OPEN Reclamation of hexavalent chromium using catalytic activity of highly recyclable biogenic Pd(0) nanoparticles R. M. Tripathi1,2 & Sang J. Chung1* Hexavalent chromium is extremely toxic and increasingly prevalent owing to industrialisation, thereby posing serious human health and environmental risks. Therefore, new approaches for detoxifying high concentrations of Cr (VI) using an ultralow amount of catalyst with high recyclability are increasingly being considered. The catalytic conversion of Cr (VI) into Cr (III) was previously reported; however, it required a large amount of catalyst to reduce a low concentration of Cr (VI); further, pH adjustment and catalyst separation had to be performed, causing issues with large-scale remediation. In this study, an unprecedented eco-friendly and cost-efective method was developed for the synthesis of Pd nanoparticles (PdNPs) with a signifcantly narrow size distribution of 3–25 nm. PdNPs demonstrated the presence of elemental Pd with the zero oxidation state when analysed by energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. The PdNPs could detoxify a high concentration of Cr (VI), without the need to adjust the pH or purify the nanoparticles for reusability. The reusability of the PdNPs for the catalytic conversion of Cr (VI) into Cr (III) was >90% for subsequent cycles without the further addition of formic acid. Thus, the study provides new insights into the catalytic reclamation of Cr (VI) for industrial wastewater treatment. Heavy-metal pollution is a serious hazard and a global issue due to the increase in industrial and agricultural activities. Several toxic pollutants, such as inorganic ions, metals, and synthetic organic matter, are released into water. Among these, Cr (VI) is the most dangerous toxic pollutant. Cr is used for various purposes, such as leather tanning, paint formulation, wood preservatives, steel fabrication, and metal fnishing, resulting in considerable Cr-based water contamination. Te oxidation state of Cr signifcantly afects its toxicity. Of the two primary oxi- dation states, i.e., Cr (III) and Cr (VI), Cr (VI) is more toxic and carcinogenic to humans and animals. A signif- icantly low concentration of Cr (III) is required by humans, as it plays an important role in glucose metabolism; however, a high concentration of Cr (III) is toxic1. Te highest acceptable concentration of Cr in drinking water is 50 parts per billion, as recommended by the World Health Organization2. Te remediation of Cr (VI) is a critical research challenge. A physicochemical method was developed for the adsorption of Cr; however, it is expensive; further, Cr (VI) is simply transferred and not removed3. Te bioremediation of Cr (VI) using bacteria is viable and cost-efective; however, the resulting waste contains bactericidal toxicants, which limits the efciency and applicability of this method4. Currently, scientists are working towards the catalytic reduction of Cr (VI) to Cr (III). Compared to Cr (VI), Cr (III) has lower toxicity and mobility; moreover, a minute amount of Cr (III) is required for sugar and lipid metabolism in humans and other animals5,6. Te nanomaterial-based catalytic reduction of Cr (VI) has attracted considerable attention because of its advantages over physicochemical- and bioremediation-based methods. Pd 7 nanoparticles (PdNPs) have been extensively applied on various support materials, such as MIL-101 , alpha-Al2O3 5 8 9 flms , polymer nanofbers , and surface-functionalised SiO2 . Te efectiveness of PdNPs in the reduction of Cr (VI) (in the presence of formic acid) is owing to their distinguished features of high selectivity and activity in catalytic hydrogenation reactions9–11. Te reduction of Cr (VI) by PdNPs in the presence of formic acid involves 12–16 two steps: the catalytic hydrogenation of formic acid (HCOOH → H2 + CO2) and the reduction of Cr (VI) to 1School of Pharmacy, Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon, Gyeonggido, 16419, Republic of Korea. 2Amity Institute of Nanotechnology, Amity University Uttar Pradesh, Sector 125, Noida, 201303, India. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:640 | https://doi.org/10.1038/s41598-020-57548-z 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Rate Quantity Quantity of constant Nanomaterial of Cr (VI) nanomaterial pH (min−1) Reference TiO nanocrystals Either 2.7 2 50 ppm 500 ppm — 31 (photocatalytic) or 7.0 Iron micro/nanostructure 100 ppm 1,500 ppm 2 0.286 32 Cobalt phosphate- 33 sensitised inverse opal TiO2 10 ppm 10 ppm 3 — (photocatalytic) Magnetic mesoporous 50 ppm 800 ppm 2 0.017 34 carbon-doped PdNPs Cobalt nanoparticles 100 ppm 100 ppm 2 0.474 35 supported on graphene Sulfur nanoparticles 200 ppm 10 ppm 1–2 0.027 36 NiO nanostructure 130 ppm 400 ppm 7 0.0026 37 (photocatalytic) Nanoscale zerovalent iron supported on mesoporous 6 ppm 180 ppm 3 & 5 0.017 38 silica PdNPs ~147 ppm 0.5 ppm 3 — 39 Biogenic PdNPs 250 ppm 0.043 ppm Not required 0.0971 Present work Table 1. Reduction of Cr (VI) by diferent catalytic nanomaterials. 2− + 3+ 5,17,18 Cr (III) through a H2 transfer pathway on the surface of PdNPs (Cr2O7 + 8H + 3H2 → 2Cr + 7H2O) . PdNPs were decorated on graphene oxide and exhibited remarkable reusability (>90% at ffh reuse)19. Generally, reusability is challenging owing to the need for recovering, purifying, and drying the catalyst. Additionally, chemical methods for synthesising nanocatalysts/photocatalysts cause environmental pollution, as hazardous chemicals are required for the synthesis process. Therefore, researchers focused on nontoxic, cost-efective, facile, and eco-friendly methods for the synthesis of nanomaterials. Typically, bacteria, fungi, and plant extracts are used to synthesise various types of nanomaterials20–24. Plant extracts have gained signifcant attention compared to bacteria and fungi because they do not require culture maintenance. Te objectives of this study were to develop a simple, cost-efective, and eco-friendly method for the biosyn- thesis of PdNPs using a leaf extract of Erigeron canadensis L (E. canadensis) and to investigate their efectiveness and reusability for the reduction of Cr (VI) without the separation of PdNPs. Te novelty of the present work is indicated by Table 1. Erigeron species is a source of γ-pyranone derivatives, favonoids, and phenolic acids25, and is important for the synthesis of nanoparticles26–28. Tis plant has medicinal value in the treatment of indigestion, hematuria, enteritis, and epidemic hepatitis29. A fower extract of E. annuus (L.) Pers was used as a reducing and capping agent for the synthesis of silver and gold nanoparticles30. Previous studies considered the separation of catalysts/photocatalysts, which resulted in a difcult, expensive, and time-consuming reusability process. Te proposed method does not require recovery, purifcation, or drying of biogenic PdNPs. Additionally, it can be applied to industrial wastewater treatment because once the biogenic PdNPs are added to the wastewater, addi- tional PdNPs need not be added for several consecutive cycles, and no further addition of formic acid is required. Results Ultraviolet–visible light spectroscopy analysis. Te synthesis of the PdNPs was monitored at 0, 5, 15, 30, and 70 min by scanning the sample by ultraviolet–visible light (UV–vis) spectroscopy. Te spectra revealed that 30 min was sufcient for synthesising the PdNPs. Figure 1a shows a distinct peak around 400 nm at 0 min, indicating the presence of Pd2+ ions in the solution; however, afer 30 min, this peak disappeared. Te sample was scanned afer 70 min; however, no change was observed in the absorbance (Fig. 1a). Fourier transform infrared spectroscopy. The synthesised nanoparticles were scanned by Fourier transform infrared (FTIR) spectroscopy in the range of 500–4,000 cm−1 (Fig. 1b). Te FTIR spectrum of the leaf extract exhibited a broad, intense peak at 3,450.56 cm−1, whereas in the spectrum of the PdNPs, this peak shifed to 3,357.98 cm−1, indicating –OH stretching40. Te peak at 2,939.44 cm−1 in the leaf-extract spectrum corre- 41 sponds to the C-H stretching of CH2 and CH3 . However, in the spectrum of the PdNPs, no peak was observed at 2,939.44 cm−1, suggesting the involvement of C-H stretching vibration in the formation of the PdNPs. A peak was observed at 1,739.74 cm−1, corresponding to C = O stretching of the aldehyde group. Te band at 1,654.88 cm−1, in the case of the leaf extract, was shifed to 1,651.02 cm−1 in the spectrum of the PdNPs, corresponding to the stretching vibration of COO−. Te leaf-extract spectrum exhibited a peak at 1,427.28 cm−1, corresponding to the N-H stretching vibration in the amide linkages of the protein; this peak was not observed for the PdNPs. Te band at 1,271.05 cm−1 for the leaf extract was similar to that at 1,240 cm−1, which corresponds to the C-N stretch- ing of amines42. Tis band was not observed for the PdNPs. Te spectra of the PdNPs and leaf extract exhibited peaks at 1,095.54 and 1,089.75 cm−1, respectively, indicating a marginal shif. Tese peaks were similar to that at 1,074 cm−1 and indicate the presence of favanones adsorbed on the surface of the nanoparticles43. Transmission electron microscopy. A sample was prepared on a carbon-coated copper grid via drop-coating, and transmission electron microscopy (TEM) was performed for analysis of the size, morphology, and crystalline nature of the biosynthesised PdNPs. TEM images were obtained at various magnifcations, which SCIENTIFIC REPORTS | (2020) 10:640 | https://doi.org/10.1038/s41598-020-57548-z 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. (a) Ultraviolet–visible (UV–vis) spectra for the biosynthesis of the Pd nanoparticles (PdNPs) as a function of time; (b) Fourier transform infrared spectra of the leaf extract of Erigeron canadensis and the biologically synthesised PdNPs.