The concentration and speciation of metals in soil solution are key factors in controlling the amount of metal uptake by plants. The processes of mobilizing and transporting heavy metals in soil and rhizosphere are of prime interest in phytoextraction. Understanding of metal speciation, bioavailability of metals and metal behaviour in soil in a phytoextraction system are important for the success of phytoremediation technology. Biosolids are solid residues from wastewater treatment after either aerobic or anaerobic digestion processes. This study was aimed to determine the fate of heavy metals and metalloids (As, Cd, Cu, Cr, Ni, Pb and Zn) in solution phase of aged biosolids stockpiled at Melbourne Water’s Western Treatment Plant (WTP), Werribee, near Melbourne, Australia.

An experimental design consisting of 18 columns was set up outdoors at the University of Melbourne’s glasshouse facility. Each column had a 70 cm layer of biosolids or a mix of biosolid and peat moss (80:20 % by wt) overlying a 20 cm layer of clay soil and a 5 cm clean sand layer at the bottom to prevent particulate leaching. The soil layer was separated from the sand layer by an 'anti-rooting mat'. Non-ionic exchange ceramic suction cups (max. pore ~ 2.5 µm) were located at four depths (15, 40, 65 and 80 cm) to sample in situ biosolid solution at regular time intervals. Leachate was drained at the base tray of each column and also sample at each sampling time. Two metal-accumulating and tolerant species of Salicaceae (Salix reichardtii and Populus balsamifera) previously grown in biosolids for 3 months were transferred into the columns. Each species was grown in triplicate in each of the two substrates. Unplanted columns were used as controls. Solution samples collected from the columns were analysed for pH, EC, dissolved heavy metals, cations, anions and dissolved organic carbon (DOC). Free metal concentrations were calculated using the geochemical modeling program Visual MINTEQ and adjusted by a calibration experiment using a Donnan Membrane Technique (DMT). Plant materials were sampled concurrently with solution samples and analysed for metal concentrations.

The inclusion of industrial waste water (30%) in the domestic sewage stream gives rise to biosolids from the WTP with elevated heavy metal and metalloid contents (mg kg^-1): Al (2950), As (28), Cd (34), Cu (960), Cr (1160), Fe (2780), Mn (127), Ni (304), Pb (881) and Zn (2800). DTPA-extractable heavy metals (mg kg^-1) were Cd (9), Cu (93), Fe (220), Mn (31), Ni (76) and Zn (782). Aged biosolids had low pH_{H2O 1:5} (4.72) and was slightly saline EC (3.73 mS cm^-1). The results of the first 3 months monitoring (Aug to Oct 2005) of the column experiment showed that dissolved heavy metals and metalloids (Al Cd, Cu, Fe, Mn, Ni and Zn) increased significantly with depth and showed tendency to increase over time. Arsenic, Cr and Pb concentrations were below the detection limit of the ICP-OES. The pH of biosolids solution slightly decreased with depth. At 65 cm depth, the dissolved metal concentrations in the columns with plants were higher than the control columns. The solution from S.reichardtii grown in 100% biosolid had the highest concentration of heavy metals compared to the other treatments. Metal concentrations (mg kg^-1 DM) in leaves were Cd (13.5), Cu (10.8), Ni (34.7) and Zn (1680) for P. balsamifera and Cd (14.1), Cu (20.7), Ni (21.9) and Zn (1300) for S. reichardtii. These preliminary results indicated that S. reichardtii has had a significant influence on the bioavailability of heavy metals and the pH of the biosolids.

Keywords: bioavailability, biosolids, heavy metals, speciation, phytoextraction