Biopolymers are the most abundant polysaccharides composed of monomeric sugar units linked via glycosidic bonds. These biopolymers play an essential role in the beneficial development of humans, as well as, the environment. In this group, there are exopolysaccharides (EPS), high molecular weight biopolymers that became attractive molecules for research in the past decades over the world. These compounds are composed of sugar units, that include also, proteins, lipids, and/or extracellular DNA as their constituents [1, 2]. Bacterial cells are able to produce two types of EPS, the capsular and the free ones [3]. EPS that are synthesized by the microbes, obtained the unique advantage over other polysaccharides isolated from different sources (starch, cellulose, pectin, glycogen, agar, alginic acids, etc.) because they are secreted outside of the cells from the host, thus avoiding the use of expensive cell lysis chemicals [4]. EPS produced from bacterial cells, as non–toxic, biodegradable, and biocompatible polymers, have found implementation in different industries around the world [5]. Hence, these biopolymers can be used as potential adsorbents for the removal of pollutants from the water.
In the last few decades, the main problem in the world has become the pollution of water, soil, and air, because of the development of unplanned industry and urban development [6]. Heavy metals are the main pollutants that are not biodegradable and have the possibility to accumulate in the human body as well as in the environment [7]. Lead, arsenic, mercury, copper, nickel, chromium, and cadmium are the most common heavy metals in the environment, they can cause many diseases, including: cancers, kidney disease, disease of the cardiovascular system, as well as gastrointestinal disturbances, and neurological disorders [8]. Removal of the metals from industrial wastewater including precipitation [9], electrodepositing [10], ion exchange, membrane separation technologies [11], etc., is economically unprofitable due to the high cost of these processes. Hence, the main advantages of the adsorption processes are the possibility to remove metals from the aqueous solution, reusing the adsorbent materials, as well as, low cost, and little use of the expensive chemicals [12].
Thus, the researches on alternative adsorbents are of great importance for the protection of the environment. EPS as adsorbents become an interesting camp for research, because of the properties that they obtain. As biodegradable, safe, with high adsorption capacity, and reusable, EPS obtained an important characteristic for adsorbents [13]. Synthesis of the novel EPS from bacterial cells, that obtained a highly efficient adsorption ability, monitoring and optimizing the adsorption conditions, and mechanism make these biopolymers very important for the development of green economy and industries [14]. Consequently, biosorption is a technique where the biomaterial is composed of different biomolecules, that occur from nature, and can adsorbed heavy metals which are presented in the environment. Removal of heavy metals from wastewater by biological methods become very attractive in the last decades, considering that microorganisms can uptake the metal actively, by bioaccumulation, or passively by biosorption.
The most common state of the nickel in the environment and biological systems is an oxidative state + 2, nevertheless, it can also appear in several oxidative states from − 1 to + 4. Nickel is an immunotoxin and carcinogen agent, that can cause a variety of the health diseases, such as cardiovascular disease, contact dermatitis, asthma, respiratory tract cancer, and lung fibrosis [15]. Hence, the accumulation of this heavy metal in the body may be responsible for various diseases in human beings. According to the toxicity and negative effect on the human health of nickel, the WHO prescribed that the acceptable limit of the Ni2+ presence in drinking water is 0.01 mgL–1 [16], while the wastewater of industrial discharge is 2 mgL–1 [17]. Consequently, researches in the camp of novel biomaterials become very interesting for researchers in the last few years. Involving these biopolymers in the system of biosorption can improve the quality of the water but, as well as, the environment.
The aim of the present study was to investigate the potential usage of the EPS for the first time extracted from the bacterial strain K. oxytoca J7 [18], as biosorbent, for the purification of contaminated water by removing Ni2+ ions. This new EPS was characterized by Scanning Electron Microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X – Ray diffraction (XRD), thermogravimetric (TG/DTG), and Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass-Spectrometry (MALDI – TOF MS) analysis. The assay of the cytotoxic activity of EPS also was performed. Likewise, all the parameters that influence the biosorption study were assessed, such as pH value of the solution, dosage, temperature, and contact time. Then, isotherm, thermodynamics, and kinetic studies were also performed and analyzed, in order to examine the potential application of EPS from K. oxytoca J7 onto Ni2+ ion in biosorption from contaminated water.