The effects of fresh versus aged biochar on the leaching of metals from multi-element contaminated soils

Biochar has been proposed as an effective soil amendment for the management of metal(loid)s [1] via a number of mechanisms: directly sorbed on its surface, by the occlusion of soil particles and, in particular, by modifying soil chemistry such as pH or dissolved organic carbon (DOC). An increase of soil pH induced by biochar has been shown to reduce metal solubility, while a release of labile compounds from biochar has resulted in soil metal leaching [2-4]. Different results may therefore be expected depending on the use of "fresh" or "aged" biochar, subject to weathering which can modify the surface and chemical properties of the biochar. Column leaching experiments were conducted in August 2014 by Frédéric Rees and Dr. Luke Beesley with the help of Aline Peregrina Puga at the James Hutton Institute of Aberdeen (UK), with the support of COST Action TD1107. One agricultural soil contaminated by repeated sewage sludge amendments was tested together with one biochar, used as purchased or aged by exposure to natural rainfall. Instead of amending soils with biochar in a single column, a new protocol was tested (Fig. 1), using one column for each material, linked in a loop system so that the retention of metals from soils by the biochar and the modifications of the biochar on the eluent fed back directly to the contaminated soils could be elucidated. Samples of the leaching solution were regularly collected after the soil column and the biochar column, which enabled the chemical evolution of the soil-biochar systems towards equilibrium to be monitored. Chemical equilibration between soils and biochar was fast, as seen by the identical final pH at the different sampling points of the system. Fresh and aged biochar retained metals released from the agricultural soil and decreased further metal release by increasing soil pH. Fresh biochar became however saturated with Zn at the end of the experiment, while aged biochar did not, indicating that aged biochar had a greater sorption capacity for metals. In conclusion, this work indicated that the simple ageing of biochar before its use as a sorbent amendment can increase its metal retention capabilities.