A suite of techniques is compared to understand the requirements, differences, synergies, and limitations of each method more comprehensively. Our review summarizes key principles to guide studies on the structure of cellulosics.

Mitigation of toxic contaminants from wastewater is crucial to the safety and sustainability of the aquatic ecosystem and human health. There is a pressing need to find economical and efficient technologies for municipal, agricultural, aquacultural, and industrial wastewater treatment. Nitrogen-doped biochar, which is synthesized from waste biomass, is shown to exhibit good adsorptive performance towards harmful aqueous contaminants, including heavy metals and organic chemicals. Incorporating nitrogen into the biochar matrix changes the overall electronic structure of biochar, which favors the interaction of N-doped biochar with contaminants. In this review, we start the discussion with the preparation techniques and raw materials used for the production of N-doped biochar, along with its structural attributes. Next, the adsorption of heavy metals and organic pollutants on N-doped biochars is systematically discussed. The adsorption mechanisms of contaminant removal by N-doped biochar are also clearly explained. Further, mathematical analyses of adsorption, crucial to the quantification of adsorption, process design, and understanding of the mechanics of the process, are reviewed. Furthermore, the influence of environmental parameters on the adsorption process and the reusability of N-doped biochars are critically evaluated. Finally, future research trends for the design and development of application-specific preparation of N-doped biochars for wastewater treatment are suggested. Graphical abstract

Sustainable waste utilization techniques are needed to combat the environmental and economic challenges faced worldwide due to the rising population. Biochars, due to their unique surface properties, offer opportunities to modify their surface to prepare application-specific materials. The aim of this research is to study the effects of biochar surface modification by oxidizing chemicals on biochar properties. Pine bark biochar was modified with sulfuric acid, nitric acid, hydrogen peroxide, ozone, and ammonium persulfate. The resulting biochars’ pH, pH at the point of zero charges, and concentration of acidic and basic sites were determined using laboratory experimentation. Instrumental techniques, such as infrared and X-ray photoelectron spectroscopy, were also obtained for all biochar samples. X-ray photoelectron spectra showed that oxygen content increased to 44.5%, 42.2%, 33.8%, 30.5%, and 14.6% from 13.4% for sulfuric acid, ozone, nitric acid, hydrogen peroxide, and ammonium persulfate, respectively. The pH at the point of zero charges was negatively correlated with the difference in concentration of acidic and basic sites in biochar samples, as well as the summation of peak components representing C=O double bonds and carboxylic groups. The results suggest that designer biochars can be prepared by understanding the interaction of oxygenated chemicals with biochar surfaces.

Eutrophication and algal blooms are serious issues plaguing the environment. Modified pine-derived biochar can be used as an innovative sorbent to address this problem by removing NO₃⁻ from aqueous solutions. In this study, pine bark nuggets were impregnated by Magnesium Chloride (MgCl₂.6H₂O) and subsequently pyrolyzed at 600 °C. The physicochemical characteristics of this modified biochar (MB) were characterized by wet chemistry techniques such as, acid value and point of zero charge. In addition, thermal, spectral, and microscopic methods like elemental analysis, specific surface area, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photon spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), elemental mapping by scanning transmission electron microscopy (STEM), and time of flight-secondary ion mass spectrometry (ToF-SIMS) were used to characterize the physicochemical properties of the prepared adsorbents. The characterization results of the unmodified and magnesium modified biochar samples were compared to understand the changes in the biochar properties due to magnesium activation. The data validated the successful magnesium (Mg) impregnation of the biochar and revealed that most of the Mg in MB was distributed in the form of MgO-flakes. In addition, MB was used to carry out batch adsorption of nitrate (NO₃⁻) from the aqueous solution, and the experimental data were fitted to different adsorption isotherm models (Langmuir, Freundlich, and Sips) and adsorption kinetic models (pseudo-first order and pseudo-second order). The results suggest a maximum NO₃⁻ adsorption capacity of 14.40 mg/g suggesting monolayer adsorption as described by the Langmuir model. Additionally, the spent biochar after nitrate adsorption was analyzed by XPS, and ToF-SIMS to validate the successful adsorption of NO₃⁻ from the aqueous solution. The spent biochar characterization and kinetics data revealed that adsorption mechanism followed a complex physicochemical pathway. Our study revealed that pine bark could be used to prepare and modify low-cost adsorbent materials for wastewater systems in industrial settings.

Efficient and economical wastewater treatment has presented itself as a global challenge. In this context, adsorption is one of the most effective methods to remove contaminants from wastewater. The present study evaluated the feasibility of chemically modified pine bark biochar’s nitrate adsorption ability. Pine bark biochar was modified with urea and sulfuric acid to remove nitrate from an aqueous solution. The physicochemical properties of the biochar samples, such as pH, pH at point of zero charges, surface atomic composition, surface morphology, and surface area, were evaluated. The equilibrium adsorption data were fitted to the Langmuir and Freundlich isotherm models. The kinetic data were fitted to different kinetic models (pseudo-first order, pseudo-second order, intraparticle diffusion, and Elovich). The adsorption data fitted well with the Langmuir and pseudo-first order models. The maximum nitrate adsorption capacity was found to be 1.548 mg g⁻¹. Mass transfer studies were conducted to identify the rate-limiting step, values of the external mass transfer coefficient, and diffusion coefficient in the nitrate adsorption process by the modified biochar. The external mass transfer coefficients were in the range of 2.2 × 10^–11–2.86 × 10^–10 m s⁻¹. The intraparticle diffusion coefficient ranged from 6.53 × 10^–10 to 1.78 × 10^–9 m² s⁻¹. The Biot number value less than 100 indicated that the adsorption was controlled by film diffusion. Interaction energies between nitrate ions and model biochar structures were calculated DFT-based quantum chemical software (Gaussian). The positive interaction energy values (2.3485–2.485 eV) suggested nitrate adsorption on model biochar structures was thermodynamically not feasible. Graphical Abstract