Sustainable development strategies, including biomass waste valorization and life cycle-based carbon management approaches, are being adopted worldwide to mitigate climate change and reduce pollution. Numerous materials or products are under development to meet the demands related to a lowered carbon footprint, higher safety, access to renewable energy, and so on. One such material is biochar, which has been employed in a plethora of applications, areas ranging from soil amendment, polymer composites, energy storage and conversion, building materials, to environmental remediation. However, no two biochars are created equal. This disparity in inherent material properties in biochar means that they should be subjected to effective engineering to tailor their properties to a specific targeted application. This means that a biochar that is suitable for concrete applications may not be appropriate for polymer composite applications. Even biochar properties that were designed for a specific application may not have the desired or expected impact. This can lead to several negative effects of biochar. Hence, this article has collated and reviewed the instances where biochar has had unintended consequences in different applications. Using the knowledge presented herein, researchers can be aware of the potential disadvantages of biochar applications and take necessary steps to abate the negative effects. 

The accumulation of post-consumer polyethylene terephthalate (PET) waste necessitates value-added recycling strategies that restore mechanical performance while maintaining environmental sustainability. In this study, recycled PET (rPET) was reinforced with 20 wt% short jute fibres subjected to untreated (UT), NaOH-treated (NT), and silane-treated (ST) surface modifications. Composites were fabricated by injection moulding and characterised using X-ray diffraction and FTIR to confirm removal of amorphous constituents and formation of interfacial siloxane linkages. Surface treatment improved consolidation, with experimental density increasing from 1.22 to 1.24 g/cm³ and porosity decreasing from 3.9% to 2.4%. Correspondingly, tensile strength increased from 38.78 to 48.29 MPa, flexural strength from 50.30 to 68.84 MPa, short-beam shear strength from 11.74 to 14.66 MPa, and hardness from 77 to 81 Shore D. One-way ANOVA confirmed statistically significant improvements (p < 0.05). Porosity-based linear regression modelling demonstrated strong structure–property relationships (R² = 0.631–0.916), with cross-validated R² values of 0.391–0.861, indicating moderate predictive robustness within the investigated domain. A cradle-to-gate life cycle assessment revealed low carbon footprints (2.49–2.51 kg CO₂-eq kg⁻¹), with fibre surface modification introducing only marginal environmental penalties. The results demonstrate that surface-engineered jute fibres effectively enhance the mechanical integrity of recycled PET while maintaining favourable sustainability performance. 

This study presents a combined experimental and numerical investigation of fused filament fabrication (FFF)-printed polyether ether ketone (PEEK) plates subjected to quasi-static and high-velocity impact loadings. Izod impact, quasi-static punch-shear (QS–PS), and high-velocity projectile impact tests were conducted on specimens with different infill patterns, namely line, grid, cubic, and hexagonal configurations. High-velocity impact experiments were performed using a two-stage gas gun at an impact velocity of 100 m/s. Infill architecture influences quasi-static and low-rate impact performance. The hexagonal pattern exhibited the highest Izod impact strength (ca. 24 kJ/m²) and punch-shear strength (ca. 12 MPa), demonstrating improved load distribution and energy absorption capability. Under high-velocity impact, infill geometry becomes less influential, indicating comparable ballistic responses. This reduced sensitivity to infill pattern is attributed to rapid stress-wave propagation and extremely short interaction times, which limit progressive deformation within the internal structure. Finite element simulations using a solid PEEK model further support these findings, showing similar stress distributions and penetration behavior across all configurations. The results demonstrate that while infill geometry plays a critical role under quasi-static loading, its effect diminishes under high-velocity impact, where the response is predominantly governed by the intrinsic material behavior of PEEK.

Increasing concerns and regulatory restrictions associated with conventional flame retardants have driven research into halogen-free and bio-based alternatives that combine fire safety with reduced environmental impact. Lignin, a macromolecule prevalent in biomass, has emerged as a promising candidate for flame-retardant applications owing to its intrinsic charring ability, high carbon content, and chemical versatility. This review provides a comprehensive and critical overview of lignin-derived flame retardants, addressing fire behavior assessment, structure–property relationships, flame-retardant mechanisms, and practical applications in polymers, coatings, textiles, and construction materials. The influence of plant origin and extraction method on thermal decomposition, char formation, and combustion behavior is discussed in detail, with particular emphasis on condensed-phase mechanisms governing fire performance. Strategies employing unmodified lignin, chemically modified lignin (phosphorus-, nitrogen-, and silicon-containing systems), lignin-containing intumescent formulations, and lignin-derived nanocomposites are systematically reviewed. Their effects on flammability metrics, heat release characteristics, smoke suppression, and mechanical performance are critically assessed. While lignin-based systems demonstrate significant potential to reduce heat release and enhance char stability, challenges remain related to dispersion, compatibility, processing stability, and the sustainability of modification routes.

Additive manufacturing using fused deposition modelling (FDM) has emerged as a versatile and resource-efficient route for producing complex polymer and composite structures. However, the quality and sustainability of FDM-printed components are strongly governed by process parameters, nozzle design, and post-processing methods. This review provides a systematic analysis of these factors and their combined influence on mechanical integrity, surface finish, and dimensional accuracy. The study highlights how optimized layer thickness, build orientation, and extrusion temperature enhance interlayer adhesion and structural performance, while advanced nozzle geometries improve melt flow and minimize material waste. Post-processing techniques such as annealing, chemical smoothing, and surface finishing are evaluated for their roles in extending product life cycles and enabling recycled or bio-based polymer feedstocks. By linking process optimization to energy efficiency and material utilization, this review positions FDM as a pathway for sustainable, waste-to-value additive manufacturing. The insights presented support the development of eco-efficient design frameworks for next-generation polymer and composite processing within circular engineering systems.

The growing accumulation of plastic and agricultural waste highlights the urgent need for sustainable material alternatives. This study investigates the incorporation of nano-biochar derived from cashew nut shell biomass to enhance the mechanical and thermal performance of recycled polylactic acid (rPLA). Nano-biochar produced via controlled pyrolysis and high-energy ball milling was incorporated into rPLA at 0–2 wt% loadings through melt compounding and injection moulding. The resulting composites were evaluated for tensile, flexural, impact, and interlaminar shear strength (ILSS), alongside UL-94 flammability testing. A one-way ANOVA followed by Tukey’s HSD post-hoc analysis confirmed statistically significant improvements (p < 0.05) across all mechanical properties. The tensile strength of virgin PLA (32.23 MPa) decreased to 25.92 MPa in recycled PLA due to polymer chain scission; however, the addition of 1.5 wt% nano-biochar increased tensile strength to 49.54 MPa and ILSS from 21.37 MPa to 36.31 MPa. Flexural and impact strengths also rose by 34.19% and 45.85%, respectively, compared to unfilled rPLA. In UL-94 testing, the rPLA1.5 composite achieved a V-0 rating with no dripping, indicating excellent flame retardancy. Overall, nano-biochar reinforcement not only restored but substantially enhanced the mechanical integrity and fire resistance of rPLA, with ANOVA validating the statistical robustness of these improvements. This work demonstrates a viable circular-economy pathway for converting biomass waste into functional nano-reinforcements for sustainable polymer composites. These composites are particularly suitable for automotive interiors, building materials, and consumer goods where improved flame resistance and mechanical durability are required.

This study introduces a novel hexagonal honeycomb lattice design incorporating integrated diagonal struts, developed to enhance compression strength and energy absorption in 3D-printed polymer structures. Five distinct lattice configurations were fabricated using polylactic acid (PLA) filament and evaluated through uniaxial compression testing. The results showed that Lattice 5, which features a hexagonal unit cell with diagonal struts from top left to bottom right, had the highest compression strength of 45.78 MPa and absorbed 14,406 J of energy. In comparison, Lattice 1, with a regular hexagonal unit cell, had 15 % lower compression strength and 20 % lower energy absorption. Analytical models based on honeycomb geometry and PLA material properties were used to predict how the structures would deform. Finite element analysis (FEA) was also conducted to study the deformation under dynamic loading, with Lattice 5 proving to be the most efficient design. The diagonal struts in Lattice 5 helped to redistribute the load more evenly, reducing stress concentrations and allowing for a more gradual deformation. The FEA results matched the experimental data closely, confirming the accuracy of the predictions. These findings offer useful insights for improving lattice structures for applications that require high performance in terms of both structural strength and energy absorption.

This study investigates the effect of Halloysite nanotube (HNT) reinforcement on the mechanical, thermal, and structural properties of recycled polylactic acid (rPLA) composites. Composites were prepared with 1-5 wt.% HNTs and characterised using tensile, flexural, compressive testing, thermogravimetric analysis (TGA), and X-ray diffraction (XRD). Tensile strength increased from 42.98 MPa for neat rPLA to a maximum of 49.39 MPa at 2 wt.% HNT, while tensile modulus improved steadily from 2423.13 MPa to 2971.26 MPa at 5 wt.%. Flexural strength peaked at 78.54 MPa (22 % improvement compared to neat rPLA) at 3 wt.%, and the highest flexural modulus of 2292.30 MPa was achieved at 4 wt.% HNT. Under compressive loading, strength and modulus increased from 100.94 MPa and 2361.52 MPa for neat rPLA to 108.69 MPa and 2479.87 MPa, respectively, at 5 wt.% HNT, showing improved resistance to deformation. Thermal degradation temperatures rose from 452.12 °C for rPLA to 465.58 °C at 5 wt.% HNT, with char residue at 600 °C increasing from 4.23 % to 9.96 %, confirming the thermal barrier effect of Halloysite. XRD analysis showed enhanced crystallinity, increasing from 57.49 % (neat rPLA) to 59.22 % at 5 wt.% HNT, indicating effective nucleation and structural ordering induced by the nanotubes. Overall, the incorporation of 2-4 wt.% Halloysite offered the most balanced improvement in strength, stiffness, and thermal stability. These results demonstrate that rPLA-Halloysite composites can be suitable for sustainable, high-performance applications in packaging, automotive interiors, and structural bioplastics.

Hydrothermal carbonisation (HTC) technology benefits the environment by lowering greenhouse gas emissions and promoting sustainable waste management practices while supporting circular economy principles. Over the past two years, research has focused on optimising HTC process parameters, broadening the range of suitable feedstocks, and enhancing the properties of hydrochar and byproducts. This review discusses the characteristics of the hydrochar, including its calorific value, surface area, and nutrient content, and the influence of these parameters by feedstock type in the HTC. Furthermore, it reviews the suitability of hydrochar derived from different feedstocks in applications such as renewable solid fuel, soil amendment and environmental remediation. By analysing the most recent research advancements and identifying the associated challenges, this review underscores the importance of HTC in promoting sustainable waste management practices and enhancing resource utilisation in a circular economy framework. 

Carbon-based materials are highly sought after due to their superior properties, making them valuable for high-performance applications. However, most carbon-based materials are derived from fossil sources, and their synthesis often involves hazardous chemicals. Therefore, it is essential to develop sustainable methods for synthesising these materials from renewable resources, using fewer solvents, catalytic reagents, and generating minimal waste. In this study, few-layer graphene oxide (GO) was directly synthesised from waste biomass, without the formation of an amorphous intermediate, and its use as a fire retardant in two bioplastics was evaluated. Waste birch wood biomass was converted directly into graphitic carbon using manganese nitrate as a catalyst, with varying concentrations (0.003 to 0.1 mol-metal/g-wood) and treatment durations (1 and 2 h). The catalyst was doped through vacuum soaking and mild heating (90 °C), which facilitated the formation of graphitic carbon at relatively lower temperatures (< 1000 °C), eliminating the need for producing amorphous biochar prior to graphitisation. After pyrolysis at 900 °C and 950 °C for 2 h, the sample containing 0.005 mol-metal/g-wood, treated at 950 °C, exhibited the highest degree of graphitisation. This sample was further processed in a planetary ball mill with melamine as a dispersant for 30 min. Characterisation showed a broad absorption peak at 230 nm and the presence of semi-transparent sheets (3–8 layers), indicating the presence of GO. To evaluate its performance as a fire retardant, 2 wt% of the synthesised GO was added to polyamide 11 and wheat gluten bioplastics, which were then subjected to cone calorimeter tests. The results showed a 42% and 33% reduction in the peak heat release rate for polyamide 11 and wheat gluten, respectively, compared to their neat counterparts. The flame retardancy index further indicated that GO had a more significant impact on improving the fire safety of wheat gluten compared to polyamide 11. This study highlights a sustainable method for the preparation of few-layer GO at lower temperatures than contemporary methods, making the process more energy-efficient, environmentally friendly, and cost-effective. Additionally, the effectiveness of few-layer GO as a fire-retardant additive for enhancing the fire safety of bioplastics has been demonstrated.