treatment factory site by four organic acids are presented and discussed. The CCA contaminated soil (pH = 5.91, carbon = 0.32, CEC = 47.84 meq/100 g) was found to contain 39.55 mg/kg As, 313.97 mg/kg Cr and 200.00 mg/kg Cu with a contamination factor greater than 6 for As, 3.14 for Cr and 4 for Cu; thus classifying the soil as very highly contaminated for As and considerably contaminated with Cr and Cu. However, speciation studies on the contaminated soil sample showed that 59% As, 19% Cr and 5% Cu were potentially bio-available and mobile. Chemical wash test using four chelating agents: oxalic, malonic, succinic and citric acid were performed at 1:10 solid/liquid ratio, 0.05M chelant concentration and at 2, 4 and 6 h contact time, respectively. At the investigated operating conditions, oxalic acid extracted the lowest amount of As, Cr and Cu from the contaminated soil and the extraction efficiency depended on the solid/liquid contact time. The amount of metal extracted at the contact time of 6 h were 10.41, 12.50, 17.71 and 18.75 mg/kg As using oxalic, malonic, citric and succinic acid, respectively; 97.05, 123.69, 140.05 and 147.28 mg/kg Cr using oxalic, malonic, citric and succinic acid, respectively; 64.00, 94.10, 106.00 and 118.00 mg/kg Cu using oxalic, malonic, citric and succinic acid, respectively. Chromium (47%) was the least to be removed of the three metals after 6 h of washing. The contamination factor of the contaminated soil was affected by the extraction test. Levels of the metals in the decontaminated soil after 6 h of washing were found to be below the target value for all metals, which is somewhat higher for Sparks in respect of As and higher than the control values for As, Cr and Cu. After the 6 h extraction test, 15.84 – 16.62 mg/kg (60 - 63%) As, 59.35 - 65.04 mg/kg (32 - 38%) Cr and 37.10 - 49.53 mg/kg (38 - 50%) Cu was found in the bioavailable fraction of the soil treated with the four different organic acid chelants.

Key words: CCA contaminated soil, decontamination, organic acids, extraction.