**1. Introduction**

Currently, environmental pollution by toxic heavy metals is one of the most alarming problems of modern society [1–3]. Since heavy metals are non-biodegradable and highly toxic, their presence in water resources poses a grea<sup>t</sup> risk to the balance of the natural environment and the health of living beings [4,5].

The divalent Ni2<sup>+</sup> is one of the most toxic heavy metals found in wastewater discharges owing to various anthropogenic activities, such as the manufacture of metal alloys, stainless steel, super-alloys, accumulators, batteries, electrical and electronic products and components, pigments, paints, coins, and ceramics, mineral processing, steel casting, nickel mining and refining, metallurgy, electroplating, leather tanning, and porcelain enameling [3,6,7]. Notably, it is evident that Ni2<sup>+</sup> is widely used in several industrial sectors, including transportation, construction, electronics, aeronautics, automotive, and telecommunications [8].

Exposure to high levels of Ni2<sup>+</sup> causes a range of harmful effects on human health, such as endocrine disorders, gastrointestinal distress, allergies, headache, anemia, dizziness, chest tightness, pulmonary fibrosis, cyanosis, rapid breathing, and encephalopathy, as well as damage to the kidneys, central nervous system, and lungs [1,4,9–11]. Moreover, Ni2<sup>+</sup> exhibits carcinogenic, embryotoxic, and teratogenic properties [9,10]. Therefore, to protect the public health from the harmful effects of Ni2+, the World Health Organization (WHO) established a reference value of 0.07 mg/<sup>L</sup> to control the concentration of nickel in drinking water [12].

The conventional methods used to remediate industrial wastewater contaminated with Ni2+, such as chemical coagulation and precipitation, adsorption onto activated carbon, ion exchange, and various electrochemical and membrane technologies [8,13] have several disadvantages. These disadvantages include high cost, ine fficient or ine ffective treatment of wastewater with low Ni2<sup>+</sup> concentrations, production of toxic chemical sludge that requires additional treatment, and/or they are highly sensitive to the operating parameters [9,14]. These disadvantages together with the increasing implementation of stricter environmental regulations have prompted the search for new treatment technologies [13]. Biosorption is a cost-e ffective, flexible, and e fficient technology for removing heavy metals from aqueous solutions, which uses plant, animal, and microbial biomass or their derived products as biosorbents [15–17]. Agricultural and forestry residues and by-products, which are mainly composed of cellulose, hemicellulose, and lignin, are abundant in nature, renewable, economical, and environmental friendly. Additionally, they are highly e fficient and e ffective for removing organic and inorganic contaminants from aqueous solutions via biosorption. Therefore, they are a viable option for bioremediation of industrial e ffluents contaminated with heavy metals [2,8,13,18,19].

Our previous studies established that the acorn shell of *Quercus crassipes* Humb. and Bonpl. (QCS) is a versatile and e ffective novel biosorbent for removing anionic and cationic heavy metals from aqueous solutions. QCS has a remarkable ability to remove hexavalent chromium (anionic heavy metal in aqueous solution) and to biosorb total chromium from aqueous solutions, both in batch [20,21] and continuous [22] systems.

Furthermore, so far, QCS is one of the best biosorbents reported for the biosorption of Ni2<sup>+</sup> (cationic heavy metal) from aqueous solutions. Therefore, it was established that the QCS performance in the biosorption of Ni2<sup>+</sup> ions is a ffected by the contact time, pH of the solution, initial Ni2<sup>+</sup> concentration, and temperature. The optimal pH for the biosorption of Ni2<sup>+</sup> by QCS is 8.0, whereas its point of zero charge is 5.4. The kinetic and equilibrium biosorption processes of Ni2<sup>+</sup> are significantly represented using the pseudo-second order and Freundlich models, respectively. Moreover, it was established that the biosorption of Ni2<sup>+</sup> by QCS is an endothermic process, non-spontaneous, and of chemical nature, in which the carboxyl, carbonyl, and hydroxyl functional groups play a major role in the removal of the heavy metal [9].

One of the critical parameters to be considered in the scaling up and large-scale application of biosorption processes is the presence of co-ions in the wastewater to be treated [20,23]. Therefore, it is important to note that most studies on biosorption of toxic heavy metals have been carried out using synthetic solutions that contain the metal of interest only. However, real industrial e ffluents are usually complex mixtures containing di fferent types of background electrolytes, such as monovalent and divalent cations and anions at di fferent concentrations [24]. The background electrolytes and their concentrations can a ffect the biosorption of the heavy metal of interest since they can: (1) compete with the heavy metal of interest for the available biosorption active sites, (2) decrease the specificity of the biosorbent by binding to sites to which the metal ion of interest does not bind, and/or (3) form chemical complexes or precipitates with the heavy metal of interest [20,23,24].

In spite of their grea<sup>t</sup> importance and relevance for the biosorption processes of toxic heavy metals, there is practically no information in the specialized literature about the e ffects of ionic strength and competing ions on the biosorptive removal of heavy metals from aqueous solutions [25]. This information is crucial for analyzing, interpreting, understanding and designing biosorption processes for heavy metal removal from aqueous solutions, meaning there is a clear need for investigation concerning the inhibitory e ffects of ionic strength and competing ions on the biosorption of the heavy metal of interest.

Therefore, the aims of the current investigation are to assess the influences of background cations, background anions, and NaCl ionic strength, on both the biosorption of Ni2<sup>+</sup> ions onto QCS in aqueous solution, and the kinetic modeling of the Ni2<sup>+</sup> biosorption process.

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