Production of magnetic sodium alginate polyelectrolyte nanospheres for lead ions removal from wastewater

Highlights

• New magnetic sodium alginate polyelectrolyte (SA@AM) nanosphere is prepared.

• SA@AM shows high adsorption capacity of Pb(II) reaching 105.8 mg g⁻¹

• Nano size (15–22 nm) and magnetic function (6.85 emu·g⁻¹) benefit to Pb(II) removal.

• SA@AM shows good recovery performance by acid elution with 76% retention.

Abstract

Polyelectrolyte composite nanospheres are relatively new adsorbents which have attracted much attention for their efficient pollutant removal and reuse performance. A novel polyelectrolyte nanosphere with magnetic function (SA@AM) was synthesized via the electrostatic reaction between the polyanionic sodium alginate (SA) and the surface of a prepared terminal amino-based magnetic nanoparticles (AMs). SA@AM showed a size of 15–22 nm with 6.85 emu·g⁻¹ of magnetization value, exhibiting a high adsorption capacity on Pb(II) ions representing a common heavy metal pollutant, with a maximum adsorption capacity of 105.8 mg g⁻¹. The Langmuir isotherm adsorption fits the adsorption curve, indicating uniform adsorption of Pb(II) on the SA@AM surfaces. Repeated adsorption desorption experiments showed that the removal ratio of Pb(II) by SA@AM was more than 76%, illustrating improved regeneration performance. These results provide useful information for the production of bio-based green magnetic nano scale adsorption materials for environmental remediation applications.

Introduction

Environmental pollution has become an issue that cannot be ignored in current world, for instance, the water health is one of serious concerns (Sonne et al., 2018). The preparation of adsorbent by modification of functional groups is a suitable and effective method to remove lead (Pb) ions (Levin et al., 2008). As a heavy metal pollutant, Pb can seriously threaten the ecosystem and bring human health risks, affecting children's intellectual development (Ghorbani et al., 2020). Pb exposure to respiratory tract, nervous system, digestive system, cardiovascular and urinary system diseases can cause harm to the human body (Levin et al., 2008). Study of Pb toxicity has been a subject of interest for environmental scientists due to its toxic effect on plants, animals, and humans (Zhang et al., 2019a).

Currently, the typical ways for treating heavy metal pollutants can be divided into ion exchange (Chen et al., 2017), membrane separation (Majooni et al., 2020), redox (Sriram et al., 2019), adsorption (Wu et al., 2018), precipitation (Li et al., 2019), and electrochemical method (Liu et al., 2019), etc. Given the advantages of low cost and easy operation, adsorption is widely employed for cleaning up the heavy metals from wastewater (Godwin et al., 2019). To ensure the adsorption performance of pollutants, the choice of adsorbent is very important and sometimes can be critical (Da'na, 2017). Now, many adsorbent materials have been researched, such as hydrogel (Larraneta et al., 2018), porous solid adsorbent (Yasmin et al., 2015), activated carbon (Xia and Shi, 2016), biochar (Ge et al., 2020), polyelectrolyte (Lee et al., 2018), etc. Polyelectrolyte is a type of material possessing an abundance of ionic functional groups which could reversibly adsorb metal ions by adjusting the pH of their solution (Kato et al., 2002). It has been reported to be good at adsorbing Pb(II) contaminants from wastewater, however, collection of the adsorbents was suffered trouble (Ewulonu et al., 2019).

To deal with the recycled adsorbents after adsorption, efforts have been made to incorporate magnetic functions to the absorbents for simply taking away by a magnet (Zhu et al., 2020). Zhang et al. (2019b) developed a novel hybrid nanoparticle owning excellent adsorption efficiency, nanostructure, low cost, and eco-friendly feature. Other magnetic adsorbents have been studied and reported, however, most of the studies focused on the inorganic materials (Jemutai-Kimosop et al., 2020).

Sodium alginate (SA) is a natural bioproduct derived from iodine and mannitol extraction from Phaeophyceae (Bhat and Aminabhavi, 2007). As a polyanionic electrolyte, SA can be used as feedstock for synthesizing polyelectrolyte. For example, chitosan and sodium alginate are assembled into polyelectrolyte composite nanoparticles (Yusif et al., 2014). Some researchers have mixed alginate with activated carbon (AC) and MnO₂ recovered from waste battery waste and used them to effectively remove p-cresol and tylosin from water (Shim et al., 2019). In this study, AN and Q-AN are promising for use as adsorbents to remove Pb(II) and then recover it from wastewater with respect to the advantages of simple preparation, high adsorption capacity, and recyclability (Qi et al., 2015). For instance, Liu and Zhao (2013) prepared chitosan/sodium alginate polyelectrolyte composite nanoparticles through ionic gel method, which presented very good pH response characteristics for adsorbing metal ion contaminants with different charges. Huang et al. (2014) explored the mechanism of polyelectrolyte complex synthesis between cellulose microcrystal and sodium alginate. As a new coupling agent or surfactant, it can overcome the defects of attachment and improve the performance. The adsorption capacity increased significantly with the increase of amino surface density. To facilitate the collection process after adsorption, Fe₃O₄ nanoparticles were introduced into the SA-based polyelectrolyte to provide magnetic function. As a magnetic absorbent, the magnetic SA-based polyelectrolyte could be easily removed by a magnet. The magnetic function can also promote the reuse of absorbent since the regenerated polyelectrolyte could also be separated from the washing solution. HCl was chosen as the regenerant because it has a good ability to elute total metal ions (Shim et al., 2019). The newly developed magnetic polyelectrolyte can be easily removed by a magnet and recovered by acid washing, which could simplify the cleaning process of wastewater and make the absorbent more sustainable. The drawbacks of hard-to-separate from water and reuse for traditional adsorbents can be addressed by this new-developed magnetic polyelectrolyte nanosphere.

In this study, magnetic SA-based polyelectrolyte nanospheres were synthesized via Ca²⁺ ion crosslinking reactions and electrostatic interactions between SA and amino modified Fe₃O₄ nanoparticles. The chemical structure, surface properties and morphology of the new magnetic polyelectrolyte were characterized and discussed. The tests and modeling of the Pb(II) adsorption were conducted, and the recovery performance was evaluated. This discovery can provide a feasible solution for the preparation of magnetic adsorbents.