Why reuse spent adsorbents? The latest challenges and limitations

Despite the abundance of published reviews over the last few years, the inconsistent data representation in regards to the use of adsorbents in each work, renders the task of comparing them challenging. Disposing the adsorbent may have adverse environmental impact, which should be mitigated through regeneration and reuse processes, such as desorption. This review discusses how the importance of desorption and regeneration equates that of the adsorption stage, and presents various regeneration methods as well as the influencing parameters, advantages, and disadvantages thereof. For the purposes of this work, the adsorbents have been categorized into four groups: (i) graphene, (ii) carbon nanotubes, (iii) activated carbon compounds and (iv) clays and polymer adsorbents as representatives in order to further study their desorption and regeneration abilities, using a variety of desorption media/eluants. The process conditions, such as pH, dose required, concentration, adsorption ability and the cost of the adsorbents were examined for further analysis. The recovery efficiency and ability to get reused through the desorption process was also evaluated. The highest adsorption capacity was observed for graphene-based adsorbents reaching between 108 and >480 mg/g, and for activated carbon materials ranging from 34 to >384 mg/g, whereas carbon nanotubes and polymer-based adsorbents indicated rather low and greatly varying adsorption capacities, between 1 and >138 mg/g and between 7 and >57 mg/g, respectively. Most of the reviewed cases appear to fit the pseudo-second order (PSO) kinetic model. These materials have demonstrated a removal effectiveness between 71% and 99%. Overall, all the aforementioned adsorbents share the advantage of being highly reusable.

Adsorption isotherm models: A comprehensive and systematic review (2010-2020)

Among numerous methods developed in purification and separation industries, the adsorption process has received considerable attention due to its inexpensive, facile, and eco-friendly nature. The importance of the adsorption process causes extraordinary endeavors for modeling the adsorption isotherms during the years; thus, myriads of research have been conducted and many reviews have been published. In this paper, we have attempted to gather the most widely used adsorption isotherms and their related definitions, along with examples of correlated work of the recent decade. In the present review, 37 adsorption isotherms with about 400 references have been collected from the research published in the period of 2010-2020. The adsorption isotherms utilized are alphabetically organized for ease of access. The parameters of each isotherm, as well as the applicable definitions, are presented in the table, in addition to being discussed in the text. Another table is provided for the practical use of researchers, featuring the usage of the related isotherms in peer-reviewed studies.

Synthesis of Nano-Magnetite from Industrial Mill Chips for the Application of Boron Removal: Characterization and Adsorption Efficacy

The present study synthesized nano-magnetite (Fe3O4) from milled steel chips using the high energy ball milling (HEBM) method, characterized it, and then utilized it as a sorbent to remediate boron concentration at various pH (4-9), dosages (0.1-0.5 g), contact times (20-240 min), and initial concentrations (10-100 mg/L). The nano-sorbents were characterized based on SEM structure, elemental composition (EDX), surface area analysis (BET), crystallinity (XRD), and functional group analysis (FTIR). The highest adsorption capacity of 8.44 mg/g with removal efficiency of 84% was attained at pH 8, 0.5 g dosage, contact time of 180 min, and 50 mg/L initial concentration. The experimental data fit best with the pseudo-second-order kinetic model with R2 of 0.998, while the Freundlich adsorption isotherm describes the adsorption process with an R2 value of 0.9464. A regeneration efficiency of 47% was attained even after five cycles of reusability studies. This efficiency implies that the nano-magnetite has the potential for sustainable industrial application.

Metallurgical Wastes Employed as Catalysts and Photocatalysts for Water Treatment: A Review

Metallurgical slags are a mass-produced industrial solid waste, often destined to landfills; the volumes disposed represent an environmental burden. Over the last three decades, applications have been found for these wastes, mainly as a low-cost additive in building materials. More recently, their unique chemical properties have attracted attention to produce high-added-value materials for environmental applications, to be used as adsorbents, catalysts, or a source of reactive species in environmental engineering. Such uses can be classified as a function of the added value generated, technological complexity, and environmental impact. This review will focus specifically on the modification and use of slags for catalysis, photocatalysis, and photocatalytic production of hydrogen, which have received relatively little attention in literature. A summary will be presented about the general requirements for using unmodified slags as well as slag processed under alkaline or acidic conditions for advanced oxidation processes. Then, an overview will be given of the use of slags as photocatalysts in water treatment, organized according to the origin of the product (steel, copper, magnesium, ferromanganese), as well as emerging reports on the photocatalytic production of hydrogen, in contrast to the use of highly specific titania-based products developed for the same purpose.