**Highlights:**


**Abstract:** In the present work, the adsorption behavior of Eriochrome Black T (EBT) on waste hemp activated carbon (WHAC) was examined. The surface of the WHAC was modified by H3PO4 acid treatment. The surface and structural characterization of the adsorbents was carried out using Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) analysis. The effect of influential adsorption parameters (pH, contact time, dosage, and initial concentration) on the adsorption of EBT onto WHAC was examined in batch experiments; some adsorption parameters such as pH, concentration and dose were improved by new mathematical models. The adsorption behavior of EBT on the surfaces of WHAC was evaluated by applying different isotherm models (Langmuir, Freundlich, Temkin and Dubinin–Radushkevich) to equilibrium data. The adsorption kinetics was studied by using pseudo-first-order, pseudo-second-order, Elovich and intraparticle models on the model. Adsorption followed the pseudo-second-order rate kinetics. The maximum removal of EBT was found to be 44–62.08% by WHAC at pH = 7, adsorbent dose of 10–70 mg, contact time of 3 h and initial dye concentration of 10 mg.L<sup>−</sup>1. The maximum adsorption capacities were 14.025 mg.g−<sup>1</sup> obtained by calculating according to the Langmuir model, while the maximum removal efficiency was obtained at 70 mg equal to 62.08% for the WHAC. The adsorption process is physical in the monolayer and multilayer.

**Keywords:** cannabis waste; isotherms; Eriochrome black T; biosorbent; mathematical models

**Citation:** El Mansouri, F.; Pelaz, G.; Morán, A.; Da Silva, J.C.G.E.; Cacciola, F.; El Farissi, H.; Tayeq, H.; Zerrouk, M.H.; Brigui, J. Efficient Removal of Eriochrome Black T Dye Using Activated Carbon of Waste Hemp (*Cannabis sativa* L.) Grown in Northern Morocco Enhanced by New Mathematical Models. *Separations* **2022**, *9*, 283. https://doi.org/ 10.3390/separations9100283

Academic Editor: Ernesto Reverchon

Received: 15 August 2022 Accepted: 29 September 2022 Published: 3 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **1. Introduction**

Hemp or (*Cannabis sativa* L.), is an annual herbaceous plant, belonging to the family of Cannabaceae, and originating in Central Asia. Regardless of its origin, hemp is commonly grown and cultivated not only in Asian countries [1] but in Africa, Europe, Canada, and the United States, and is one of the oldest cultivated plants known to humans. Since 5000–4000 BC, [2], it has been used as a food and fiber source, as well as playing an important role in medical use. It contains several chemically bioactive compounds, such as cannabinoids, terpenoids, flavonoids, and alkaloids [3]. It contains more than 100 active chemical compounds well-known as 'cannabinoids' [4].

Waste is a major worldwide problem and produces severe ecological and socioeconomic troubles. The exploitation of waste has become of vast scientific and industrial importance in order to reduce ecological harm, attain sustainability and progress, apropos a circular economy. Energy, fuels, and other value-added products can be recuperated from waste [5]. Moreover, owing to the fast growth in the quantity and nature of agricultural waste biomass, which comes from the growth of population and rigorous agricultural behaviors, these wastes are deemed as a consequential cause of pollution with an annual growth frequency of 5–10%. People produce about 150 billion metric tons of agricultural waste [6].

As a biological and ecological alternative, the application of agro-waste as precursors of materials with high adsorptive abilities is being explored [7]. Bio-adsorbents (bioproducts) are natural elements suitable for water treatment in view of the many benefits and remarkable characteristics of these assets. They are ample, inexhaustible, decomposable, and economic [8], moreover, they have macromolecular chains with various extremely reactive chemical functions [9]. In addition, adsorption is designed within the effective methods for the elimination of water contaminants because of its facility of function and the aptitude to eliminate various kinds of contaminants, providing larger application in water quality management [10].

Amid the diversity of adsorbents, activated carbon has been demonstrated to remain efficient in the elimination of contaminants from water and even in a gaseous atmosphere [11] Activated carbon, a commonly used adsorbent in industrial procedures, is constituted of a pored, consistent organization with a great surface area and displays radiation stability [12]. On the other hand, its application is restricted because it is expensive and very difficult to be regenerated [13,14]. Therefore, an investigation of the fabrication of activated carbon from inexhaustible, inexpensive native farming waste has won interest due to its reduced cost and extremely plentiful qualities [15,16]; in this context, certain of the farming wastes that have been investigated as an alternative source of activated carbon are barley seeds [17], orange peel [18], cassava peel [19], eucalyptus bark [20], coconut shells [21], hazel nuts [22], and tobacco steam [23].

Eriochrome Black T (EBT) is one of the colorants that are employed in water hardness resolution, in the production of paper and textiles, and also in the biomedical research field [24]. It was identified as a water pollutant which causes substantial damage to the environment. EBT is an organic azo colorant that is tolerant to natural treatment as most azo colorants are, due to their aromatic rings and sulfonate groups. Even at reduced amounts, it is not easily decayed by chemical or bacterial deterioration [25].

The overall objective of this study is to report the synthesis and characterization of hemp waste (*Cannabis sativa* L.) and its activated carbon. The as-synthesized materials were used as adsorbents for the removal of Eriochrome Black T dye in an aqueous solution. The effects of various experimental parameters on the adsorption of the dye were examined. The adsorption parameters (pH, concentration, and dose) were improved by new mathematical models obtained with a polynomial interpolation.
