Heavy metal and phenol adsorptive properties of biochars from pyrolyzed switchgrass and woody biomass in correlation with surface properties

Abstract

In this work, the surface structures of biochars, derived from three types of biomass, switchgrass (SG), hardwood (HW) and softwood (SW) through either fast pyrolysis (FP) in a fluidized-bed reactor (at 500 °C) or slow pyrolysis (at 500° and 700 °C), were studied in detail, and compared with that of the activated carbons obtained by steam activation of the slow pyrolyzed biochars (at 500 °C). The surface acidic functional groups were determined quantitatively by the Boehm Titration method. The adsorptive properties of heavy metals, Zn²⁺ and Cu²⁺ onto the biochars and the activated carbons were investigated by the adsorption isotherms and SEM images, and correlated with the surface properties. ATR-FTIR and GC techniques were used to analyze the adsorptive behavior of phenol onto the biochars and activated carbons, and the results demonstrated that phenol adsorption capability is directly proportional to the micropore surface area as well as the combined level of the accessible carboxylic and lactonic groups. The relative adsorption capacity with respect to the biomass precursor follows the order: SW > HW > SG.

Introduction

Despite their low concentration, heavy metals are some of the most toxic inorganic pollutants found in water and soils, and can accumulate slowly. Heavy metals can be absorbed by plants and can leach into groundwater resulting in poisonous and harmful effects on many forms of life (Alumaa et al., 2002; Bradl, 2004; Mohan et al., 2007; Naidu et al., 1998; Veeresh et al., 2003). Among all heavy metals, zinc (Zn) and copper (Cu) are some of the most widely used industrially, and thereby constitute materials with the most potential source of pollution and hence causing the most concerns (Yu et al., 2000, 2001) to environmental researchers worldwide. Numerous treatments have been developed for cleaning water and/or soils contaminated with zinc and copper, among them the application of biochar has been shown to be particularly promising due to its low cost. Biochars have also been demonstrated to be some of the most suitable adsorbent materials for these metals (Regmi et al., 2012).

Uchimiya et al. (2011) investigated the retention of Cu²⁺, Ni²⁺, Cd²⁺ and Pb²⁺ by biochars produced from broiler litter manure at 350 °C and 700 °C with and without steam activation, and concluded that higher pyrolysis temperature with activation and higher pH enhanced the immobilization of heavy metals. In an earlier study, Uchimiya et al. (2010) selected phosphoric acid activated carbon and broiler litter biochar as adsorbents to compare Cu²⁺ sorption onto two distinct soil types: clay-rich alkaline soil and eroded acidic soil. Their results suggested that pH and nutrient leaching have an impact on long-term desorption of Cu²⁺ from amended soils. Paradelo and Barral (2012) examined the immobilization of Cu²⁺, Pb²⁺ and Zn²⁺ with biochars produced from two types of municipal solid waste and indicated that the sorption for these three elements took place in the following affinity sequence: Pb²⁺ > Cu²⁺ > Zn²⁺, which was in agreement with those reported by Veeresh et al. (2003) and Peric et al. (2004). Chen et al. (2011) and Arias et al. (2006) claimed there would be competitive adsorption when different metal ions co-existed. Their study indicated that the presence of increasing concentration of Cu²⁺ resulted in severely depressed adsorption of Zn²⁺, but the adsorption of Cu²⁺ was not greatly reduced by the presence of increasing concentrations of Zn²⁺. Mohan et al. (2007) tested the adsorption of the toxic materials from water with fast pyrolysis biochar produced from oak bark, pine bark, oak wood and pine wood in an auger-fed reactor at 400 and 450 °C and concluded that for these samples, the maximum adsorption occurred over a pH range of 3–4 for As, and between 4 and 5 for Pb and Ca. In the same study, they found that ion exchange was the dominating mode by which the biochars adsorbed the metal ions.

In comparison with the heavy metals, the effective removal of organic pollutants that commonly exist in industrial wastewater, e.g., phenol and substituted phenols (Cichy and Szymanowski, 2002; Jung et al., 2001; Li et al., 2002), and dyes (Faria et al., 2004; Li et al., 2010; Xu et al., 2011) etc. is another major task of great importance and interest. These phenolic compounds are toxic to humans and aquatic life, causing oxygen depletion in receiving waters (Kumar et al., 2009). Research efforts have focused on developing adsorption processes and adsorbent materials for separating organic pollutants from wastewater (Khalid et al., 2004; Kujawski et al., 2004). It has been suggested that activated carbon is one of the most suitable adsorbents for these pollutants because of their finely developed porous structure and various highly effective functional groups (Jung et al., 2001; Qi et al., 2004).

The application of biochar and activated carbon in metal and organic compound adsorption depends on the substrates' structure and chemical properties, which, in turn, are a function of pyrolysis and activation processes that create them. The aim of this study is to fabricate highly effective biochar and activated carbon materials from agricultural feedstocks, i.e. switchgrass, hardwood and softwood, systematically investigate and correlate the immobilization properties of two heavy metals, Zn²⁺ and Cu²⁺, and the adsorption behavior of phenol in aqueous solution to these new materials with their unique surface characteristics such as the pore structures and surface functional groups.