Elsevier

Chemosphere

Volume 287, Part 3, January 2022, 132287
Chemosphere

Sunlight promoted removal of toxic hexavalent chromium by cellulose derived photoactive carbon dots

https://doi.org/10.1016/j.chemosphere.2021.132287Get rights and content

Highlights

  • Photoactive nano carbon dots (CD) are synthesized from cellulose by carbonization.

  • Carbonization is performed at ∼90 °C for 30 min.

  • CD shows significant photocatalytic activity by sunlight illumination for Cr(VI) removal.

Abstract

A scalable synthetic procedure for fabricating photoactive carbon dots (CD) from microcrystalline cellulose (MCC) is presented. The MCC was transformed into a photoactive nanosized CD by a one-step acid-assisted thermal-carbonization (~90 °C for 30 min). The efficiency of the obtained CD was determined by photo-removal of toxic hexavalent chromium (Cr(VI)) ions from wastewater. CD obtained from cellulose completely removed 20 ppm of Cr(VI) wastewater within ∼120 min under sunlight illumination. No Cr(VI) removal was observed in dark conditions and with control cellulose material as reference samples. The Cr(VI) removal follows pseudo-first-order kinetics along with a half-life of ∼26 min. Furthermore, the Cr(VI) removal from wastewater was supported via cyclic voltammetry analysis. Using a low-cost, naturally available cellulose material and sulfuric acid, the world's most-used chemical, creates techno-economic prerequisites for a scalable process of photoactive carbon dots.

Introduction

Carbon dots (CD) (Sun et al., 2006) are explored and developed worldwide, mainly due to their attractive properties such as high specific surface area, low toxicity, excellent photoluminescence properties, biocompatibility (compared to heavy metal-based nanoparticles) (Han et al., 2018; Liu et al., 2020a). With such properties, CD are envisaged to be part in the development of new materials in several areas such as biosensors (Dai et al., 2014), optoelectronic devices (Choi et al., 2013), solar cells (Dao et al., 2016), photocatalysis (Aggarwal et al., 2020), chemical sensing (Wu et al., 2018; Anand et al., 2019; Ma et al., 2019; Ghosh et al., 2021; Sun et al., 2021), electrocatalysis (Dong et al., 2014; Chellasamy et al., 2022), waste water treatment (Perumal et al., 2022) and bioimaging applications. (Zhu et al., 2013). CD can be synthesized using several different procedures such as; ablation (Gonçalves et al., 2010), hydrothermal (Anand et al., 2019; Shukla et al., 2020; Zhou et al., 2021), solvothermal treatment (Sun et al., 2021), ultrasonic treatment (Dang et al., 2016), microwave irradiation (Saini et al., 2020), electrochemical carbonization (Hou et al., 2015), electrochemical oxidation (Ming et al., 2012), etc. Another advantage is their ease of surface modification to induce passivation by introducing various surface-functionalities which may increase surface area and surface catalytic sites. (Yan et al., 2018; Yuan et al., 2018). However, apart from using the chemical precursors, CD and doped-CD can be fabricated from waste (Hernández-Cocoletzi et al., 2020), biomass (Chen et al., 2018; Chai et al., 2021), and biomass-derived waste that are inexpensive carbonaceous precursors, thus, significantly lowering overall cost and minimizing the use of any complicated experimental set-ups (Jonoobi et al., 2015; Meng et al., 2019).

Cellulose is a readily available natural resource with intrinsic renewable properties among the naturally occurring polymers. It is a natural polymeric material usually fractionated from coniferous and deciduous wood or cotton and isolated from various marine animals, fungi, bacteria, algae, amoeba, plants, etc (Siddhanta et al., 2013; Jonoobi et al., 2015). Depending on the cellulose source used, the hierarchical structure, especially the amount of amorphous and crystalline regions and cross section are different. Therefore, upon carbonization, the formed CD structure may show slightly different sizes and properties depending on which cellulose source has been used. Presently, cellulose has gained much attention in developing new materials (Hasanpour et al., 2021; Hong et al., 2021; Yap et al., 2021) as it is an economic biomass-material and environmentally friendly and does not compete with the food supply chain. At a low cost and as being a natural source, cellulose is a promising precursor for producing carbon-based nanomaterials.

CD has been used in various photocatalytic and electrocatalytic applications (Zhang et al., 2020b), but few reports are available on CD photocatalytic applications concerning Cr(VI) removal. Cr(VI) mainly occurs in areas where chromium-based leather tanning occurs and in electroplating, mining, and dyeing industries, leading to soil and water contamination (Aggarwal et al., 2019). Mainly, chromium shows two oxidation states, trivalent state (III) (a stable form) and hexavalent state (VI) (an oxidizing agent). Cr(III) is a vital trace element found in glucose and insulin metabolism, whereas Cr(VI) is toxic (Kan et al., 2017). Cr(VI) in the form of CrO42−or HCrO4 diffuse cell membranes and reach the bloodstream. The uptake damages the liver, kidney, and blood cells via different oxidation reactions (Jin et al., 2016). On a global scale, environmental clean-up and remediation processes for the proper management of brownfield lands demand Cr(VI) reduction or removal.

Various literature reports on Cr(VI) removal has been presented, but many of them are using expensive processes and require complex technical methods such as membrane nano-filtration (Ren et al., 2010; Yang et al., 2018), chemical precipitation (Xie et al., 2017; Wang et al., 2018), ion-exchange (Alvarado et al., 2013), nanocomposite (Wang et al., 2018) etc. As a consequence, various reports on CD regarding Cr(VI) treatment as a practical and economical solution has been published using various approaches to improve the efficacy of the CD's (Choi et al., 2018; Bhati et al., 2019; Liu et al., 2020b; Zhang et al., 2020a).

Nevertheless, there are still demands for a simple, cost-effective, sustainable, and eco-friendly Cr(VI) removal method. Herein, we present a simple acid-assisted carbonization methodology for the fabrication of photoactive CD. The acidic hydrolysis pathway using sulfuric acid gives highly functional CD within 30 min at ~90 °C, which are considerable shorter reaction time and lower temperature than commonly used for CD preparation. After the carbonization and washing procedure, the black charred product was used to remove toxic Cr(VI) for wastewater treatment under sunlight illumination.

Section snippets

Materials

Microcrystalline cellulose (MCC), Avicel ® PH 101, was purchased from Sigma Aldrich. Sulfuric Acid (H2SO4), Potassium Dichromate (K2Cr2O7), Diphenyl Carbazide (DPC), Absolute Acetone, para-Benzoquinone (p-BZQ), Potassium Persulphate (K2S2O8), Sodium salt of Ethylenediaminetetraacetic acid (Na2-EDTA), tert-Butanol (t-BuOH) (99% assay) and tetra-n-Butyl Ammonium Bromide (TBAB) were of analytical grade and purchased from RANKEM. Absolute Ethanol for TEM. analysis was purchased from EMSURE.

Method for CD synthesis

Carbon

Result and discussion

The synthesis of photoactive CD from MCC (microcrystalline cellulose) is presented in Scheme 1. Cellulose was carbonized in concentrated sulfuric acid at ~90 °C for 30 min. During carbonization, cellulose follows a complex chemical reaction pathway likely by a simultaneous combination of polymerization (condensation and addition), dehydration and aromatization, formation of the carbon material with sulfonic groups, and other gaseous products such as H2O, CO, and CO2 (Carlson et al., 2009; Lin

Conclusion

Cellulose is a low-cost, green, and universally available biomass that forms photoactive CD under simple acid-assisted carbonization. The successful carbonization of cellulose is performed at ~90 °C in 30 min which is much lower and shorter than normal carbonization conditions (Temperature >250 °C, Time > 2hrs). The obtained CD was reusable and removed hexavalent chromium Cr(VI) from wastewater by sunlight irradiation. The removal of Cr(VI) was monitored by UV–visible spectrometer and supported

Author contributions

G. W., A. K. S. and S. K. S. planned and supervised all experiments of the work; R. A and D. S. executed most of the experiment work and performed all the experiments. All authors reviewed the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

R.A. thanks MNIT Jaipur for the doctoral fellowship, D.S. thanks DST, India (IF180133) for Inspire doctoral fellowship, and S.K.S. admires DST (SB/EMEQ-383/2014) and CSIR[ (01(2854)/16/ERMII)] for funding and the Material Research Centre (M.R.C.), MNIT Jaipur. G.W and A.K.S would like to acknowledge the Knut and Alice Wallenberg Foundation for financial support. The research has been carried out jointly in the Wallenberg Wood Science Center (WWSC).

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