Application of olive mill waste-based biochars in agriculture: Impact on soil properties, enzymatic activities and tomato growth

Abstract

The olive oil industry is an important economic sector in Mediterranean countries. However, oil production is unfortunately accompanied by the generation of huge amounts of olive mill solid wastes (OMSW) and olive mill wastewater (OMWW). In the present study, a strategy is proposed for converting these olive mill wastes into biochar through pyrolysis, for their later use as an organic amendment in agriculture. Specifically, two biochars were prepared from the pyrolysis of OMSW at 500°C, either alone or impregnated with OMWW (OMSW-B and I-OMSW-B).The characterization of the OMSW and I-OMSW samples and their derived biochars showed that the fixed carbon and ash contents in the feedstocks increased by 38% and 11% respectively for OMSW-B, and by 37% and 12% respectively for I-OMSW-B. Interestingly, the impregnation process significantly increased Na, P, K, Ca and Fe contents in the produced biochars. The effect of OMSW-B and I-OMSW-B amendments at different application dose (1%, 2.5% and 5% wt/wt) on the enzymatic activity of an agricultural soil was performed at laboratory scale with a pot test. The experimental results showed that phosphatase and urease activity increased with biochar application rate; amendment with I-OMSW-B at 1%, 2.5% and 5% enhanced the phosphatase activity by 63%, 142% and 285% and urease activity by 50%, 116% and 149%, respectively. On the other hand, dehydrogenase and protease activities were higher for the application rate of 2.5 % biochar. Biochar amendment promoted tomatoes seedling growth after 10 weeks, which was highest in the application rates of 2.5% and 5% for both OMSW-B and I-OSMW-B. Thus, the produced biochars had great potential to be used as biofertilizers in agriculture.

Olive Mill by-Products Thermochemical Conversion via Hydrothermal Carbonization and Slow Pyrolysis: Detailed Comparison between the Generated Hydrochars and Biochars Characteristics

“…with the release of CO, CO 2 and H 2 O species. The rest organic fragments such as phenols, lignin can lead also to the release other species such as CH 4 , H 2 [42]. A similar outcome was reported by Jian et al [43] when studying the production of biochars from rice husk under different temperatures.…”

“…On the other hand, the peaks corresponding to aromatic groups, notably the aliphatic =C-H at 2870 cm −1 , C-H carbohydrates and lignin deformation group at 1650 cm −1 and aromatic band at 876 cm −1 remained unchanged even at higher pyrolysis temperature. According to Zhang et al [77], in case of an initial abundance of functionalities, this behavior could be attributed to the pyrolysis mechanism for the condensation of carbon in the solid fraction and the release of oxygen in the form of CO 2 gas [42,78], suggesting the maintenance of the aromatic skeleton of the lignin structure at pyrolysis temperatures varying between 400 and 600 • C. Therefore, it appears that the biochar materials have significantly poorer surface chemistry than hydrochars.…”

“…Processes 2022, 10, x FOR PEER REVIEW 13 of 19 bon in the solid fraction and the release of oxygen in the form of CO2 gas [42,78], suggesting the maintenance of the aromatic skeleton of the lignin structure at pyrolysis temperatures varying between 400 and 600 °C. Therefore, it appears that the biochar materials have significantly poorer surface chemistry than hydrochars.…”

“…Consequently, oxygen-based groups concentrations decrease significantly with increasing carbonization temperature, thus yielding a surface with less acidic groups; (ii) the rearrangement of the aromatic structure into more fixed carbon content and even towards graphite-structure goes along with the immobilization of alkaline and alkaline earth metals (Ca 2+ , Mg 2+ , K + , Na + , etc.) in crystalline forms [42,72,80]. The presence of these ions, even at low concentrations, leads to more alkaline biochar surface.…”

Thermal behaviour of impregnated olive stones with phosphoric acid via TGA-MS

“…These biomass materials can be turned into valuable materials through pyrolysis producing solid biofuels (e.g. charcoal, chips), biochar [24][25][26], liquid products (ethanol, pyrolytic oils vegetable oil ester, agricultural water source) [27,28] or gaseous fuel (biogas, poor gas) [27,28]. It could be also transformed into activated carbon widely used in industry for water and waste gaseous treatment [1,14,16,29].…”