Testing graphene versus classical soil improvers in a sandy calcisol

In this study for the first time, Graphene was tested in a leaching batch experiment on a calcareous sandy soil (Calcisol) to assess its capacity to ameliorate soil's properties in comparison with traditional soil improvers (Compost, Biochar, and Zeolites). A first set of experiments was performed in triplicates using the saturation soil extraction method (5:1 liquid–solid ratio) with deionized water as liquid phase and two different rates (minimum and maximum) of each improver. A second set of experiments (5:1 liquid–solid ratio) was performed to understand the fate of fertilizers, using the maximum amount of soil improvers and two different liquid phases: NPK solution from a synthetic fertilizer and fertigation water from a wastewater treatment plant. EC, pH, ORP and dissolved O2 were monitored daily for two weeks, and water samples were obtained at the end of the experiment to analyze DOC, major ions, and trace elements. Results showed that Graphene neither altered the physical–chemical parameters nor released nutrients and heavy metals in the solution in comparison to control and other improvers. Biochar increased EC and pH beyond recommended limits for some plants growth, while Compost released NO3– above threshold limits. Batches with the NPK solution showed NH4+ accumulation in the solute for controls, and to a less extent for Zeolites, Graphene and Biochar, while no accumulation was recorded for Compost. NH4+ accumulation was triggered by microbial N remineralization in absence of a C source. Batches with fertigation water showed NO2– accumulation in the solute due to incomplete nitrification in all the improvers and control batches, since the available C source consumed O2 that in turn hampered the nitrification process. Despite the above-mentioned limitations, Graphene performed well in comparison to the other soil improvers in reducing nutrients and metals leaching.