Collaborative strategy for elevated reduction of Cr(VI) through pyrolyzed graphite-based biosynthetic Schwertmannite composite catalyzed by oxalic acid

Abstract

Graphite has the potential to mediate the reduction process of Cr(VI) by oxalic acid (OA), but a reasonable modification is required to enhance the mediation of electron transfer. In this study, biosynthetic Schwertmannite (Sch) modified graphite (Sch@G) was pyrolyzed at 700℃ for Cr(VI) remediation. Biosynthetic Sch particles were successfully loaded on the graphite, providing high specific surface area and abundant O-containing functional groups. The removal efficiency of Cr(VI) reached 90.42% within 60 min, facilitated by the synergistic between 1 g/L Sch@G and 1 mmol/L OA. Additionally, the comparative experiments exhibited a significant capacity of Sch@G in a wide pH range (pH 2−10), the removal efficiency was 97.9% within 60 min even at pH 10. Furthermore, the catalyst presented superior environmental adaptability in solutions containing various types of anions (Cl−, SO4^2−, NO3−, H2PO4−). Mechanism analysis revealed that the catalyst greatly promotes the transfer of electrons from OA to Cr-contaminants, along with the release of low-valent Fe from Sch, enabling efficient electrons transfer to the Cr-contaminant. Meanwhile, the addition of OA could complex OA-Cr(VI) compound, lowering the activity of Cr(VI) and facilitating the subsequent Cr(VI) removal. Generally, the synergistic effect of the catalyst and OA can form an efficient system that enables rapid and effective remediation of Cr(VI) contamination across a wide pH range. Thus, the catalyst presents as a promising graphite-based biomaterial for the rapid and effective remediation of Cr(VI) contaminants from wastewater.

1. Introduction

The rapid development of modern industry has led to the discharge of substantial quantities of industrial wastewater containing heavy metals into aquatic and terrestrial environments, presenting a significant environmental challenge [1]. Chromium (Cr), known as one of the most harmful heavy metal ions, predominantly exists in two main forms: trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] [2, 3]. Cr(VI), found in the form of chromate (CrO4^2−) or dichromate (Cr2O7^2−), is more toxic than Cr(III) [4]. It exhibits high toxicity, and low biodegradability, and can easily accumulate in living organisms, thereby posing a substantial threat to human health and environmental ecosystems [4−6]. Consequently, Cr(VI) is recognized as a prominent heavy metal pollutant requiring to be tackled urgently.

The reduction of Cr(VI) to the less toxic Cr(III) is considered an effective method for treating Cr-contaminant. Various conventional methods, including physical adsorption, chemical reduction, membrane fouling, and bio-remediation, have been evaluated and applied [7−11]. Among these methods, the adsorption-based process has emerged as a superior technology due to its advantages of high removal efficiency, low-cost instruments, and straightforward implementation, thus it is widely used in the treatment of heavy metal wastewater [12, 13]. Based on the wide applications of traditional adsorption methods for the removal of Cr-pollutants, the integration with novel materials is expected to address the complex challenges posed by various pollutants in industrial wastewater.

Graphite, a widely used adsorbent, possesses superior electrical conductivity due to its ability to form three covalent bonds between different carbon atoms, with each retaining one free electron for charge transfer. However, graphite's efficacy is limited by its underdeveloped pore structure and limited adsorption sites [14]. Consequently, fu...