Enhanced adsorption of Cu(II) and Zn(II) from aqueous solution by polyethyleneimine modified straw hydrochar

Highlights

• Desired hydrochars after acid/alkali activation and PEI loading were synthesized.

• Higher Cu(II) and Zn(II) adsorption ability was obtained by alkali-PEI-HC.

• Energy of straws converted into hydrochar precursor was consumed as 1.08 kWh kg⁻¹.

Abstract

Heavy metals removal from aqueous phase by adsorption technique has recently attracted a considerable interest. Although various adsorbing materials have been developed, introducing more functional groups is considered as the most efficient way to promote the adsorption capacity of the selected adsorbent. However, this approach is usually limited in costly modification precursor and unguaranteed loading efficacy. In this study, waste corn straw was converted to adsorbent precursor by hydrothermal carbonization. The obtained hydrochar (HC) was chemically activated before being modified by polyethyleneimine (PEI). Multiple analysis methods including Scanning Electron Microscopy, Fourier Transform Infrared analysis, and X-ray Photoelectron Spectroscopy analysis verified the alkali activated hydrochar (alkali-HC) was more efficacy to enhance PEI grafting than acid activation. Based on this, the modified HC materials obtained a better adsorption performance. The sorption process of Cu(II) and Zn(II) on the acid-PEI-HC, alkali-PEI-HC, and pristine HC fitted the pseudo second order kinetic and Freundlich model well, and was dominated by chemisorption. Among these adsorbents, the adsorption capacity of alkali-PEI-HC to metal ions was the maximum, which was 207.6 mg/g to Zn(II) and 56.1 mg/g to Cu(II) at 298 K. Regeneration tests showed a result of no less than 60% of its removal capacity was achieved after five cycles. Therefore, alkali-PEI-HC performed as a promising composite sorbent for metal ions. In addition, the study described here has provided a new basis for the utilization of hydrochar (1.08 kWh kg⁻¹) derived from agricultural resources as a promising adsorbent precursor.

Introduction

Heavy metal is one kind of the most concerned pollutants in aquatic environment. Recently, the industrial effluents, rural sewage, livestock farming wastewater, etc., has been the major emission source of heavy metals (Oh et al., 2019; Chakraborty et al., 2020). The heavy metal ions concentration in various aquatic environments is approaching dangerous levels (Anastopoulos et al., 2019; Chen et al., 2019). Even the essential trace metal elements, such as Cu(II) and Zn(II), have risk to spread through food chain, and could be absorbed and accumulated in organisms excessively to trigger diseases, like energy metabolism disruption, and cognitive loss (Sorouraddin and Nouri, 2016; Kepp and Squitti, 2019; Leong et al., 2018). Therefore, it is necessary to do the decrement of heavy metals from contaminated sources before discharging.

Diverse techniques have emerged to remediate heavy metals from effluents, such as adsorption, oxidation/electrochemical oxidation, chemical precipitation, membrane filtration, reverse osmosis, coagulation/flocculation (Gao et al., 2019; Pang et al., 2019; Suzaimi et al., 2019). However, some of their shortcomings could limit their scope of application: precipitation and coagulation/flocculation, chemical processes to remove heavy metals, need high operation costs and are easy to cause secondary pollution. Membrane filtration and reverse osmosis, as typical membrane separation technology, have high cost for membrane preparation and specific requirements for water quality. Biological method has higher ecological stability, but it has significant seasonal differences, and its processing period is longer. By comparison, adsorption, a physiochemical method can avoid those drawbacks (Zhang et al., 2020a; Wang et al., 2017). It is widely used due to the high efficiency, eco-friendly features, and proneness to chemical modifications (Jinendra et al., 2019; Khan et al., 2017; Ng et al., 2017). Moreover, considering the principles of “Green Chemistry”, the use of biochar generated from biowaste as sorbent have aroused growing interests (Das and Goud, 2020; Anastopoulos et al., 2019; Cao et al., 2017).

Hydrothermal carbonization (HTC) is one promising process to convert biomass into carbonaceous solid, i.e. hydrochar, effectively (Cao et al., 2017), which is conducted at temperatures of 180–280 °C in water. Recently, it has been widely used in agricultural field for the agricultural wastes conversion to valuable chemicals. Among various agricultural wastes, straws produced from crops with high contents of extractives and lignocellulose, are suitable for hydrochar (HC) production via HTC (Reza et al., 2013). Hydrochars are made up of carbon spheres and present a higher degree of aromatization with larger number of oxygen-containing groups on the surface, which benefits its affinity for water contaminant. Most recent studies mainly focus on the sorption capacity of host hydrochar, for example, Li et al. (2019) reported the adsorption amount of Zn (II) by straw hydrochar was up to 112.8 mg/g. Although progress has been made, the adsorption capacity of host hydrochar was still restricted by insufficient surface functionalities, there is still a lot of space for improvement.

Prior studies have demonstrated that combining adsorbent matrix with the functionality polymers could significantly promote the adsorption performance (Suzaimi et al., 2020). Polyethyleneimine (PEI), which is a hydrosoluble cationic polyelectrolyte, has been recognized that its branched macromolecule and the primary (1/4), secondary (2/4), and ternary amine (3/4) groups exhibit outstanding affinity to metal ions (Wang et al., 2020). However, it is difficult to applicate PEI as adsorbent directly because of its free molecule form. Therefore, it is necessary to immobilize PEI on host material to form a suitable and engineered adsorbent for environmental pollution control (Ma et al., 2020). Some studies have verified that PEI modified materials possessed excellent adsorption ability to remove heavy metal ions from aqueous. Wang et al. (2020) reported that PEI modified peanut shell possessed the adsorption quantity toward Cr(VI) was up to 42.5 mg/g at 293 K. Jin et al. (2017) synthesized PEI modified bacterial cellulose that showed the maximum adsorption capacity on Pb(II) was 141 mg/g. And as reported by Zhang et al. (2016), PEI modified nanofibril adsorbent had good Cu(II) removal ability and its maximum capacity was 52.32 mg/g. Compared with these host matrices, hydrochar has more abundant organic functional groups. It is expected that the hydrochar as the host would have a higher adsorption potential. Nevertheless, the procedure of grafting and crosslinking is necessary for PEI immobilization onto the host material. It is difficult to guarantee the PEI loading efficiency and desired properties. Therefore, the regeneration of the precursor of hydrochars via chemical activation is considered to promote mass transfer and catalyst/modifier loadings. A few studies have demonstrated the effect of chemical reagents (NaOH, KOH, HCl, etc.) on the activation of carbon materials (Zhang et al., 2020a; Jain et al., 2016). However, less have concentrated on the role of acid/alkali activation in PEI loadings.

Our hypothesis is to strengthen the surface functionalization of pristine HC through chemical activation, thereby increasing the grafting amount of nitrogen-containing functional groups on the surface of HC during PEI modification, and finally achieve the goal of improving the removal efficiency of the HC matrix to metal ions. The combination of physical chemistry and instrumental chemistry was used to verify this hypothesis. The present study examined the synthesis of PEI loaded hydrochars beyond a chemical activation pretreatment and applied them to remove metal ions in aqueous media. Specifically, it mainly includes (1) to evaluate the influence of acid and alkali pretreated hydrochar to enhance PEI grafting; (2) to examine the adsorption behaviors of acid- and alkali-PEI-HC as functional adsorbents comprehensively; (3) to investigate the potential mechanism for removing Cu(II) and Zn(II). Furthermore, the views on the prospects of HTC products applicated as absorbent precursor are discussed, and come up with suggestions.