Investigation of slow pyrolysis and hydrothermal carbonization processes effects on the stabilizing properties of cypress cones in order to stabilize nickel in a calcareous soil

In the present study, the effects of biochar (produced by slow pyrolysis process) and hydrochar (produced by hydrothermal carbonization process) of cypress cones application on nickel (Ni) stabilization in a soil spiked with this element were investigated. For this purpose, the raw cypress cone and its biochar and hydrochar are added separately at 1.5 and 3% (w/w) to a Ni-spiked calcareous soil (350 mg kg-1) and after 3 months of incubation process, under field capacity moisture condition, desorption kinetics and chemical forms of Ni, in the laboratory of environment department, graduate university of advanced technology, Kerman (2020), were measured and investigated. According to the results, hydrochar had higher specific surface area and functional groups containing reactive oxygen, as well as more irregular porosity morphology compared to biochar. The values ​​of final desorbed Ni in the treated soils showed a decrease of 27-37% and 11-16.5% of desorbed Ni in the soil treated with hydrochar and biochar, respectively, compared to the control sample. The fitness of the two first-order reaction model on Ni desorption data in all treated samples showed the high accuracy of this model (coefficient of determination>99%) in predicting Ni desorption. Evaluation of Ni mobility factor obtained from the Ni chemical forms shows a decrease in this factor in samples of soils treated with hydrochar (29.2-30.2%) and biochar (31.9-33.3%) in comparison with control soil sample (38.8%). In general, the presence of functional groups containing reactive oxygen and higher specific surface area of ​​hydrochar compared to biochar has resulted in more stabilization of Ni in the soil compared to treated soils with biochar. Due to lower production costs and reduced production of destructive greenhouse gases in hydrochar production compared to biochar, the need for more attention to hydrochar and engineered hydrochar application is required in future studies of heavy metals stabilization in the soil environment.