**1. Introduction**

Chemical process industries generate tremendous quantities of wastewater that significantly contribute to aquatic environmental pollution. This has demanded the development of sustainable and effective treatment methodologies for industrial effluents [1]. Design and development of innovative process technology to efficiently handle large volumes of polluted water streams in a relatively shorter time are highly desirable and promising for industrial applications. Phenol is the most commonly present contaminant in the industrial effluent stream. It is known to be highly toxic and causes harmful chronic effects on humans and animals alike [2]. This raises concern over the traces of phenolic content present in drinking water and the negative externalities caused by their discharge

from the effluent treatment plants on the environment [3]. Many technologies have been developed for the decontamination of phenol-polluted wastewater streams, including membrane separation, advanced oxidation, activated sludge, ion exchange and adsorption [4–6]. Out of the various specified technologies, adsorption proves to be the most viable option due to its economic feasibility, ease of scalability and strategic removal of the target compound(s) [7,8]. It offers a wide variety of highly selective adsorbents with a good regeneration potential [9,10]. Since it does not yield any formation of sludge [11], adsorption does not pose any threat to the environment and achieves a high product quality [12,13]. Batch-mode and column-mode are the two major categories of the adsorption operation.

Column adsorption types consist of four main types: a fixed-bed type; continuous moving-bed type; fluidized-bed type and pulsed-bed type. Of these, the fluidized-bed type is widely adopted in industries due to its ability to handle large feed volumes and its easier process control [9]. The fluidized-bed technology offers the salient advantages of improved heat/mass transfer rates, enhanced interfacial contact area and isothermal operation [10]. Within the fluidized-bed column adsorption, the circulating fluidized-bed (CFB) offers effective liquid-solid contact, uniform temperature, high throughputs and better solid holdup control as compared to the conventional fluidized-bed type [11]. Although gas-solid-type CFBs have been vastly investigated from the 1960s, research studies on a liquid-solid CFB (LSCFB) are very scant. The concept of a LSCFB gained momentum around the 1990s and has been explored widely in the last ten years [10]. The LSCFB technique is very promising for various industries, such as pharmaceuticals, biotechnology catalytic refining, wastewater treatment, etc. [9]. In this work, we primarily focused on phenol adsorption studies via a liquid-solid circulating fluidized bed (LSCFB). The usage of LSCFB for phenol adsorption provides significant advantages over the conventional fluidized beds. These advantages include an enhanced adsorption efficiency, high operational simplicity, better yields, high liquid residence times, low chemical consumption, applicability to non-clarified streams and an increase in the interfacial area between the solid and liquid phases [9,12].

With the use of the appropriate solid particles, the LSCFB system would help overcome the limitations of a conventional fluidized bed. To aid the process of fluidization in the column, the adsorbent used in this study is glass beads [13] coated with commercial activated carbon, primarily because of its high density. Epoxy resin was used for coating these beads with activated carbon due to its strong adhesive properties. Before the column studies, batch investigations were conducted to identify the optimized operating conditions for the LSCFB.
