Changes in soil structure and sorption capacity after mixed treatment with macromolecular compounds and orange peels-derived activated carbons

Soil provides 95% of our food and provides other essential ecosystem services, such as water purification, biodiversity, and climate regulation. Unfortunately, numerous agroecological functions of soil are increasingly threatened by the intensifying, primarily anthropogenic, processes of soil degradation. This deteriorates the surface, sorption, and buffering properties of soils, the spread of pollutants into watercourses and groundwater, and adverse changes in porosity, organic matter composition and content, wettability, aggregation, and microbial community, resulting in soil partially or completely losing its ability to function properly. Therefore, it is so important to develop new soil conditioners that can reduce the effects of anthropogenic pressure and make soils more resistant to negative phenomena.

The main aim of this study was to estimate the impact of newly developed biochars and activated carbons from orange peels as well as water-soluble polymers (exopolysaccharide of bacterial origin (Rhizobium leguminosarum bv. trifolii), ionic polyacrylamides) on the structure and sorption capacity of the selected soil. Haplic Luvisol, the most common Polish soil, was collected from 0–20 cm depth of arable land in Poland (Parchatka, Lublin Upland, N 51°22′54″ E 21°59′54″). It was derived from loess parent material. It was modified with 1 wt.% of solid modifier (biochar, or activated carbon) by mixing. Macromolecular compounds (of initial concentration 100 mg/L) were added in the form of solutions. The following parameters: pH, ash content, total organic carbon content, porosity, variable surface charge of the soil were measured before and after modification to estimate effectiveness of the performed treatment. The soil sorption capacity was examined towards copper (Cu) and cadmium (Cd) using a batch adsorption method. The metal concentration was determined using a atomic absorption spectrometer working in the graphite cuvette technique (ContrAA 800, Analytik Jena, Germany). Porosity of the soils was examined using a mercury porosimetry (autopore IV 9500, Micrometrics INC, USA).

It was observed that all modifications using carbonaceous materials improved total pore area, average pore diameter, and total porosity of the soil, which was mainly associated with highly porous structure and relatively large specific surface area of the applied solids. The modification with activated carbon and cationic polyacrylamide resulted in the highest increase in total pore area. Carbon-rich materials could not only increase specific surface area and porosity of the soil, but also form organo-mineral connections, improving the number of active centers. Consequently, they increase soil sorption capacity towards Cu and Cd. The activated carbon application improved their 3- and 1.9-fold adsorption, respectively. The presence of polymers further increased their adsorption on the soil.

The research was founded by National Science Centre, Poland (2021/41/B/NZ9/03059).