Fabrication of bimetallic Ag/Fe immobilized on modified biochar for removal of carbon tetrachloride

Abstract

As an effective conventional absorbent, biochar exhibited limited adsorption ability toward small hydrophobic molecules. To enhance the adsorption capacity, a novel adsorbent was prepared by immobilizing nanoscale zero-valent iron onto modified biochar (MB) and then the elemental silver was attached to the surface of iron (Ag/Fe/MB). It's noted that spherical Ag/Fe nanoparticles with diameter of 51 nm were highly dispersed on the surface of MB. As the typical hydrophobic contaminant, carbon tetrachloride was selected for examining the removal efficiency of the adsorbent. The removal efficiencies of carbon tetrachloride by original biochar (OB), Ag/Fe, Ag/Fe/OB and Ag/Fe/MB were fully investigated. It's found that Ag/Fe/MB showed higher carbon tetrachloride removal efficiency, which is about 5.5 times higher than that of the OB sample due to utilizing the merits of high adsorption and reduction. Thermodynamic parameters revealed that the removal of carbon tetrachloride by Ag/Fe/MB was a spontaneous and exothermic process, which was affected by solution pH, initial carbon tetrachloride concentration and temperature. The novel Ag/Fe/MB composites provided a promising material for carbon tetrachloride removal from effluent.

Introduction

Carbon tetrachloride, a hydrophobic organic matter, has been widely used in many industries such as petrochemical, pharmaceutical, and allied industries (Saleh et al., 2015). Carbon tetrachloride is a priority pollutant listed by United States Environmental Protection Agency (EPA), because of its toxicity (Poursaberi et al., 2014), persistence, and relative hydrophobicity (Maithreepala and Doong, 2004). However, the disclosure and emission of carbon tetrachloride has caused serious contamination of environmental matrices in many regions (Beatriz et al., 2012, Penny et al., 2010). For example, Qiligou water source (Xuzhou, East China) was contaminated by the leakage of carbon tetrachloride, the peak concentration up to 3909 μg/L, which leaded to the water source closure from November 2000 (Han et al., 2006). Carbon tetrachloride can also contribute to smog formation, ozone destruction and harmful odor generation which can result in chronic toxicity for humans, animals and vegetation (Saleh et al., 2015). Many techniques such as advanced oxidation technology (Lee et al., 2012), thermal oxidation (Xu et al., 2015), biodegradation (Kwon et al., 2016), adsorption (Liu et al., 2010) and reduction (Huang et al., 2009) are adopted to solve the environmental issues caused by carbon tetrachloride. Among them, adsorption strategy provides a cost-effective and efficient approach to remove carbon tetrachloride.

Biochar can be derived from many kinds of agricultural by-products, sludge and animal wastes under lower temperature (< 700°C) compared to activated carbon (AC) (Chen et al., 2011, Jiang et al., 2016). With large surface area, good porosity and abundant surface functional groups, biochar reveals a higher adsorption capacity than other carbon adsorbents (Luo et al., 2011). It was reported that biochar is a cost-effective and high-efficient adsorbent for heavy metals (Chen et al., 2011), anions (Jung et al., 2015) and organic compounds (Feng et al., 2013). However, biochar usually possesses lower aromaticity and hydrophobicity than AC since it contains more noncarbonized organic matter (Chen et al., 2008). There is a limited chance to contact with carbon tetrachloride by biochar in aqueous solution due to that carbon tetrachloride is a hydrophobic and dense substance. The hydrophobicity, pore volume and varieties of functional groups of biochar can be changed by physicochemical modifying, such as steam activation, chemical modification, impregnation and heat treatment (Ahmed et al., 2016, Liu et al., 2012). Therefore, it is expected that the adsorption capability of hydrophobic pollutants on biochar can be enhanced by suitable modification.

Over the past decades, nanoscale zero-valent iron (nZVI) and bimetallic systems (Ni/Fe, Pd/Fe, Ni/Si and Ag/Fe) have been proved to be valid ways for the removal of chlorinated hydrocarbons from water (Devi and Saroha, 2015, Lu et al., 2014, Wu et al., 2013, Xu et al., 2013). Noble metals or transition metals are usually used as the second metal and catalyst to enhance the dechlorination rate as well as alter the dechlorination mechanism (Parshetti and Doong, 2012). However, due to van der Waals attraction and high energy level of surface state, nanoparticles are apt to aggregate together to be more stable state but less reactivity (Xu et al., 2013). Generally, there are two approaches to solve the above issues: (1) adding surfactants such as starch (He and zhao, 2005), carboxymethyl cellulose (He et al., 2009), guar gum (Xin et al., 2015) and PVP (Vijayakumar et al., 2012) during the preparation process; (2) stabilizing nanoparticles on the matrix with lots of functional groups (amino, hydroxyl and sulfonic group), such as polymeric resins (Jiang et al., 2011), active carbon (Choi et al., 2008, Wu et al., 2013) and biochar (Devi and Saroha, 2015, Han et al., 2015, Yan et al., 2014). Wu et al. (2013) found that the removal efficiency of bromate by nZVI supported on modified AC was 95.66% (after 5 min), while the efficiency of nZVI/AC was just 54.12%. Han et al. (2015) and Quan et al. (2014) stabilized nZVI on acid modified biochar (MB) for methyl orange dye degradation. Ag is relatively cheaper (compared with conventional catalysts Au, Pd and Pt) and possesses a higher standard potential (0.799 V), indicating that Fe (E^θ_Fe(II)/Fe = − 0.44 V) and Ag can form a galvanic couple with higher potential and faster electron transfer rate (Denis et al., 2013). Luo et al. (2010) described that the removal efficiency of tetrabromobisphenol A by Ag/Fe nanoparticles was much higher than that of nZVI, owing to the existence of Ag as the catalyst. However, there is still limited study about Ag/Fe nanoparticles loading on MB for carbon tetrachloride removal. The primary objective of the work is to utilize the high adsorption and reduction by loading Ag/Fe nanoparticles on MB (Ag/Fe/MB) to enhance carbon tetrachloride removal efficiency.

In this study, a novel adsorbent Ag/Fe/MB has been successfully prepared and investigated in carbon tetrachloride removal in water. The surface morphology, porosity, distribution patterns and surface functional groups of immobilized Ag/Fe nanoparticles were characterized by high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The recyclability and leaching studies were implemented to evaluate the stability of Ag/Fe/MB. The stabilized Ag/Fe nanoparticles were then used as an adsorbent for the removal of carbon tetrachloride. The effects of initial concentration and pH value on the removal efficiency were investigated. In addition, the reaction kinetics and thermodynamics were also investigated to uncover the adsorption and dechlorination mechanisms.