4. Conclusions
CES presented a liming capacity able to shift soil pH to values higher than 6. The environmental availability of Pb and Zn in the contaminated soil was ameliorated, when considering the metals mobile and mobilisable fractions. However, in acid conditions CES effectiveness was only clearly proven for Zn immobilisation. Reducible and residual soil fractions were the major sinks for zinc retention in soil amended with CES at higher application rates (CES_T3), but in this case the CO2 emissions were about 6-fold higher than in untreated soil. It is important to strengthen that the environmental risk decreased from “medium” to “low level” for both heavy metals at any application rate of CES.
In comparison to CWES, CES showed more effective characteristics to promote soil acidity correction and metals immobilisation. Nevertheless, the ability of ES to promote remediation of the tested soil was not effectively overcome by CES.
In summary, CES was found to be an effective liming material and revealed intrinsic properties for in situ remediation of soil contaminated with Pb and Zn. Nevertheless, due to the stable organic matter present within CES, its use could be more beneficial to metal-contaminated soils that also require restoration of organic matter levels, thus contributing to a long term soil quality and fertility.