Insights into interfacial interaction mechanism of dyes sorption on a novel hydrochar: Experimental and DFT study

Molecular simulation-based insights into dye pollutant adsorption: A perspective review

Growing concerns about environmental pollution have highlighted the need for efficient and sustainable methods to remove dye contamination from various ecosystems. In this context, computational methods such as molecular dynamics (MD), Monte Carlo (MC) simulations, quantum mechanics (QM) calculations, and machine learning (ML) methods are powerful tools used to study and predict the adsorption processes of dyes on various adsorbents. These methods provide detailed insights into the molecular interactions and mechanisms involved, which can be crucial for designing efficient adsorption systems. MD simulations, detailing molecular arrangements, predict dyes' adsorption behaviour and interaction energies with adsorbents. They simulate the entire adsorption process, including surface diffusion, solvent layer penetration, and physisorption. QM calculations, especially density functional theory (DFT), determine molecular structures and reactivity descriptors, aiding in understanding adsorption mechanisms. They identify stable adsorption configurations and interactions like hydrogen bonding and electrostatic forces. MC simulations predict equilibrium properties and adsorption energies by sampling molecular configurations. ML methods have proven highly effective in predicting and optimizing dye adsorption processes. These models offer significant advantages over traditional methods, including higher accuracy and the ability to handle complex datasets. These methods optimize adsorption conditions, clarify adsorbent functionalization roles, and predict dye removal efficiency under various conditions. This research explores MD, MC, QM, and ML approaches to connect molecular interactions with macroscopic adsorption phenomena. Probing these techniques provides insights into the dynamics and energetics of dye pollutants on adsorption surfaces. The findings will aid in developing and optimizing new materials for dye removal. This review has significant implications for environmental remediation, offering a comprehensive understanding of adsorption at various scales. Merging microscopic data with macroscopic observations enhances knowledge of dye pollutant adsorption, laying the groundwork for efficient, sustainable removal technologies. Addressing the growing challenges of ecosystem protection, this study contributes to a cleaner, more sustainable future.

Development of a sustainability-oriented university laboratory: Insight into adsorption kinetics models for the removal of pollutants from aqueous solution

The aim of the present research is to show the development of a sustainability-oriented lab that teaches adsorption concepts in a virtual environment based on the premise "learning-through-play". Kinetic results in the virtual environment are contrasted to those obtained experimentally when diverse adsorbents prepared from Agave Bagasse (Raw Fibers, Hydrothermal Fibers, and Activated Fibers) were synthesized. Comparison between virtual and real-life experiments involving removal of methylene blue in solution showed that a pseudo-first-order model could describe adsorption kinetics satisfactorily. The study is complemented with a characterization of the adsorbents through SEM, nitrogen adsorption isotherms, FTIR and Raman. In addition, the environmental impact of the synthesis of adsorbents was evaluated through well-known methodologies (GAPI, NEMI, and Eco-Scale), which agree that raw fibers are the most eco-friendly material. This research provides an exciting opportunity to advance our knowledge on developing new technologies for teaching in engineering and to compliment real-life practices that consider environmental impacts with virtual experiments.

Enhanced adsorption of dye wastewater by low-temperature combined NaOH/urea pretreated hydrochar: Fabrication, performance, and mechanism

The highly stable biomass structure formed by cellulose, hemicellulose, and lignin results in incomplete conversion and carbonization under hydrothermal conditions. In this study, pretreated corn straw hydrochar (PCS-HC) was prepared using a low-temperature alkali/urea combination pretreatment method. The Mass loss rate of cellulose, hemicellulose, and lignin from pretreated biomass, as well as the effects of the pretreatment method on the physicochemical properties of PCS-HC and the adsorption performance of PCS-HC for alkaline dyes (rhodamine B and methylene blue), were investigated. The results showed that the low-temperature NaOH/urea pretreatment effectively disrupted the stable structure formed by cellulose, hemicellulose, and lignin. NaOH played a dominant role in solubilizing cellulose and the combination of low temperature and urea enhanced the ability of NaOH to remove cellulose, hemicellulose, and lignin. Compared to the untreated hydrochar, PCS-HC exhibited a rougher surface, a more abundant pore structure, and a larger specific surface area. The unpretreated hydrochar exhibited an adsorption capacity of 64.8% for rhodamine B and 66.32% for methylene blue. However, the removal of rhodamine B and methylene blue by PCS-BC increased to 89.12% and 90.71%, respectively, under the optimal pretreatment conditions. The PCS-HC exhibited a favorable adsorption capacity within the pH range of 6-9. However, the presence of co-existing anions such as Cl-, SO42-, CO32-, and NO3- hindered the adsorption capacity of PCS-HC. Among these anions, CO32- exhibited the highest level of inhibition. Chemisorption, including complexation, electrostatic attraction, and hydrogen bonding, were the primary mechanism for dye adsorption by PCS-HC. This study provides an efficient method for utilizing agricultural waste and treating dye wastewater.

A review: Hydrochar as potential adsorbents for wastewater treatment and CO2 adsorption

To valorize the biomass and organic waste, hydrothermal carbonization (HTC) stands out as a highly efficient and promising pathway given its intrinsic advantages over other thermochemical processes. Hydrochar, as the main product obtained from HTC, is widely applied as a fuel source and soil conditioner. Aside from these applications, hydrochar can be either directly used or modified as bio-adsorbents for environmental remediation. This potential arises from its tunable surface chemistry and its suitability to act as a precursor for activated or engineered carbon. In view of the importance of this topic, this review offers a thorough examination of the research progress for using hydrochar and its modified forms to remove organic dyes (cationic and anionic dyes), heavy metals, herbicides/pesticides, pharmaceuticals, and CO2. The review also sheds light on the fundamental chemistry involved in HTC of biomass and the major analytical techniques applied for understanding surface chemistry of hydrochar and modified hydrochar. The knowledge gaps and potential hurdles are identified to highlight the challenges and prospects of this research field with a summary of the key findings from this review. Overall, this article provides valuable insights and directives and pinpoints the areas meriting further investigation in the application potential of hydrochar in wastewater management and CO2 capture.

Significant Differences in the Effects of Nitrogen Doping on Pristine Biochar and Graphene-like Biochar for the Adsorption of Tetracycline

To improve the adsorption efficiency of pollutants by biochar, preparing graphene-like biochar (GBC) or nitrogen-doped biochar are two commonly used methods. However, the difference in the nitrogen doping (N-doping) effects upon the adsorption of pollutants by pristine biochar (PBC) and GBC, as well as the underlying mechanisms, are still unclear. Take the tetracycline (TC) as an example, the present study analyzed the characteristics of the adsorption of TCs on biochars (PBC, GBC, N-PBC, N-GBC), and significant differences in the effects of N-doping on the adsorption of TCs by PBC and GBC were consistently observed at different solution properties. Specifically, N-doping had varied effects on the adsorption performance of PBC, whereas it uniformly improved the adsorption performance of GBC. To interpret the phenomenon, the N-doping upon the adsorption was revealed by the QSAR model, which indicated that the pore filling (VM) and the interactions between TCs with biochars (Ead-v) were found to be the most important two factors. Furthermore, the density functional theory (DFT) results demonstrated that N-doping slightly affects biochar's chemical reactivity. The van der Waals (vdWs) and electrostatic interactions are the main forces for TCs-biochars interactions. Moreover, N-doping mostly strengthened the electrostatic interactions of TCs-biochars, but the vdWs interactions of most samples remained largely unaffected. Overall, the revealed mechanism of N-doping on TCs adsorption by biochars will enhance our knowledge of antibiotic pollution remediation.

The opposite influences of Cu and Cd cation bridges on sulfamethoxazole sorption on humic acids in wetting-drying cycles

Wetting-drying cycles in the environment could change the inner- or outer-sphere complexation of heavy metal cations on natural organic matter (NOM) and then influence ternary interactions with organic contaminants - a rarely-discussed essential geochemical process. In this work, the sorption of sulfamethoxazole (SMX) on humic acids (HAs) mediated by cations (Cu2+ and Cd2+) was investigated. Considering that outer-sphere complexation could be transformed into inner-sphere complexation during vacuum freeze-drying, the role of inner- or outer-sphere complexation on SMX sorption was explored. The experimental sorption results and density functional theory (DFT) calculations suggested that Cu2+ and Cd2+ sorption on HAs was mainly outer- and inner-sphere complexation, respectively. Cd2+ consistently promoted SMX sorption on HAs, while Cu2+ promoted and inhibited SMX sorption before and after freeze-drying. The structure of HA-Cu complexes with inner-sphere complexation was more compact than those with outer-sphere complexation, which reduced the accessibility of sorption sites for SMX on HA-Cu and inhibited SMX sorption. However, the greater number of coordination sites of Cd2+ may provide more sorption sites and the structure of HA-Cd was looser. These findings provide a groundbreaking understanding of the sorption of organics on natural adsorbents in the presence of cations.

Removal of crystal violet dye using a three-dimensional network of date pits powder/sodium alginate hydrogel beads: Experimental optimization and DFT calculation

Biodegradable and very low-cost adsorbent beads were prepared from date pits powder (DP) and sodium alginate (SA). DP to SA ratios was varied (1/2, 1/4 and 1/6) and used to eliminate Crystal violet (CV) a cationic dye. Adsorbents were characterized by FTIR, SEM-EDS, UV-vis DR, TGA and the point of zero charge (pHPZC). The optimal composite beads SA@6DP show high adsorption capacities of 83.565 mg/g toward CV than SA@2DP and SA@4DP. The kinetics investigation showed that the adsorption is well described by the pseudo-second-order kinetic (R2 = 0.998). The thermodynamics and isotherms studies exhibit that the adsorption phenomenon for SA@6DP adsorbent is endothermic and significantly fitted with the Redlich-Peterson model. The experimental adsorption tests were optimized by the Box-Behnken design (BBD) which led to conclude the maximal CV removal efficiency achieved by SA@6DP was 99.873 % using [CV] = 50 mg/L, adsorbent mass = 20 mg and 48 h of contact time. The theoretical calculation proved that the CV molecules favor the mode of attack due to their electrophilic character and can accept the SA@6DP adsorbent electrons more easily to form an anti-bonding orbital. SA@6DP hydrogel beads are therefore an exceptional bio-adsorbent that offers excellent adsorption performance.

From Waste to Resource: Valorization of Lignocellulosic Agri-Food Residues through Engineered Hydrochar and Biochar for Environmental and Clean Energy Applications-A Comprehensive Review

Agri-food residues or by-products have increased their contribution to the global tally of unsustainably generated waste. These residues, characterized by their inherent physicochemical properties and rich in lignocellulosic composition, are progressively being recognized as valuable products that align with the principles of zero waste and circular economy advocated for by different government entities. Consequently, they are utilized as raw materials in other industrial sectors, such as the notable case of environmental remediation. This review highlights the substantial potential of thermochemical valorized agri-food residues, transformed into biochar and hydrochar, as versatile adsorbents in wastewater treatment and as promising alternatives in various environmental and energy-related applications. These materials, with their enhanced properties achieved through tailored engineering techniques, offer competent solutions with cost-effective and satisfactory results in applications in various environmental contexts such as removing pollutants from wastewater or green energy generation. This sustainable approach not only addresses environmental concerns but also paves the way for a more eco-friendly and resource-efficient future, making it an exciting prospect for diverse applications.

Microwave assisted and conventional hydrothermal treatment of waste seaweed: Comparison of hydrochar properties and energy efficiency

Waste seaweed is a valuable source for converting into value-added carbon materials. In this study, the production of hydrochar from waste seaweed was optimized for hydrothermal carbonization in a microwave process. The produced hydrochar was compared with hydrochar synthesized by the regular process using a conventional heating oven. The results show that hydrochar produced with a holding time of 1 h by microwave heating has similar properties to the hydrochar produced in a conventionally heated oven for 4 h (200 °C and water/biomass ratio 5): carbon mass fraction (52.4 ± 3.9 %), methylene blue adsorption capacity (40.2 ± 0.2 mg g^-1) and similar observations on surface functional groups and thermal stability were made between hydrochars produced by both methods. The analysis of energy consumption showed microwave assisted carbonization consume higher energy in compare to conventional oven. The present results suggest that hydrochar made from waste seaweed and using the microwave technique could be an energy-saving technology for producing hydrochar with similar specifications to hydrochar produced by conventional heating methods.

Biomass carbon mining to develop nature-inspired materials for a circular economy

A transition from a linear to a circular economy is the only alternative to reduce current pressures in natural resources. Our society must redefine our material sources, rethink our supply chains, improve our waste management, and redesign materials and products. Valorizing extensively available biomass wastes, as new carbon mines, and developing biobased materials that mimic nature's efficiency and wasteless procedures are the most promising avenues to achieve technical solutions for the global challenges ahead. Advances in materials processing, and characterization, as well as the rise of artificial intelligence, and machine learning, are supporting this transition to a new materials' mining. Location, cultural, and social aspects are also factors to consider. This perspective discusses new alternatives for carbon mining in biomass wastes, the valorization of biomass using available processing techniques, and the implementation of computational modeling, artificial intelligence, and machine learning to accelerate material's development and process engineering.

A novel preparation process of straw-based iron material for enhanced persulfate activation of reactive black 5 degradation

In this study, a new straw-iron composite material (ST@Fe) was synthesized through impregnation and freeze-drying process for persulfate (PS) activation to degrade reactive black 5 (RB5). Scanning electron microscope, Brunauer-Emmett-Teller, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy confirmed that straw owns huge pore structure and varieties of organic functional groups, including hydroxyl carboxyl groups, which could effectively adsorb and complex iron ions. The interaction between the active iron particles in ST@Fe and straw generated Fe²⁺ for PS activation, effectively degrading over 94.80% of RB5 at an initial concentration of 20 ppm in 100 min with a specific degradation capacity of 18.97 min^-1 per unit of iron ions. ST@Fe/PS system demonstrated high tolerance in a wide initial pH range, which could gradually attack the RB5 molecular structure and significantly reduce the mineralization of water. Quenching experiments and electron paramagnetic resonance demonstrated the efficient generation of ROS including sulfate radicals, hydroxyl radicals, and singlet oxygen, and confirmed the dominance of sulfate radicals in the degradation process. The continuous degradation capacity and reusability of ST@Fe were also evaluated, which proved that the contaminant could be effectively degraded even after multiple cycles in the simulated textile wastewater, indicating its potential for use in practical remediation. This work provided a new method for the preparation of modified functional materials for the degradation of organic pollutants in textile wastewater and posed a novel strategy for the utilization of waste biomass.

Modification of hydro-chars by non-thermal plasma to enhance co-anaerobic digestion and degradation of sewage sludge pyrolysis oil

The pyrolysis oil produced from the sewage sludge pyrolysis process is a complex admixture of organic substances, which is difficult to be degraded in a normal anaerobic digestion (AD) process. In this study, the hydro-chars produced at 200, 240, and 280 °C were modified by non-thermal plasma (NTP) and then they were used to promote pyrolysis oil degradation and biogas production in a co-AD digester. The experimental results revealed that after NTP modification, the specific surface areas of the hydro-chars produced at 200 °C (SW200+P) and 240 °C were increased from 28.0 to 39.3 m²g^-1 and from 36.2 to 45.4 m²g^-1, respectively. Their pore volumes also increased by more than 10%. The SW200+P hydro-char exhibited the highest chemical oxygen demand (COD) removal rate (60.49%) and the highest CH₄ yield, which is 6.3 times of the digester with pyrolysis oil but without hydro-char addition (PO + CC). Additionally, the benzene series in the pyrolysis oil can be completely degraded in all digesters with the hydro-char addition. With addition of the SW200+P hydro-char, the Clostridia increased most significantly to become the predominant bacteria community at the class level, and the Methanosarcina became the predominant archaea community at the genus level, which contributed to the increased CH₄ yield. The hydro-char addition also increased Dietzia and Cellulosimicrobium, which promoted the degradation of benzene series in the pyrolysis oil. The investigation results suggest that the NTP modification technique can be a potential solution to effectively utilize the hydro-char and help pyrolysis oil degradation via the co-AD process.

Graphene-Based Materials Immobilized within Chitosan: Applications as Adsorbents for the Removal of Aquatic Pollutants

Graphene and its derivatives, especially graphene oxide (GO), are attracting considerable interest in the fabrication of new adsorbents that have the potential to remove various pollutants that have escaped into the aquatic environment. Herein, the development of GO/chitosan (GO/CS) composites as adsorbent materials is described and reviewed. This combination is interesting as the addition of graphene to chitosan enhances its mechanical properties, while the chitosan hydrogel serves as an immobilization matrix for graphene. Following a brief description of both graphene and chitosan as independent adsorbent materials, the emerging GO/CS composites are introduced. The additional materials that have been added to the GO/CS composites, including magnetic iron oxides, chelating agents, cyclodextrins, additional adsorbents and polymeric blends, are then described and discussed. The performance of these materials in the removal of heavy metal ions, dyes and other organic molecules are discussed followed by the introduction of strategies employed in the regeneration of the GO/CS adsorbents. It is clear that, while some challenges exist, including cost, regeneration and selectivity in the adsorption process, the GO/CS composites are emerging as promising adsorbent materials.