Synthesis and evaluation of biochar based zinc oxide(ZnO) composite as a slow release fertilizer

Abstract

Biochar is a carbon-rich material produced from organic residues through the pyrolysis process under limited oxygen supply. The versatile properties of biochar include high porosity, high surface area, and an abundance of surface functional groups, making it beneficial as a soil amendment, for carbon pool, for heavy metal adsorbent, as well as slow-release fertilizer. The study entitled “Synthesis and Evaluation of Biochar-Based Zinc Oxide Composite as a Slow- Release Fertilizer” was conducted in the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellanikkara during 2022-2024. This study comprised of two experiments. The first experiment, the coconut fronds (CF) were used as feedstocks for biochar production at two different temperatures, viz 450℃ and 650℃. The biochar obtained at respective temperatures was evaluated for their physicochemical properties, including pH, EC, volatile matter, ash content, and macro- and micronutrient composition, to determine variations based on temperature. The finely sieved biochar undergone surface characterisation techniques such as Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) analysis, and X-ray Diffraction (XRD) to assess surface morphology, functional groups, surface area, and crystallinity, respectively. In the experiment, from CF 450 and CF 650 biochar, corresponding biochar composites (BBZOC 450 and BBZOC 650) were prepared using a biochar-sodium hydroxide solution and a zinc nitrate-starch solution in a 1:1 ratio. The prepared BBZOC were also analysed for their physicochemical properties. The physical properties showed that CF 450 biochar had a higher recovery percentage (35.80%) than CF 650 biochar (28.30%). CF 650 biochar exhibited higher ash content (17.90%), bulk density (0.131 g/cm³), pH (10.70), EC (6.7 dS/m), and pHzpc (10.80), while CF 450 biochar had lower values for these parameters (16.66% ash content, 0.13 g/cm³ bulk density, pH 10.34, EC 4.50 dS/m, and pHzpc 9.50). The macro- and micronutrient concentrations decreased with increasing pyrolysis temperature. The mineral composition of CF 650 biochar were 55.95% carbon, 1.63% hydrogen, 0.56% nitrogen, 25.20% oxygen, 0.25% S, 0.28% P, 1.142% K, 1.54% Ca, and 2.52% Mg, along with trace amounts of Fe, Mn, Zn, Cu, and B. CF 450 biochar contained 50.86% carbon, 2.36% hydrogen, 0.4% nitrogen, 28.14% oxygen, 0.22% S, 0.22% P, 0.90% K, 1.25% Ca, and 2.26% Mg, along with trace amounts of Fe, Mn, Zn, Cu, and B. The FTIR characterisation techniques depicted the presence of more functional groups in CF 450 biochar as compared to CF 650 biochar. The XRD spectra confirmed the presence of KCl and SiO₂ in both CF 450 and CF 650, with the indication of broad wide spectra. The BET analysis exhibited a greater surface area for CF 650 biochar, with more number of pores concentrated below the 10 nm range, whereas CF 450 biochar showed more number of pores concentrated in the 10– 100 nm range. The elemental ratio analysis of biochar exhibited higher aromaticity, hydrophilicity, and polarity for CF 650 biochar than CF 450 biochar. The physico-chemical analysis of BBZOC showed a decline in the pH, with values viz. 7.36 for BBZOC 650 and 7.91 for BBZOC 450. The results also showed an increase in bulk density in the BBZOC (from 0.13 to 0.23 g/cm³ in BBZOC 650 and from 0.131 to 0.19 g/cm³ in BBZOC 450) due to the formation of zinc oxide. The surface characterisation techniques revealed an enhanced surface area with a decreased pore diameter in BBZOC 450, whereas BBZOC 650 exhibited a decreased surface area. The presence of white deposits on the BBZOC surface in SEM analysis and the appearance of 536 cm⁻¹ and 499 cm⁻¹ peaks in FTIR spectra confirmed ZnO bonding and the formation of zinc oxide on the biochar. Elemental ratio analysis indicated increased aromaticity, hydrophilicity, and polarity in BBZOC due to oxygen addition. Water absorbance, swelling ratio, equivalent water content, and water retention studies showed that BBZOC 450 exhibited slightly better water absorbance and retention compared to corresponding CF 450 biochar. In the second experiment, biochar and its corresponding BBZOC were used to study the slow-release pattern of zinc in both water and soil. In the water column study, leachates were analyzed at 1, 3, 5, 10, 15, and 30 days. Similarly, in the soil column study, leachate were analyzed at 1, 3, 5, 10, 15, 30, and 60 days using five different treatments (10 g, 20 g, 30 g, and 40 g of BBZOC). The results of soil column showed a cumulative release of zinc and zinc content were estimated as 1665.53 mg/L and 2054.09 mg/L for BBZOC 650 and BBZOC 450, respectively. In the water column study, cumulative zinc release was 273.608 mg/L and 411.480 mg/L for BBZOC 650 and BBZOC 450, respectively. The data thus obtained from both the soil and water column study picturizes that the BBZOC 450 exhibited greater zinc release compared to BBZOC 650, with the highest release at an application rate of 40 g BBZOC treatment. The results confirmed that BBZOCs can function as an efficient slow-release fertilizer, providing gradual zinc release over time.