Clean Technol., Volume 7, Issue 2 (June 2025) – 10 articles

Ultrasound-assisted extraction (UAE) using water as a green solvent is a promising non-thermal technique for the extraction of total dietary fiber (TDF) and betaine from red beetroot (Beta vulgaris L.) peel. Compared to conventional thermal extraction (CE), UAE has proven to be a more efficient alternative method for the extraction of TDF and betaine. The pretreatment of beet was carried out using pulsed electric field (PEF) technology, with the specific energy of the PEF treatment set at 1.6 kJ/kg. To achieve the maximum betaine concentration of 24.80 µg/mL, the optimum UAE parameters were 50% amplitude with an extraction time of 3 min using distilled water as extraction solvent. The optimum TDF yield of 44.07% was achieved at 75% amplitude, 6 min treatment time and 50% ethanol solution as extraction solvent. These conditions can effectively supplement UAE, especially in the extraction of bioactive compounds from red beetroot peel. However, the TDF obtained in the residue must be evaporated for further use, which increases energy consumption. Ethanol concentration had no statistically significant effect (p > 0.05) on the TDF results, suggesting that distilled water could replace ethanol as a solvent in UAE. This substitution offers environmental and economic advantages, as water is more environmentally friendly and less expensive than ethanol. In addition, the use of distilled water eliminates the need to evaporate ethanol, which is particularly advantageous when the extracted material is intended for fortification or improvement of the technological and functional properties of food products. Full article

Transitioning to clean energy in off-grid remote locations is essential to reducing fossil-fuel-generated greenhouse gas emissions and supporting renewable energy growth. While hybrid renewable energy systems (HRES), including multiple renewable energy (RE) sources and energy storage systems are instrumental, it requires technical reliability with economic efficiency. This study examines the feasibility of an HRES incorporating solar, wind, hydrogen, and biofuel energy at a remote location in Australia. An electric vehicle charging load alongside a residential load is considered to lower transportation-based emissions. Additionally, the input data (load profile and solar data) is validated through statistical analysis, ensuring data reliability. HOMER Pro software is used to assess the techno-economic and environmental performance of the hybrid systems. Results indicate that the optimal HRES comprising of photovoltaic, wind turbines, fuel cell, battery, and biodiesel generators provides a net present cost of AUD 9.46 million and a cost of energy of AUD 0.183, outperforming diesel generator-inclusive systems. Hydrogen energy-based FC offered the major backup supply, indicating the potential role of hydrogen energy in maintaining reliability in off-grid hybrid systems. Sensitivity analysis observes the effect of variations in biodiesel price and electric load on the system performance. Environmentally, the proposed system is highly beneficial, offering zero carbon dioxide and sulfur dioxide emissions, contributing to the global net-zero target. The implications of this research highlight the necessity of a regional clean energy policy facilitating energy planning and implementation, skill development to nurture technology-intensive energy projects, and active community engagement for a smooth energy transition. Potentially, the research outcome advances the understanding of HRES feasibility for remote locations and offers a practical roadmap for sustainable energy solutions. Full article

The occurrence of heavy metals in aquatic ecosystems is a serious environmental hazard, and their effective removal is imperative. In this regard, the feasibility of living microalga Chlorella vulgaris (C. vulgaris) to remove heavy metals (Ni, Pb, Zn, Cd, and Cu) is investigated by using 1, 5, and 10 ppm concentrations of single- and multiple-metal-treated (MT) cultures. Experiments were performed in controlled laboratory conditions, and metal removal analysis was performed through atomic absorption spectroscopy (AAS). The cultures were also examined by means of optical microscopy, UV-Vis spectrophotometry, and Fourier transform infrared (FTIR) spectroscopy to follow the cultures’ pigment content, cell population, and functional group changes during cultivation. The removal efficiency results of both single and multiple MT cultures were evaluated using the Langmuir isotherm model. The results indicate that C. vulgaris presents potential for heavy metal bioremediation, even towards multi-MT conditions, despite the influence of a competitive uptake in multi-MT cultures. In mono-MT cultures, the removal efficiency of C. vulgaris presents values of 65–99% on Day 3 and 72–99% on Day 7 of cultivation, while the results for the multi-MT cultures are 49–99% and 62–99% for Days 3 and 7 of cultivation, respectively. The research illustrates the potential for C. vulgaris as a promising biosorbent for heavy metal remediation along with its post-treatment use in applications supporting the green circular economy. Full article

Adsorption represents an effective strategy for water remediation applications, particularly when utilising eco-friendly materials in a circular economy framework. This approach offers significant advantages, including low cost, material availability, ease of operation, and high efficiency. Herein, the performance of cadmium ion adsorption onto hydroxyapatites, derived through a calcination-free process from shells of two mollusc species, Queen Scallop (Aequipecten opercularis) and Pacific Oyster (Magallana gigas), is examined. The phase and morphology of the synthesised adsorbents were investigated. The results showed that hydroxyapatites obtained from mollusc shells are characterised by high efficiency regarding cadmium removal from water, exhibiting rapid kinetics with equilibrium achieved within 5 min and high adsorption capacities up to 334.9 mg g⁻¹, much higher than many waste-based adsorbents reported in literature. Structural investigation revealed the presence of Cadmium Hydrogen Phosphate Hydrate in the hydroxyapatite derived from oyster shells loaded with Cd, indicating the formation of a solid solution. This finding suggests that the material not only has the capability to decontaminate but also to immobilise and store Cd. Overall, the results indicate that hydroxyapatites prepared via a synthetic route in mild conditions from waste shells are an economical and efficient sorbent for heavy metals encountered in wastewater. Full article

This study investigates the size effect on the Seebeck coefficient (SC) in cement composites incorporating silicon carbide (SiC). Two specimen shapes, cubic (50 × 50 × 50 mm³) and beam (40 × 40 × 160 mm³), were analyzed with varying SiC substitution ratios (0%, 50%, and 100%) for fine aggregates. Thermal and electrical conductivities were measured to assess their influence on the SC. The results showed that a higher SiC content increased porosity, which reduced mechanical strength but significantly improved thermal and electrical conductivities. Thermal conductivity increased from 1.88 W/mK (0% substitution) to 11.89 W/mK (100% substitution), while electrical conductivity showed an improvement from 0.0056 S/m to 0.065 S/m. Cubic specimens exhibited higher SC values compared to beam specimens, with a maximum SC of 1374 μV/K at 100% SiC substitution, attributed to shorter thermal diffusion distances. The findings suggest that optimizing member size and SiC content can significantly improve the thermoelectric performance of cement composites, potentially enhancing energy efficiency in construction applications. Full article

The expansion of fisheries and aquaculture in recent decades has led to a substantial increase in fish by-products. This study investigates the extraction and characterization of calcium phosphates from the by-products of representative species in these industries, aiming to identify potential sources for biotechnological and pharmaceutical applications. Clean bones obtained by enzyme hydrolysis from the heads, central skeletons, and/or tails of Atlantic horse mackerel, blue whiting, hake, mackerel, and farmed turbot were subjected to calcination to obtain calcium phosphates. The clean bone content in terms of nitrogen, lipids, organic matter, total protein, and amino acids was evaluated together with the chemical bonds, structures, and elemental composition of calcium phosphates. Results indicated a significantly higher yield of wet bone recovery (23%, p < 0.05) for the central skeleton of Atlantic horse mackerel and the highest mineral fraction for the heads of Atlantic horse mackerel (73.2%), followed by that of blue whiting (72.6%). Hake and turbot presented the lowest mineral fractions and, therefore, the highest protein content (27–31%, p < 0.05), with significant levels of collagen-related amino acids (p < 0.05). X-ray diffraction (XRD) and Fourier-transform Raman spectroscopy (FT-Raman) confirmed the biphasic calcium phosphate composition for most samples based on hydroxyapatite with contributions of whitlockite/β-tricalcium phosphate. The highest contribution to the non-apatite phase was made by the central skeletons of both mackerel and Atlantic horse mackerel. Full article

This study investigates the drying behavior and rheological properties of dehydrated sewage sludge from various wastewater treatment plants in the Liege region (Belgium). Emphasizing the characterization of key parameters to enhance sludge management strategies, a series of experiments were conducted, including total solid content (TSC) determination, volatile solid content (VSC) analysis, texture profile analysis (TPA), penetrometry, and oscillatory rheology tests. Results showed no significant trends between specific evaporation capacity and the analyzed variables, cohesiveness, TSC, VSC, hardness, yield, and flow point. However, a clear trend indicated that higher G′ values are associated with improved drying rates. This aligns with the existing literature, suggesting that the viscoelastic properties of sludge, represented by G′, could potentially predict drying performance. A strong correlation between G′ and cohesiveness was also observed, recommending the use of G′ as the primary parameter due to the standardization and reliability of rheological tests. Despite the limited sample size, the study provides a valuable starting point for future research. Further investigations with larger sample sizes and controlled laboratory conditions are recommended to validate these findings and establish ranges within which the studied properties can be useful for future calculations and analyses. These efforts will contribute to optimizing sludge drying processes and promoting sustainable wastewater treatment practices. Full article

The ammonia adsorption capacity of lignin-rich biomass solids was tested for the first time at low partial pressures (<1.5 kPa) and 20 °C. The biomass samples included untreated tree barks, husks, and peats, as well as the biochars produced by their slow pyrolysis. Proximate and ultimate analyses, lignin content, and metal content are also presented. The untreated biosolids had higher VM/FC ratios, molar H/C, and O/C than the treated biosolids (biochars and treated biochars). A novel methodology is described for the safe generation of gaseous ammonia at predictable low partial pressures from tabletop-scale batch reaction experiments of NaOH with (NH₄)₂SO₄ in aqueous solution, leading to the determination of ammonia adsorption capacities from low-cost experiments. Statistically significantly larger NH₃ adsorption capacities were obtained for the untreated biosolids than for their biochars (p < 0.001). In contrast, the biochars were found to be poor NH₃ adsorbers without further treatment. The NH₃ adsorption capacities from this study’s biosolids were compared with those of common adsorbent types in the same conditions using the existing literature through equilibrium model interpolation (Dubinin–Astakhov, Toth, and Freundlich) or cubic spline fit from graphical isotherms. Controls consisting of commercially sourced activated carbons (AC) had low adsorption capacities, close to those derived from the literature in the same conditions for similar materials, confirming the methodology’s robustness. The untreated biosolids’ NH₃ adsorption capacities were in the same range as those reported for silica, gamma-alumina, and some of the treated or doped ACs. They also performed better than the undoped, untreated ACs. The work suggests lignin-rich untreated biosolids such as barks and peats are competent low-cost ammonia adsorbents. Full article

Chert gravel, a byproduct of sand quarrying, remains an underutilized material in construction due to its low microwave (MW) absorption and high mechanical strength. The present study deals with the potential of MW irradiation as a novel, energy-efficient method for processing chert gravel into high-quality aggregates, reducing reliance on virgin materials. The research systematically examines MW exposure duration (1–2.5 min), rock size (150–800 g), moisture conditions, and cooling methods (air vs. water quenching) to optimize fragmentation. Experimental results indicate that larger rock sizes (600–800 g) yield coarser, less uniform aggregates, while prolonged MW exposure (>2 min) induces extensive micro-fracturing, producing finer, well-graded particles. Water quenching significantly intensifies fragmentation, generating irregular but highly fragmented aggregates, whereas pre-wetted samples exhibit finer and more uniform breakage than dry samples. The findings introduce a novel approach for optimizing chert gravel fragmentation, a material previously considered unsuitable for MW treatment. The study proposed a customizable methodology for tailoring aggregate properties through precise control of MW parameters, offering a sustainable alternative to conventional crushing. The results contribute to resource conservation, reduced energy consumption, and climate change mitigation, paving the way for more sustainable construction practices. Full article

This study assessed the techno-economic and environmental feasibility of a grid-connected PV system on a university building, with a focus on potential revenue from carbon credit sales. The analysis assumes a regulated CO₂ emissions market in Ecuador and references carbon credit prices from the European Union, New Zealand, China, and the Republic of Korea. Seven PV system configurations, varying in size and capacity, were modeled using Homer Pro and assessed for their techno-economic feasibility and environmental performance. The results indicated that the 166 kWp system was the most promising, supplying approximately 74% of the building’s electricity demand. Thus, this system was selected as the baseline for evaluating potential revenues from carbon credit sales in international markets, based on average carbon prices in 2022. The selected markets generated annual revenues of USD 4410.68, USD 2587.55, USD 446.34, and USD 958.37, respectively. While these additional revenues improved the Net Present Value (NPV) of the 166 kWp system, the overall NPV remained negative due to the high initial investment costs. Full article