Characterization of graphene/pine wood biochar hybrids: Potential to remove aqueous Cu²⁺

Highlights

• Cu²⁺ removal capacities of graphene-biochar hybrids were assessed.

• All hybrids have higher surface areas than raw biochar.

• The hybrid with the lowest graphene surface area had the highest sorption capacity.

• Larger graphene surface areas resulted in lower surface area hybrids.

• Evidence for enhanced Cu²⁺ adsorption by thick graphene sheets is presented.

Abstract

Biochar-based hybrid composites containing added nano-sized phases are emerging adsorbents. Biochar, when functionalized with nanomaterials, can enhance pollutant removal when both the nanophase and the biochar surface act as adsorbents. Three different pine wood wastes (particle size < 0.5 mm, 10 g) were preblended with 1 wt% of three different graphenes in aqueous suspensions, designated as G1, G2, and G3. G1 (S_BET, 8.1 m²/g) was prepared by sonicating graphite made from commercial synthetic graphite powder (particle size 7–11 μm). G2 (312.0 m²/g) and G3 (712.0 m²/g) were purchased commercial graphene nanoplatelets (100 mg in 100 mL deionized water). These three pine wood-graphene mixtures were pyrolyzed at 600 °C for 1 h to generate three graphene-biochar adsorbents, GPBC-1, GPBC-2, and GPBC-3 containing 4.4, 4.9, and 5.0 wt% of G1, G2, and G3 respectively. Aqueous Cu²⁺ adsorption capacities were 10.6 mg/g (GPBC-1), 4.7 mg/g (GPBC-2), and 5.5 mg/g (GPBC-3) versus 7.2 mg/g for raw pine wood biochar (PBC) (0.05 g adsorbent dose, Cu²⁺ 75 mg/L, 25 mL, pH 6, 24 h, 25 ± 0.5 °C). Increased graphene surface areas did not result in adsorption increases. GPBC-1, containing the lowest nanophase surface area with the highest Cu²⁺ capacity, was chosen to evaluate its Cu²⁺ adsorption characteristics further. Results from XPS showed that the surface concentration of oxygenated functional groups on the GPBC-1 is greater than that on the PBC, possibly contributing to its greater Cu²⁺ removal versus PBC. GPBC-1 and PBC uptake of Cu²⁺ followed the pseudo-second-order kinetic model. Langmuir maximum adsorption capacities and BET surface areas were 18.4 mg/g, 484.0 m²/g (GPBC-1) and 9.2 mg/g, 378.0 m²/g (PBC). This corresponds to 3.8 × 10⁻² versus 2.4 × 10⁻² mg/m² of Cu²⁺ removed on GPBC-1 (58% more Cu²⁺ per m²) versus PBC. Cu²⁺ adsorption on both these adsorbents was spontaneous and endothermic.

Introduction

Industry releases large amounts of toxic inorganic and organic substances into the environment worldwide, driving the need for remediation (Kim et al., 2016). Heavy metals discharged in industrial effluents pose serious health threats since they persist and bioaccumulate in nature (Agrafioti et al., 2014). Many metals have critical biological roles at low concentrations but cause acute and chronic illness at higher concentrations. Toxicity and carcinogenic effects caused the USEPA to establish limits for 13 heavy metal priority pollutants (Ag, As, Cu, Pd, Zn, Hg, Cd, Be, Cr, Ni, Sb, Se, and Tl) (Hsieh et al., 2004). The permissible limit for Cu²⁺ in industrial effluents is 1.3 mg/L (US EPA, 2015). The World Health Organization has announced that drinking water Cu²⁺ concentrations should be less than 2 mg/L (WHO, 2011).

Of the metals with EPA effluent limits, copper is produced in the greatest quantity (US EPA, 2015; BGS, 2019; World Steel Association, 2019). World copper production increased by 29% from 2007 to 2017 (15.5–20 million metric tonnes) (ICSG, 2018). Copper contributes more to water pollution than Cd, Pb, Cr, and Hg because of its widespread use in plumbing, electrical wiring, air conditioning, tubing, and roofing (Aksu and İşoğlu, 2005). Wastewater from these industries contains high copper concentrations (ICSG, 2019; Selvanathan et al., 2017), and industrial process waters have copper ion concentrations as high as 2000 mg/L (Cséfalvay et al., 2009). Copper is an essential trace element (adult daily dietary uptake of copper, 1–2 mg). It regulates many enzyme functions involving cell respiration, melanin synthesis, protection against free radicals, and formation of connective tissues. Nevertheless, high copper concentrations may cause human liver damage (LD₅₀, 220.5 ± 23.8 μg/mL copper sulfate, upon 48 h exposure) (Tchounwou et al., 2008), nausea, abdominal pain, vomiting, and other gastrointestinal symptoms (≥3 mg/L, as copper sulfate) (Nancharaiah et al., 2015; Pizarro et al., 1999). Copper can inhibit the metabolic activity of denitrifying bacteria when concentrations reach up to 0.95 mg/L (IC₅₀) (Ochoa-Herrera et al., 2011) and can also be carcinogenic (Lowndes and Harris, 2005). Higher serum copper levels were reported for advanced breast cancer versus early breast cancer patients (177.9 μg/dL vs. 130.4 μg/dL) (Gupta et al., 1991). Therefore, monitoring and controlling of wastewater copper concentrations is essential.

Traditionally, copper remediation from water employs chemical precipitation (Mirbagheri and Hosseini, 2005), reduction (Gómez-Lahoz et al., 1992), filtration (Aziz et al., 2001), flotation (Rubio and Tessele, 1997), coagulation (Hargreaves et al., 2018), adsorption (Chen et al., 2011), ion exchange (Doula and Dimirkou, 2008), or reverse osmosis (Cséfalvay et al., 2009). However, these methods can be expensive and/or ineffective when treating copper ion concentrations below 100 mg/L. Chemical-mechanical polishing effluents can contain dissolved copper at 5–100 mg/L (Pérez-Marín et al., 2007; Yang et al., 2017). Lime precipitation effectively removes copper only if metal concentrations exceed 1000 mg/L (Kurniawan et al., 2006), and requires large quantities of precipitating agents. High coagulation-flocculation costs hinder its widespread use in wastewater plants (Kurniawan et al., 2006). Ion exchange is a good option for treating metals as low as 10 mg/L; however, finding a suitable ion exchanger can be problematic (Keane, 1995).

Smooth operation and low cost make adsorption an option for removing copper from wastewater (Chen et al., 2011; Inyang et al., 2011; Karunanayake et al., 2018; Navarathna et al., 2019; Wang et al., 2015). Adsorbents can provide excellent Cu²⁺ binding capacities (q_max of 53.2 mg/g by chitosan-cellulose hydrogel-beads) (Li and Bai, 2005). Recently, biological materials (Yang et al., 2017), clay minerals (Sarı et al., 2007), layered double hydroxides (Kameda et al., 2005), synthetic nanomaterials (Dave and Chopda, 2014), activated carbon (Imamoglu and Tekir, 2008; Rao et al., 2009) and biomass (Acar and Eren, 2006) have been employed and optimized to adsorb copper ions in water streams. High capital and regeneration costs of many commercially available adsorbents limit large-scale applications, so growing efforts seek novel low-cost adsorbents. A large sorbent surface area and an abundance of active functional groups on adsorbent surfaces improve heavy metal ion adsorption (Mosa et al., 2016; Yin et al., 2019). The low-cost, efficient Cu²⁺ removal from wastewater with environmental-friendly adsorbents is of great interest to the wastewater treatment community.

Biochar produced by pyrolysis under oxygen-limited conditions can adsorb Cu²⁺ ions from water (Chen et al., 2011). Partial biomass carbonization leaves oxygenated functions, –COOH, phenolic, keto, ether, and hydroxyl groups on biochar surfaces. These bind or ion exchange with metal cations (Tong et al., 2011). Fast pyrolysis (400–500 °C for 1–40 s) (Lima et al., 2010) biochar surface areas (10–100 m²/g) are substantially lower than those of activated carbon (1000–2500 m²/g) (Imamoglu and Tekir, 2008; Mohan et al., 2014a; Mohan and Pittman, 2006). Biochar is produced for decontamination applications at low cost from abundant and cheap agricultural and forestry biomass wastes but is often overlooked (Chen et al., 2018; Mohan and Singh, 2002; Qian et al., 2015; Tong et al., 2011). Biochar has been used as a support for zeolites to remove aqueous phosphates and humates (Mosa et al., 2020). Additionally, oxidized biochar has been employed to scavenge the phytotoxicity caused by excessive Pb²⁺ accumulated on plants (El-Banna et al., 2019).

Graphene has a theoretical surface area of 2630 m²/g for a completely exfoliated single-layered sample (Lloyd-Hughes and Jeon, 2012). Graphene-based biochar hybrids with increased surface functional groups, surface areas, micro, and mesopore volumes, adsorbed higher amounts of Pb²⁺, Cu²⁺, and methylene blue (Ren et al., 2012, 2011; Tang et al., 2015; Zhang et al., 2012). Introducing a graphene/pyrene derivative onto cottonwood at 600 °C for 1 h generated a biochar hybrid with a methylene blue 174 mg/g removal capacity, 20 times greater than unmodified cottonwood biochar (Zhang et al., 2012). Graphene-pretreated wheat straw biochar's surface became increasingly negatively charged with graphene's addition (Tang et al., 2015). No previous work has been published regarding copper adsorption with graphene-biochar hybrids.

This paper presents the synthesis and comparison of three graphene-biochar hybrids to remove Cu²⁺ from aqueous suspensions. A sample of thick graphene stacks, G1 (8.1 ± 0.7 m²/g), was produced by a 1 h ultrasonication of synthetic graphite dispersed in DI water. G1 and two commercial graphene samples, G2 (312.0 ± 4.0 m²/g), and G3 (712.0 ± 7.0 m²/g), were employed to modify the biochar. These graphene type samples G1, G2, and G3 were mixed and co-pyrolyzed with pine wood under N₂ at 600 °C for 1 h to produce three hybrid adsorbents (GPBC-1, GPBC-2, and GPBC-3) to evaluate their Cu²⁺ sorptions against pine biochar (PBC) pyrolyzed identically. Higher adsorption capacity was reported for GPBC-1 compared to other hybrids. The Cu²⁺ sorption behavior of GPBC-1 over PBC was characterized using different analytical techniques.