Synthesis, characterization and evaluation of biochar based slow release multi nutrient fertilizer

Abstract

Biochar is a black, finely-grained, extremely porous carbon-rich solid material usually produced by pyrolysis which involves the thermal decomposition of the material in the partial or complete absence of oxygen. It is mainly obtained from plant-derived residues and is an easy and cost-effective method to convert agricultural wastes into value-added products. The high porosity and pore structure of the biochar enable to adsorb nutrients and release slowly to avoid loss of nutrients. The study entitled “Synthesis, characterization, and evaluation of biochar based slow release multi nutrient fertilizer” was carried out in the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellanikkara during 2021-22 to synthesize a biochar based slow release fertilizer and to understand its release mechanism in water and soil. The study consisted of two experiments. First experiment was the synthesis and characterization of biochar based slow release fertilizer. Three different biochars, viz., coconut biochar (CB), banana biochar (BB), and sugarcane biochar (SB) was prepared from the raw materials coconut husk, banana pseudo stem, and sugarcane trash respectively. Among three biochars, CB had highest recovery per cent of 23.25%. The moisture content (10.42%) and bulk density (0.22 Mg m-3) was highest for CB, whereas, ash content (35.7%), particle density (1.17 Mg m-3), pore space (68.08%), and water holding capacity (590 %) was highest for BB. The pH ranged from 9.48 to 9.82 showing the alkaline nature of the biochar and the range of EC, CEC, and pHpzc was 1.73 to 3.46 dS m-1, 11.6 to 17.67 cmol (p+) kg-1, and 8.4 to 9.1 respectively. From the adsorption study, it was found that, all the nutrients (N, Zn, Fe, Mn, Cu, B and Mo), except K, was adsorbed by all the three biochars. The best ratio of biochar and multi nutrient mixture for the adsorption was 10:0.5. The data was fitted into Freundlich and Langmuir models. The coefficient of determination (R2) value of these models showed that, the adsorption of all nutrients except Mo by CB was fitted best to the Langmuir isotherm model showing monolayer adsorption on homogeneous sites. Adsorption of all the nutrients by BB and SB fitted best to Freundlich isotherm model showing multilayer adsorption on heterogeneous sites. From the kinetics study, 180 minutes was selected as the optimum time for the adsorption of most of the nutrients into biochar. The kinetics data was switched into pseudo-first-order and pseudo-second-order kinetics models. The three biochars followed pseudo-second-order model, which showed the chemical adsorption of nutrients. The biochar based multi nutrient fertilizers (BBMFs) was prepared with 10:0.5 ratio and 180 minutes as optimum time and its characterization was carried out. The Brunauer-Emmet-Teller (BET) analysis showed that, SB has the highest surface area and total pore volume. The Barrett-Joyner-Halenda (BJH) plots indicated the existence of nano sized pores (1-100 nm). The C/H ratio showed that, CB had more aromaticity, O/C and (O+N)/C ratios indicated that BB had more hydrophilicity and polarity, and C/N ratio was wider for CB. The moisture content, BD, PD, porosity, and all the nutrient content were found high for BBMFs than the normal biochar, whereas, pH and EC were decreased. Also, the BBMFs had more water absorbance, swelling ratio, and equilibrium water content than the normal biochar with a maximum of 89.5 per cent for BBBMF, 5.96 g g-1 for SBBMF, and 95.8 per cent for SBBMF respectively. The water retention study proved that, the per cent of water retained was high when the soil was mixed with BBMF than the soil taken alone. The Scanning Electron Microscopy (SEM) images showed the porous structure of all biochar and the appearance of white coating and reduction in pore size in BBMFs due to the adsorption of nutrients. Atomic Force Microscopy (AFM) confirmed that the size of the particle’s growth was up to 45.8 nm for CBBMF, 27.3 nm for BBBMF, and 86 nm for SBBMF and the average surface roughness was 3.01 nm for CBBMF, 1.76 nm for BBBMF, and 3.83 nm for SBBMF. The Fourier Transformed Infra-Red (FTIR) spectra identified the functional groups, mainly carboxyl, hydroxyl, and carbonyl groups on the surface of biochar and shift in peaks and appearance of new peaks in the BBMFs spectra due to the impregnated nutrients. X-ray diffraction (XRD) analyzed the crystal structure of biochar and showed that the biochar had an amorphous carbon structure and highly crystalline inorganic fraction, whereas, for BBMFs, most peaks was disappeared due to reduction in intensity because of the adsorption of nutrients into amorphous C structure. The Thermogravimetric Analysis (TGA) proved that, prepared BBMFs had more thermal stability compared to the normal biochar. In the second experiment, the release pattern of nutrients in water and soil was analyzed at specific intervals of 1, 3, 5, 10, 15, and 30 days and 1, 2, 3, 5, 10, 15, 30, 45, and 60 days respectively. From the water column study, it was found that, BBMFs released nutrients in water upto 30th day and when nutrients were given alone, the release was only for 3 to 15 days. Also, the days of release was increased with the increase in the amount of BBMF. In soil column study, treatments containing BBMF released nutrients upto 60th day which was more than all other treatments. Thus, the dissolution study revealed that the BBMFs acted as slow release fertilizer. In the present study, BBMFs were synthesized using a simple and cost-effective methodology. All the BBMFs had high water retention, water absorbance, equilibrium water content and stability compared to biochars. The BBMFs had great potential to be used as a slow release fertilizer. Among the three BBMFs, SBBMF was found to be the best compared to other BBMFs in nutrient adsorption and dissolution due to the innate properties of sugarcane biochar.