**1. Introduction**

Excessive population growth, urbanization, and industrial development have increased the pollution of the planet and altered ecosystems. Of all environmental pollution, the contamination of water is the most worrisome because of affecting the primordial element on which life is based. The main source of water pollution is the discharge of industrial wastewater with diverse toxic substances, among which heavy metals are of particular concern [1].

Cobalt is a heavy metal found in the Earth's crust, being a natural component of volcanic emissions, as well as surface and subterranean water. It is released into the environment through anthropogenic activities: Burning fossil fuels, applying fertilizers, mining, electroplating, manufacturing batteries, and producing commodities with industrial processes involving cobalt-containing compounds, among others.

Although cobalt is an essential nutrient in human metabolism and the principal component of vitamin B12 [2], it is harmful to our health beyond trace levels, competing with other elements that constitute integral parts of a proper metabolic function [3]. In excess, it can give rise to skin irritation and problems in bone development, as well as respiratory, cardiac, thyroid, liver, and gastric disorders [4,5]. Due to being hazardous to humans and ecosystems [6], cobalt-contaminated wastewater should be treated prior to being released into the environment.

Since low concentrations of cobalt are difficult to remove from water by conventional physicochemical treatments, it is necessary to apply innovative technology characterized by safety, efficiency, and versatility. One alternative is biosorption, a process independent of cell metabolism [7]. This technique, which has been little studied as a remedy for Co2<sup>+</sup> pollution, can be carried out by living, dead, or inactive biological material [8,9].

Biosorption is a process of capturing heavy metals by physical adsorption (physisorption), ionic interchange, chemisorption (e.g., complexation, coordination, and chelation), and microprecipitation [10]. Diverse biological materials are capable of biosorption, including agroindustrial waste, microbial biomass, and biopolymers. These economical materials are available in great quantities, and the respective processes are environmentally friendly [11,12]. Unlike physicochemical methods, biosorption techniques can efficiently remove low concentrations of metals from aqueous solutions. If biosorption is followed by desorption, the metals can be recovered and the biosorbents regenerated for later use [13].

The current contribution focuses on the biosorbent potential of *Lemna gibba*, a macrophyte of universal distribution commonly known as duckweed. This plant, which quickly proliferates to double its biomass in about two days, lends itself to the bioremediation of aquatic systems, due to its small size (2–4 mm) and ability to bioaccumulate toxic compounds (e.g., heavy metals) [14]. Because eutrophication has provoked an excessive spread of *Lemna gibba*, it is now a plague in many places. Its excessive growth in the form of a thick mat on the aquatic body leads to navigation problems, harbors harmful fauna, and prevents sunlight from reaching photosynthetic species in the water below, thus interrupting the correct oxygenation of its environment [15,16]. Apart from being abundantly available, the plant material holds promise as a sustainable biosorbent for treating wastewater contaminated with cobalt and other heavy metals.

According to a previous report, pretreatment of *L. gibba* with K2HPO<sup>4</sup> substantially improves the availability of sorption sites on the surface of plant cells, and therefore, their capacity for Co2<sup>+</sup> biosorption, which is achieved by removing salts and producing a higher negative charge (−35 vs. −26 mV). The zero point of charge (ζ0) was 2.37 for unpretreated and 1.62 for K2HPO4-pretreated *Lemna gibba*, thus creating a greater attraction in the latter for positively charged Co2+. The ATR-FTIR analysis of K2HPO4-pretreated *Lemna gibba* revealed an important role of its hydroxyl and carboxyl groups in the removal of Co2<sup>+</sup> [17]. The aim of the present study was to analyze the performance of K2HPO4-pretreated *L. gibba* as a biosorbent under distinct conditions of pH, particle size, and the initial concentration of Co2+. Various theoretical models were tested to find the best one for describing the experimental data on biosorption. To determine the best eluent solution for desorption, saturated *L. gibba* was processed with strong and weak acids, as well as some alkaline compounds. Considering that recyclability is a prerequisite for the practical application of biosorption technology, three biosorption/desorption cycles were herein evaluated.

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

#### *2.1. Reagents*

The reagents employed in the experiments were all of analytical grade (JT Baker®, Monterrey, Mexico). During the biosorption experiments, the pH of the solutions was maintained constant by adding HCl and NaOH in the solution at a concentration of 0.1 M and 0.01 M, respectively. The different concentrations of Co2<sup>+</sup> were prepared by making dilutions of a stock solution of CoCl2·6H2O (>98% purity) containing 1 g L−<sup>1</sup> of Co2+.

#### *2.2. Preparation of the Biosorbent*

*Lemna gibba* was collected from the Xochimilco canals in Mexico City (19◦15031.800 N 99◦05005.300 W). It was cleaned with running tap water and then deionized water before being dried in a Luzeren® oven (Proveedor de Laboratorios, Mexico) at 60 ◦C for 48 h. Afterward, the material was ground in a hammer

mill (Glen Creston, Ltd., London, UK) and sieved (U.S. ASTM) to obtain fractions of the biosorbent, each with a particular particle size between 0.3 and 2.0 mm (0.3–0.5, 0.5–0.8, 0.8–1.4, and 1.4–2.0 mm). The fractions were all pretreated with K2HPO4. Briefly, 5 g (dry weight) of *Lemna gibba* per liter were exposed to K2HPO<sup>4</sup> (0.3 M) at 18 ◦C for 30 min. During the pretreatment, the material was agitated at 140 rpm in an orbital shaker (All Sheng™, Hangzhou Allsheng Instruments Co, Ltd., Hangzhou, China). Upon completion of the exposure time, the biosorbent was washed with deionized water. When the resulting wash water had a pH near the deionized water being used, the material was dried in an oven at 60 ◦C for 48 h [17]. Each fraction of dried, pretreated *Lemna gibba* (*PLEM*) was stored in a separate, well-labeled, hermetically-sealed bottle at room temperature (rt).
