**1. Introduction**

Heavy metal pollution is currently a serious environmental problem. Heavy metal ions are not biodegradable and tend to accumulate in living organisms, thereby causing many serious health and environment problems [1,2]. Therefore, it is of grea<sup>t</sup> significance to remove heavy metal ions in water. In recent years, many methods such as ion-exchange, chemical oxidation and reduction, membrane filtration, chemical precipitation, and adsorption have been employed for removing heavy metal ions from water media [3–7]. Among these treatment methods, adsorption attracts more attention due to its simple operation, low price, recyclability of the adsorbent, and high efficiency in treating low-concentration wastewater [8].

The exploration of efficient adsorbent materials starts by providing additional binding sites for adsorbed ions, improving the diffusion coefficient during the adsorption process, and enhancing the overall mechanical properties of adsorbent materials. Adsorbents with a large specific surface area, additional active groups, such as carboxyl and hydroxyl groups, and rational diffusion path for metal ion during adsorption are required. Deze et al. demonstrated the effect of porosity in heavy metal ions sorption [9].

In our previous work, we synthesized pure calcium alginate and chitosan-calcium alginate hybrid aerogels for removing heavy metal ions in water [10,11]. The results showed that calcium alginate could efficient removal of Pb2+ and Cu2+ from wastewater due to its rich carboxyl ( −COOH) and hydroxyl (−OH) groups. However, dry alginate bead that has been used for heavy metal ion sorption is a rigid material without controlled porous structure, and given the low diffusion coefficient of the material, the sorption kinetics is relatively slow. Furthermore, pure alginate also exhibits several unsatisfactory structural properties, such as weak mechanical strength, structural instability, and fragile collapse [12], thus limiting their applications in the actual setting.

Graphene oxide (GO) is a new carbon material with excellent properties, high specific surface area, and rich surface functional groups. GO also exhibits a considerable potential in reinforcing fillers given its outstanding mechanical properties, excellent binding capacity, and superior flexibility [13]. The unsatisfactory properties of alginate structure and collapse of a porous material structure can be easily solved by adding GO sheets as ideal reinforcing fillers for composites. Jiao et al. presented that GO, as a reinforcing filler, exhibits excellent mechanical strength and elasticity in adsorbing heavy metal ions [13]. Recently, Yang et al. successfully prepared double network hydrogel beads by directly mixing GO and sodium alginate solutions. The as-prepared hydrogel beads showed good affinity to cationic metals and the theoretical maximum adsorption capacity for Mn2+ reached 56.49 mg/g [14]. However, preparing adsorbent materials with controlled pore structure and excellent mechanical properties has not been reported.

In this study, a novel macro-porous (mp) calcium alginate/graphene oxide composite aerogel (mp-CA/GO) with controlled pore structure was prepared by introducing macropores within the composite aerogel using polystyrene (PS) colloidal particles as sacrificial template and GO as reinforcing fillers. The as-prepared mp-CA/GO was characterized and used for heavy metal ions (Pb2+, Cu2+ and Cd2+) sorption. In addition, adsorption capacity, kinetics and thermodynamics properties, adsorption mechanism, and reutilization were also explored.

#### **2. Materials and Methods**
