Journal of Hazardous, Toxic, and Radioactive Waste

The objective of the present study was the biosynthesis of nonhazardous and cost-effective silver nanoparticles (AgNPs) using plant extracts of Azadirachta indica (neem), for the removal of hexavalent chromium [Cr (VI)] ions from an aqueous solution. Fourier transform infrared (FT-IR) and energy dispersive X-ray (EDX) spectroscopy characterization results confirmed successful biosynthesis and the presence of elemental silver in the AgNPs. Further, scanning electron microscopy (SEM) and particle size analysis indicated that AgNPs have an amorphous structure and are highly monodispersed. Batch adsorption studies were conducted to investigate the impact of parameters, like pH, time, and initial concentration on Cr (VI) removal efficiency at a fixed AgNP dose of 50 mg and 150 rpm. From the batch study, maximum chromium removal of 87.68% was achieved at an initial concentration of 5 mg/L, pH of 1, and time of 50 min. Results from equilibrium and kinetic study indicated pseudo-second-order reactions kinetics with monolayer coverage. The maximum adsorption capacity of 7.2 mg/g was obtained at an equilibrium time of 50 min and temperature of 30°C. These synthesized AgNPs also displayed significant antibacterial activity against both Gram-positive Bacillus subtilis and Gram-negative Escherichia coli. Based on the results obtained, biosynthesized AgNPs from extracts of A. indica could be utilized for the removal of water pollutants.

Abstract

This research involves the blending of readily accessible waste materials, namely diaper cellulose (DC) and sugarcane bagasse (SB), for a comprehensive thermodynamic and kinetic analysis during co-pyrolysis. Thermogravimetric analysis (TGA) has been conducted to undertake a co-pyrolysis investigation across different ratios of DC and SB. The analysis covered a temperature sequence of 20°C–800°C with varying heat flow rates of 10°C/min, 20°C/min, and 30°C/min. For activation energy determination, the Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW) methods were employed. The average activation energy, calculated using the OFW and KAS methods, was found to have values in the range of 209.46 and 226.109 kJ/mol for DC, 139.08 and 157.41 kJ/mol for 50DC-50SB, and 102.19 and 109.758 kJ/mol for SB, respectively. The activation energy data of the mixtures were shown to be decreasing from the pristine DC. Notably, the mixture exhibited a lower activation energy, indicating a positive synergistic effect resulting from co-pyrolysis. Thermodynamic parameters such as ΔH, ΔS, and ΔG for the mixture were also evaluated. The Gibbs free energy (ΔG) across samples varied between 288.71 and 123.05 kJ/mol for pure DC, DC-SB mixtures, and pure SB, demonstrating a notable influence on co-pyrolysis. The observed decrease in energy levels in the mixture suggests that the combination of sugarcane bagasse with diaper cellulose results in a reduced energy demand.

Practical Applications

The activation energy, determined through thermogravimetric analysis (TGA), delivers valuable insights into the thermal decomposition behavior of materials. The TGA data help in illustrating the thermal constancy of materials, allowing researchers to assess the suitability of substances for specific applications. In industries such as pharmaceuticals, polymers, and chemicals, TGA with activation energy determination can be employed for quality control purposes. The calculated thermodynamic data are crucial in designing and optimizing pyrolysis and gasification processes. They aid in predicting the rate of thermal decomposition, optimizing reaction conditions, and improving the efficiency of these processes for bioenergy production or waste treatment. TGA with activation energy analysis is extensively used in the polymer industry. It assists in understanding the degradation kinetics of polymers, enabling the development of heat-resistant materials for specific applications. Also, these values are essential in catalyst design for chemical reactions. They guide the selection and optimization of catalysts for various processes, ensuring efficient and economical chemical transformations. Determining the activation energy of fuels provides critical information for combustion processes. This is useful in optimizing combustion conditions for energy production, as well as in assessing the stability and safety of fuel storage.

Abstract

Hydrothermal carbonization (HTC) is a promising technology for the conversion of waste biomass into an energy-dense coal-like product known as hydrochar. This study evaluated the application of HTC to generate a renewable solid fuel resource from hazardous oily sludge. The influence of process parameters on the energy characteristics of hydrochar was investigated. A design matrix of 20 experimental HTC runs was used as input for response surface methodology (RSM). The central point for this HTC operation was at a temperature of 200°C, with a duration of 75 min and a concentration of 5.5%. The higher heating value (HHV) increased from 18.18 to 28.7 MJ/kg. A linear correlation was derived between yield and HHV. The influence of process parameters on energy densification and energetic retention properties revealed that energy storage properties were enhanced. In total, 66.6%–80% of energy was retained in hydrochar. The enhancement of the fuel characteristics of hydrochar was demonstrated by the elevated fuel ratio of 0.31 obtained at a temperature of 250°C, thereby indicating the improvement brought about by the HTC process. The quantification of energy losses due to convection and radiation yielded 309 and 1,854 kJ/h, respectively. Maximizing the efficiency of internal heat sources is crucial to conserving energy and reducing costs in HTC.

Practical Applications

The activities of the oil industry have several impacts on the environment due to the large amounts of oily waste generated. Oily sludge is a prevalent hazardous waste generated in the petroleum industry, and effectively treating it remains a key challenge for the petroleum and petrochemical sectors. Hydrothermal carbonization is an important thermochemical conversion process that can be used as an energy-efficient alternative to enhance the dewaterability of sludge and meanwhile to convert sludge into high-value-added products, such as clean biofuel, organic fertilizers, and precursors of functional materials. Optimizing the process conditions is essential to successfully implement the HTC processing of oily sludge in real field conditions. In this study, waste oily sludge was converted to an energy-rich product, namely, hydrochar. For this, the study investigated the hydrochar characteristics for hydrothermal conditions including a reaction temperature of 175°C–250°C, a reaction time of 30–120 min, and a solid load of 3%–8%. The study also found that the majority of energy loss occurred during the cooling process and therefore suggests that this process might be operated as a continuous process rather than a batch process.

In this study, we employed synthesized titanium oxide/zirconium oxide (TiO₂/ZrO₂) nanoparticles as activated catalysts to enhance the ozonation process during hydroxyl radical (OH°) generation using atrazine (ATZ) as a pollutant. The TiO₂/ZrO₂ nanoparticles were synthesized using the coprecipitation digestion method and were characterized using scanning and transmission electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller theory, and energy dispersive X-ray spectroscopy. The experiments were conducted under different initial pH conditions (3, 7, and 10), with an initial ATZ concentration of 4.64 × 10⁻⁶ M, initial TiO₂/ZrO₂ nanocatalyst concentrations of 100, 200, and 300 ppm, and initial ozone (O₃) concentrations of 1.35 × 10⁻⁴, 1.55 × 10⁻⁴, and 2.27 × 10⁻⁴ M, respectively. The key operational parameters were optimized, and the optimal OH° results were obtained at pH 10, 200 ppm TiO₂/ZrO₂ concentration, 4.64 × 10⁻⁶ M ATZ concentration, and 1.55 × 10⁻⁴ M O₃ concentration. The potential of TiO₂/ZrO₂ nanoparticles to enhance OH° generation during catalytic ozonation was demonstrated. The catalytic ozonation mechanism using TiO₂/ZrO₂ nanoparticles was governed by the optimal formation of OH° radicals at about 1 × 10⁻¹¹ mol/L and an excellent nanocatalytic activity with an R_ct value of 2.83 × 10⁻⁸.