Adsorption behavior and mechanism of Lead (Pb²⁺) by sulfate polysaccharide from Enteromorpha prolifera

Abstract

Lead (Pb²⁺) pollution poses severe healthy and ecological risks to humans. In this work, sulfate polysaccharide from Enteromorpha prolifera (SPE) was utilized for Pb²⁺ adsorption from simulated intestinal fluid. In order to evaluate its adsorption behaviors comprehensively, batch adsorption of Pb²⁺ was investigated under different conditions. Results showed that SPE presents high adsorption ability for Pb²⁺ through chemical adsorption process and the maximum adsorption capacity for Pb²⁺ was 278.5 mg/g. And SPE exhibited higher removal efficiency (≥60%) for trace Pb²⁺ (<10 mg/L) compared to that of other adsorbents based on polysaccharide. Besides, its adsorption can be described by Langmuir isotherm and pseudo-second-order kinetic models. Further, XRD, FTIR, and XPS were used to characterize the possible interaction of Pb²⁺ with SPE, and the results showed that carboxyl and hydroxyl groups in SPE play more important role than that of sulfate group. Our work represents the first assessment of Pb²⁺ adsorption properties of SPE. This investigation highlights the potential application of SPE to protect the body from hazard of food-derived heavy metals.

Introduction

At present, human health faces potential risk because of the increasing chemical contamination from dyeing pigment, heavy metal irons, and other toxic suspended particles [1]. Especially, due to the toxicity, long persistence, non-biodegradable, and biological magnification in foods chain, the contamination of heavy metals has been a serious threaten worldwide [2]. Among the various heavy metals, lead (Pb²⁺), a carcinogenic contaminant, is the second in the priority list of hazardous substances to human beings [3] and its level of environment has increased more than a thousand-fold in the past 300 years with the development of urbanization and industry [4]. There is no safe threshold for Pb²⁺ exposure and even low Pb²⁺ level (<10 μg/dL blood) could result in adverse health outcomes [5]. Dietary Pb²⁺ is adsorbed and transported by epithelial cells in the gastrointestinal tract, and then distributes to various organs through the blood [3]. The Pb²⁺ can disrupt the homeostasis of essential element, and even inhibit Ca²⁺ transport [6]. Expect that, Pb²⁺ could fasten the formation of reactive oxygen species (ROS) which causes oxidative stress and damages in living cells. Further, it causes severe damage to central nervous, hematopoietic, hepatic, and renal systems of human body [7].

Without doubt, the management of Pb²⁺ exposure and toxicity remains a major public health worldwide. As a consequence, the development of safe strategies for Pb²⁺ excretion from human body is one area of ongoing researches [8]. In order to protect body from Pb²⁺ poisoning, it's the most common therapeutic strategy to use ethylenediaminetetraacetic acid (EDTA) and dimercaptosuccinic acid (DMSA) to promote Pb²⁺ excretion. However, a series of side-effects of EDTA and DMSA were showed and these chelators are not suitable for the treatment of high-dose and long-term [9]. Thus, micronutrients, plant extracts, and probiotics, as safe dietary supplements, gain increasing attention to intervene and alleviate Pb²⁺ intoxication. Sulfated polysaccharide from Enteromorpha prolifera (SPE), a linear polymer, is extracted from E. prolifera, which is one of the most widely distributed green algae and collected for consumption [10]. Yu et al. [11] reported that the backbone of SPE is D-GlcAp-α-(1 → 4)-3-sulfate-L-Rhap-β-(1 → 4)-D-Xylp-β-(1 → 4)-3-sulfate-L-Rhap units. The SPE shows a variety of bioactivities including haematopoiesis, anti-tumor, gastrointestinal protection, anticoagulant properties, antioxidation, and immunomodulation [12]. In addition, SPE shows adsorption ability for metal cations because of its mass of hydroxyl, carboxyl, and sulfate groups [13]. The results of Li et al. [14] indicated that SPE adsorbs Fe³⁺ efficiently and the SPE‑iron complex ameliorates the anemia symptoms. Besides, the chromium (III) adsorption of SPE was investigated and the results showed that SPECr³⁺ derivative is a promising nutraceutical for type 2 diabetes mellitus [15], [16]. Obviously, the aforementioned researches focused mainly on the bioactivities of SPE combining with beneficial metal. To the best of our knowledge, there is a dearth of literature dealing with the role of SPE to remove hazardous heavy metal from the intestinal system related to organism detoxification. Although a few researches reported the adsorption of SPE for metal irons, the adsorption mechanism such as isotherm, kinetic, and thermodynamic was not studied, especially in the simulated intestinal fluid. Besides, E. prolifera forms green tide easily because of water eutrophication and many oceans are suffering from its massive accumulation [17]. Its frequent outbreaks have became serious threat to marine ecosystem and affected marine transportation and tourism adversely [18]. Application of E. prolifera in functional or health promoting foods is significant for its exploitation.

Consequently, the aim of this study was to investigate the role of SPE exposed to Pb²⁺ ions in vitro. Firstly, the SPE was purified and then used for the subsequent research. The conditions influenced the adsorption of SPE were evaluated in the batch mode as function of SPE concentration, pH, initial Pb²⁺ concentration, temperature, and contact time. Moreover, the experimental biosorption data were further interpreted by kinetic and isotherm equations to estimate the mechanism. Finally, the role of surface functional groups present in the SPE during Pb²⁺ biosorption was examined using FTIR and XPS techniques. The morphology of SPE and SPEPb²⁺ was determined by SEM analysis to get better insights of biosorption process. These results were of potential significance for the practical usage of SPE for heavy metal detoxification.