Life cycle assessment for selective the mGO-NH2 nanoadsorbent disposal used in removing Hg (II) from aquatic solutions

Abstract

Introduction

Pollutants including heavy metals can affect aquatic organisms and human health all around the world. The reduction of toxic metal ions such as Hg (II), is of specific concern due to their nonbiodegradable and harmful properties in the environment. Recently, various physical and chemical separation processes for the removal of toxic ions from water and wastewater have been employed. Regarding the various available adsorbents for the adsorption of toxic metal ions, nano-adsorbents are preferred due to their high adsorption capacity, low waste production, and simplicity in design and operation. Among nano-adsorbents, functionalized Graphene oxide (GO) has been the most popular and widely estimated as an adsorbent to remove metal ions from aqueous solutions. Meanwhile, the use of GO is an emerging technology and is in the early stages of development and the environmental assessment of its application and disposal needs specific attention. Life cycle assessment (LCA) is an applicable method for estimating the environmental impacts associated with the life cycle of a product or process from the early stage of production to its final disposal. Life cycle assessment can be used to determine the impacts of a product or process on the environment from the raw material extraction or production to its final disposal LCA is also applied to provide a guideline for the development of emerging technologies considering their environmental performance Hence, the LCA of mGO-NH2 disposal, utilized for Hg (II) removal, was investigated in two scenarios including desorption and landfill.

Materials and Methods

The LCA of mGO-NH2 disposal was evaluated based on ISO 14040:2006, considering a functional unit of one kg of mGO-NH2 nano-adsorbent. The ReCiPe (H) 2016 midpoint and endpoint were used to assess the environmental impacts using SimaPro 9.5.5.0 and the Ecoinvent 3.4 datasets in desorption and landfill scenarios. Moreover, greenhouse gas (GHG) emissions, cumulative energy demand (CED), and ecological footprint (EF) were applied.

Results and discussion

Comparing two disposal scenarios, 17 environmental impacts of the desorption scenario were significantly higher than the landfill scenario. Whereas the landfill scenario showed higher non-carcinogenic toxicity (9.29 kg 1,4-DCB) than the desorption process with a value of 6.29 kg 1,4-DCB. Evaluation of effective parameters and processes in the environmental impact categories for the desorption scenario, the most important factor was electricity consumption, relating to the fuel type used for electricity generation. Since more electricity in Iran is produced from diesel and oil fuel, it can pose a significant effect on increasing the environmental burdens, especially global warming. Regarding the required electricity for nano-adsorbent synthesis and its desorption, it can play a substantial role in intensifying the considered impacts compared to the landfill scenario. Optimization of the electricity consumption during the desorption process can be achieved by the reduction of reaction time without the change of the nano-adsorbent characteristics and its performance because the purpose of nano-adsorbent desorption is the possibility of its reuse during the metal ions adsorption process. The use of renewable energy sources in electricity production can also play an impressive role in emissions decrement. The assessment of endpoint impacts of nano-adsorbent disposal revealed severe impacts of the desorption scenario on human health, ecosystems, and resources. The results of CED also indicated that fossil fuels had the highest energy consumption with 94.29% and 90.50% contributions in two scenarios of desorption and landfill, respectively. The GGP index also demonstrated that the desorption scenario has a higher contribution to global warming. Since the energy supply of nano-adsorbent desorption was from fossil fuel combustion, therefore, this factor accounts for almost the entire share of global warming potential. The amount of CO2 release, land occupation, and nuclear energy during the ecological footprint analysis elucidated that the landfill scenario had a much lower ecological footprint than the desorption scenario.

Conclusion

In this study, the environmental impacts of the mGO-NH2 nano-adsorbent disposal used for the removal of Hg(II) ions were investigated by using life cycle assessment in order to choose the appropriate method within desorption and landfill scenarios on a laboratory scale using LCA. The comparison of two mGO-NH2 disposal scenarios showed that the landfill process induced lower environmental impacts compared to the desorption scenario. Evaluation of the midpoint and endpoint impacts, CED, GGP, and EP indicated the effective role of electricity consumption in the impacts of the desorption scenario. As a result, the landfill scenario caused lower environmental impacts. Moreover, the application of the landfill process can be limited considering the lack of available land. On the other hand, due to the high cost of mGO-NH2 synthesis and also its suitable potential in metal ions removal, the possibility of desorption and reuse can reduce the environmental burdens compared to its re-synthesis. Therefore, for the desorption of nano-adsorbent on an industrial scale, the electricity consumption should be optimized. On the other hand, the impacts of electricity consumption can also be reduced by replacing renewable energy sources for electricity production.