Elsevier

Environmental Pollution

Volume 240, September 2018, Pages 1-9
Environmental Pollution

Phosphoric acid pretreatment enhances the specific surface areas of biochars by generation of micropores

https://doi.org/10.1016/j.envpol.2018.04.003Get rights and content

Highlights

  • Phosphoric acid treatment enhances surface area due to the generated micropores.

  • Cellulose produces more micropores than lignin after phosphoric acid treatment.

  • Acid catalysis and crosslinking contribute to micropore generation.

  • Large surface area and pore volume of the treated biochars enhanced the sorption.

  • Phosphoric acid modified biochars could be applied in P-depleted soil.

Abstract

Biochars are being increasingly applied in soil for carbon sequestration, fertility improvement, as well as contamination remediation. Phosphoric acid (H3PO4) pretreatment is a method for biochar modification, but the mechanism is not yet fully understood. In this work, biochars and the raw biomass were treated by H3PO4 prior to pyrolysis. Due to an acid catalysis and crosslink, the micropores of the pretreated particles were much more than those without H3PO4 pretreatment, resulting in the dramatical enhancement of specific surface areas of the pretreated particles. Crystalline cellulose (CL) exhibited a greater advantage in the formation of micropores than of amorphous lignin (LG) with H3PO4 modification. The formation mechanisms of micropores were: (a) H+ from H3PO4 contributes to micropores generation via H+ catalysis process; (b) the organic phosphate bridge protected the carbon skeleton from micropore collapse via the crosslinking of phosphate radical. The sorption capacities to carbamazepine (CBZ) and bisphenol A (BPA) increased after H3PO4 modification, which is ascribed to the large hydrophobic surface areas and more abundant micropores. Overall, H3PO4 pretreatment produced biochars with large surface area and high abundance of porous structures. Furthermore, the H3PO4 modified biochars can be applied as high adsorbing material as well as P-rich fertilizer.

Introduction

Biochars are carbon-rich materials prepared by pyrolysis under limited oxygen condition, and have attracted increased attention for potential application in carbon sequestration and soil quality improvement (Wu et al., 2013; Xiao et al., 2014). Although biochars are proposed as pollution remediation materials to adsorb various contaminants (Chu et al., 2017; Uchimiya et al., 2010), they generally have low surface area in the range of pyrolytic temperatures below 500 °C (Lian and Xing, 2017), which limited their sorption of contaminants as an adsorbent (Gomez-Eyles et al., 2013). In addition to surface area, other factors, such as pore structure, contribute to the sorptive properties (Li et al., 2017b). It is therefore useful to explore effective techniques to further optimize biochar properties to facilitate their applications.

Various chemical treatment techniques, such as H2SO4, KOH and ZnCl2 methods, have been proposed and tested to modify biochars with improved sorption capacity (Aghababaei et al., 2017; Lau et al., 2017; Xia et al., 2016). Compared to these methods, phosphoric acid (H3PO4) treatment exhibits several advantages: relatively low pyrolysis temperature, low corrosivity to the equipment, minor pollution, and low costs (Girgis and El-Hendawy, 2002; Li et al., 2010). Previous studies have investigated the impact of H3PO4 treatment on biochar characteristics, mostly on the application as adsorbent for contaminants. For instance, Fernandez et al. (2015) characterized activated hydrochars (char substrates prepared by hydrothermal carbonization) from orange peels after H3PO4 modification at 600 °C. They observed better developed porosity and investigated sorption of emerging organic contaminants on hydrochars after H3PO4 modification. Wu et al. (2017) studied the sorption of Cr(VI) on modified biochar from pomelo peel, and also pointed out that modified biochar possessed the developed porous structure after H3PO4 treatment at 450 °C. Taha et al. (2014) studied the removal of a mixture of 15 different pesticides from water using biochars treated with H3PO4 at 80 °C. The surface functional groups and aromatization of treated biochars was increased and pesticides could be easily removed from contaminated water using biochars treated by H3PO4.

These papers, however, did neither discuss possible modifying mechanisms of H3PO4 treatment, nor the effect of precursor structural characteristics on the producted biochars. A lot of papers reported that pore structure of H3PO4-treated biochars was dominantly mesoporous (Guo and Rockstraw, 2006; Jagtoyen and Derbyshire, 1998). But, Zhao et al. (2017) recently reported that micropores were largely generated on modified biochars through the interaction between H3PO4 and carbon structure. It was generally agreed that the type of pore structure depended on the nature of feedstock and pyrolysis temperature (Lian and Xing, 2017). The observation was the generation of pore structure, the decrease of pore size and the increase of porosity as a result of the escape volatile matter during carbonization (Tan et al., 2015). These pore structure in biochars plays an important role in sorption process (Chu et al., 2017). However, the discussion of which kind of pores (macro-, meso-, or micro) are central in the general sorption properties of biochars is under debate. For instance, Phuong et al. (2015) reported that the macropores (≥50 nm) apparently have a significant effect on the water sorption capacity of the biochar. Contrarily, Li et al. (2017b) attributed the sorption capacity of biochars to mesopores (2–50 nm), rather than micropores. A third group of researchers claim that micropores (<2 nm) are central in the sorption process of inorganic and organic pollutants as reported by Pituello et al. (2015) and Liu et al. (2012).

A great deal of researches has demonstrated that sorption is an effective treatment for organic contamination in soils (Ghaffar et al., 2015; Jin et al., 2016). Carbamazepine (CBZ), an antiepileptic drug, can be frequently detected at notable concentrations in environment (Wang et al., 2013). Bisphenol A (BPA) is widely applied for plastic production and is well known for its endocrine disrupting potential (Pan et al., 2008). Both chemicals have been paid wide attention due to their health risk to humans (Gomez et al., 2007; Wanda et al., 2017). They can be discharged into the agricultural soils in different ways, including fertilization and irrigation (Knight et al., 2017; Novo et al., 2018). Therefore, CBZ and BPA were selected as representatives of organic compounds. Both chemicals were selected as the adsorbates also owing to their comparable hydrophobicity and molecular masses, but different molecular structures and functional groups (Table S1).

The main objective of this study was to illustrate the development of porous structure of the biochars as affected by H3PO4 treatment. Specifically, different precursors of different structural characteristics will be modified and thus the mechanisms of H3PO4 treatment will be discussed, with a major focus on the formation of porous structure of treated biochars. We hypothesize that particularly micropores are beneficial for the sorption of CBZ and BPA.

Section snippets

Materials

Pine sawdust (PS), obtained from the local wood processing mill (Kunming, Yunnan, China), were used to produce biochars. The collected PS were air-dried for 3 days, oven-dried overnight at 60 °C, and then ground to pass through a 60 mesh sieve. The representative compositions of biomass, cellulose (CL) and lignin (LG), were purchased from Sigma-Aldrich Chemical Co. for biochar preparation. CBZ and BPA were purchased from Beijing Chemicals Reagent Company (Analytical grade reagents). Selected

Pore structure development

With increases in the pyrolysis temperature, the SSA increased for all three types of charred samples, BCPS, BCPS-H, and PS-H (Fig. 1a). Clearly, without H3PO4 treatment, the SSA of BCPS was much lower than that of the other two types of H3PO4-treated biochars at the same temperature. For instance, BCPS3 has a SSA of 9.15 m2/g, while the SSA of BCPS3-H3 and PS-H3 increased to 795 m2/g and 1148 m2/g, respectively. The elevated SSA of modified biochars from H3PO4 treatment can be associated with

Conclusions

The phosphoric acid treatment was used to prepare modified biochars from pine sawdust. The two types of modified biochars had much larger specific surface area than that of the pristine biochars, which was attributed to the micropore development. Micropores were generated extensively on modified biochars through hydrogen protons catalysis effect. The specific surface area as well as the micropores of modified biochars from pine sawdust may greatly be attributed to the crosslink of cellulose

Acknowledgements

This research was supported by National Natural Scientific Foundation of China (41663013, U1602231 and 41703121), and Yunnan applied basic research project (2016FA040).

References (49)

  • M. Jagtoyen et al.

    Activated carbons from yellow poplar and white oak by H3PO4 activation

    Carbon

    (1998)
  • J. Jin et al.

    Properties of biochar-amended soils and their sorption of imidacloprid, isoproturon, and atrazine

    Sci. Total Environ.

    (2016)
  • A.Y.T. Lau et al.

    Surface-modified biochar in a bioretention system for Escherichia coli removal from stormwater

    Chemosphere

    (2017)
  • J. Li et al.

    The role of ash content on bisphenol A sorption to biochars derived from different agricultural wastes

    Chemosphere

    (2017)
  • K. Li et al.

    Characterization and lead adsorption properties of activated carbons prepared from cotton stalk by one-step H3PO4 activation

    J. Hazard Mater.

    (2010)
  • M. Novo et al.

    Endocrine disruptors in soil: effects of bisphenol A on gene expression of the earthworm Eisenia fetida

    Ecotoxicol. Environ. Saf.

    (2018)
  • H. Peng et al.

    Adsorption of ofloxacin on carbon nanotubes: solubility, pH and cosolvent effects

    J. Hazard Mater.

    (2012)
  • H. Peng et al.

    Adsorption of ofloxacin and norfloxacin on carbon nanotubes: hydrophobicity- and structure-controlled process

    J. Hazard Mater.

    (2012)
  • H.B. Peng et al.

    Enhanced adsorption of Cu(II) and Cd(II) by phosphoric acid-modified biochars

    Environ. Pollut.

    (2017)
  • F. RodríGuez-Reinoso et al.

    Preparation of activated carbon cloths from viscous rayon. Part II: physical activation processes

    Carbon

    (2000)
  • D.K. Shen et al.

    The mechanism for thermal decomposition of cellulose and its main products

    Bioresour. Technol.

    (2009)
  • M.S. Solum et al.

    Evolution of carbon structure in chemically activated wood

    Carbon

    (1995)
  • N.V. Sych et al.

    Porous structure and surface chemistry of phosphoric acid activated carbon from corncob

    Appl. Surf. Sci.

    (2012)
  • S.M. Taha et al.

    Adsorption of 15 different pesticides on untreated and phosphoric acid treated biochar and charcoal from water

    J. Environ. Chem. Eng.

    (2014)
  • Cited by (0)

    This paper has been recommended for acceptance by Joerg Rinklebe.

    View full text