Synthesis and characterization of modified mineral-biochar composite and its application for removing perfluorooctanoic acid from water

Per-and polyfluoroalkyl substances (PFAS) are a large group of anthropogenic compounds known as ‘forever chemicals’ due to their extremely persistent nature which makes them difficult to remove from the environment. Perfluorooctanoic acid (PFOA), one of the most well-studied PFAS, is widespread in the environment and known to be toxic to wildlife and humans. The US EPA has set a maximum contaminant level (MCL) of PFOA in drinking water at 4 ng/L. Therefore, it is urgently required to remove PFOA from water. There are many technologies involved in removal of PFOA from water such as advanced oxidation processes, photocatalysis, UV-Fenton reaction, and adsorption processes. However, most of these processes are expensive and energy intensive. On the other hand, adsorption is simple, easy, efficient and widely used in recent years. Conventional adsorbents such as granular activated carbon, io exchange resins, organoclays and iron oxides have been used to remove PFAS from water but most of them suffer from low adsorption capacity, slow rate, high desorption capacity and energy intensive synthesis process. Hence, there is a need to develop inexpensive and efficient adsorbents to remove PFOA from aqueous systems using naturally available minerals and biowaste. In this study, we synthesized wood chip biochar (WCBC) through pyrolysis reaction at 500? using wood biomass and prepared montmorillonite (MMT) clay-biochar (WCBC-MMT) in 50:50 ratio using ball mill. The composite was further modified with a long-chained surfactant (di(hydrogenated tallow)dimethylammonium chloride) on the basis of cation exchange capacity of the composite (27.2 cmol (p+)/kg) and it was renamed as modified clay-biochar (MWCBC-MMT) composite. The modified mineral-biochar composite was synthesized successfully and confirmed through X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier-transformed infrared spectroscopy (FTIR). The specific surface area of MMT, WCBC, WCBC-MMT composite and MWCBC-MMT composite was 242.3, 364.1, 315.7, and 61.4 m²/g. The kinetic experiment suggested that the MWCBC-MMT composite reached PFOA sorption equilibria within 2 h at initial PFOA concentration of 20 mg/L. The maximum potential of PFOA adsorption was 143.35 mg/g for MWCBC-MMT compositewhich was comparatively better than unmodified MMT, raw WCBC and WCBC-MMT composite. The modified clay-biochar composite adsorbents had better PFOA sorption capacity due to its porous nature and enhanced functional groups present on the surface. The mechanism of interaction between PFOA and the MWCBC-MMT composite was predominantly hydrophobic. The pH experiment at 20 mg/L initial concentration revealed that PFOA adsorption was more effective at low pH between 2 and 4 and it decreased when pH was increased to 8 and 10. This phenomenon can be described by electrostatic attraction of negatively charged PFOA to the positively charged sites on the adsorbent at low pH. However, the desorption was 60% at first (24 h = 1 cycle) and reached up to 99% after 3 cycles. This study suggests that surfactant-modified biochar-clay composite can be an efficient adsorbent for PFOA removal from water and warrants future research to evaluate the application and performance of the material under natural wastewater conditions.

Audience Take Away Notes: 

• The audience of this presentation would get detail information on how to design an inexpensive adsorbent using naturally available biowaste and mineral to remove extremely persistent contaminant like perfluorooctanoic acid (PFOA)
• This study will definitely smoothen the job of engineer to design an efficient adsorbent material without spending high energy, cost and labor for wastewater remediation
• This study shows a way forward to practically remediate PFAS contaminated wastewater and soil using the inexpensive surfactant modified wood chip biochar-mineral composite
• This study would also help the faculty to further expand their research work in the area of mineral adsorbent which can be easily implemented by the wastewater treatment plant stakeholders and the land reclamation experts to remediate PFAS contaminated soil