Synthesis and characterization of graphene quantum dot nanomaterials for the removal of dyes and metal ion and its environmental life cycle assessment/

Abstract

Utilizing biomass waste in the production of nanomaterial adsorbents can serve as a sustainable and valuable option that integrates the concept of circular economy into the existing system. This study aims to synthesize, characterize, and evaluate the performance of activated carbon (AC), graphene oxide (GO) and graphene quantum dot (GQD) derived from rubber seed shells for the removal of metal ion and dyes from wastewater. The synthesised adsorbents (AC, GO and GQD) were characterised to examine their morphological, physical, and chemical properties using XRD, EDX mapping, FT-IR, FESEM, Zeta potential, Raman spectroscopy, and TEM analyses. The characterization showed that all the adsorbents were highly porous and had uniform surfaces, with GO and GQD having a significant presence of hydroxyl, carboxyl, and carbonyl functional groups while AC had single-layered planes. The kinetic results showed that AC had high α values with a strong tendency to adsorb dyes while GO and GQD have high R 2 values which indicated great adsorption and an enhanced bonding of adsorbent and adsorbates with effective interactions. AC, GO and GQD adsorbents’ performance were also found to efficiently remove dyes (congo red and methylene blue) and metal ion (cadmium) with at least 70% removal rate with the adsorption being spontaneous and exothermic process having excellent reusability. The photoluminescence properties of the GQD were also analysed through spectral analysis and GQD degraded the dyes under sunlight irradiation with an efficiency of over 90% and fitted a first-order Langmuir-Hinshelwood (L-H) dynamic kinetic model. These results indicated that AC and GO are promising as adsorbents along with GQD also being a great adsorbent and photocatalyst for the degradation of dyes. In addition, the environmental performance of the overall GQD production which involved the syntheses of AC, GO and GQD was also evaluated through life cycle assessment (LCA) using ReCiPe 2016 method and SimaPro software, adopting a ‘cradle-to-gate’ approach. At the midpoint level, global warming potential (GWP) had the highest impact (18.22 kg CO2 eq), followed by land use (LUC) (13.71 m2 a crop eq) and human non-carcinogenic toxicity (HTPnc) (12.49 kg 1,4- DCB), while at the endpoint level it affected human health by 5.08 x 10-5 DALY, the ecosystem by 1.96 x 10-7 species⋅year and resources by 1.01 USD2013. The synthesis of AC contributed the highest to the LUC with almost 90% of the total impact due to the land use transformation of rubber plantation while GO contributed the highest in the GWP with 54% contributions, whilst GQD contributed the least to the environment, with only less than 6% of the overall impacts. Since the overall production of GQD is highly related to energy and chemical consumption, the environmental impacts of several energy sources and chemical usages were evaluated by comparing the baseline scenario with two alternative scenarios. The environmental impacts of energy consumption were found to significantly decrease in the following order: Grid > Grid/Natural gas > Grid/PV. For the chemical usage, it was revealed that using ammonia as a precursor in GQD activation fared better than urea and citric acid. This study can assist decision-makers in developing sustainable strategies for the use of nanomaterials, specifically the high-performing GQD adsorbent in improving drinking water quality and treating wastewater. This will contribute to the promotion of waste management and adopting a circular economy concept in Malaysia. Thus, future research should also focus more on converting biomass waste into valuable carbon nanomaterials by adopting the waste-to-wealth concept.