# Efficient Removal of Lead Ions from Water by a Low-Cost Alginate-Melamine Hybrid Sorbent
## 1 Introduction  
Lead is a non-essential element of the human body [1]. Lead that enters the human body through the food chain cannot be degraded, and also accumulates in the human body for a long time, thereby causing damage to the intestinal system and nervous system, resulting in a series of physiological diseases such as colic, headache, insomnia and convulsions. In addition, lead has greater impacts on children in their development period. Previous studies have indicated that children are 30 times more sensitive to lead ion than adults, and excessive lead can severely affect their ability to speak, to remember and to pay attention [2]. Therefore, it is important to study the removal of lead ions from water.  
Currently, many methods such as ion exchange, chemical precipitation, electrochemistry, membrane filtration, adsorption and biochemical treatment (bio-adsorption, flocculation, microbial metabolism) have been employed for the removal of Pb\({}^{2+}\) from wastewater [3; 4; 5]. Among these methods, the adsorption method has attracted greater attention due to its simple operation, large processing capacity, can be used repeatedly, and has low-concentration wastewater treatment efficiency (\(\leq\)10 mg/L). The most widely used adsorbent at present is activated carbon. Activated carbon has high specific surface area and reaction activity, and therefore it can remove lead ions from water. However, the high-quality activated carbon has a high preparation cost and its regeneration process is cumbersome, which limit the large-scale production and applications of activated carbon.  
Over the past decade, many low-cost lead ion adsorbents have been developed to replace activated carbon [6; 7; 8], such as clay minerals, zeolites, chitosan and its derivatives, industrial waste and so on. However, these adsorbents often have one or more of the following deficiencies in the applicationprocess, for example: (1) less binding sites (functional groups such as -COOH, -NH\({}_{2}\), -OH, -COO, -CHO, etc.) on the surface of adsorbent materials, and a longer time required for adsorption to reach equilibrium; (2) low adsorption capacity for Pb\({}^{2+}\) ions due to a small specific surface area; (3) the adsorbent is a powder material and is not easy to recollect and recycle; (4) the stability is not high. The morphology and adsorption performance of adsorbent decrease rapidly after repeated use; (5) the preparation or modification process is more complicated, and organic solvents (such as toluene, ethanol, etc.) are usually used. To overcome these disadvantages of traditional adsorbents, the macroscopic porous monolithic adsorbents are highly desired.  
In our previous study, we used biocompatible sodium alginate as a raw material to prepare a green and efficient calcium alginate aerogel (CAA) by a simple one-step gel method and vacuum freeze-drying technique [9]. The fabricated CAA sorbent could uptake 96.4% of lead ions from Pb\({}^{2+}\)-containing solutions (1.5 nmol/L). The maximum adsorption capacity for lead ions was 390.7 mg/g. However, after repeated use, the adsorption performance and morphology of CAA changed greatly. In order to solve this problem, in this work, the chemically stable melamine is modified on the surface of CAA to improve the physical and chemical properties of the aerogel. In addition, to the best of our knowledge, there have been no reports on the adsorption of Pb\({}^{2+}\) by combined melamine and sodium alginate. The physical and chemical properties (such as adsorption ability and capacity, selectivity, etc.) of the synthetic alginate-melamine hybrid sorbent (Alg-Mel) are also evaluated by scanning electron microscopy (SEM), infrared spectrometry and inductively coupled plasma-optical emission spectrometry (ICP-OES).