Recent Advancements in Chitosan-based Adsorbents for Hexavalent Chromium Removal from Aqueous Media

Contamination of aqueous environments by Cr(VI) has become a matter of concern owing to its detrimental impacts on human health with its long-term exposure. Thus, effective treatment of adulterated aqueous media is critical in terms of the health and safety of humans together with flora and fauna. Currently, chitosan is considered an excellent adsorptive material for theremediation of Cr(VI) contamination owing primarily to its biodegradability, non-toxicity, abundance, and ability to modify its microstructure. The present review focuses on the up-to-date progression of chitosan-based sorbents that can be utilized in the mitigation of Cr(VI) oxyanions from aqueous media. This paper provides an overview of pristine chitosan along with recentadvancements and insights into structurally and chemically modified chitosan. Chitosan has been chemically modified through cross-linking, grafting, and/or combination with other adsorptive materials to enhance its performance in Cr(VI) removal. Structurally modified chitosan-based hybrid materials that are commonly used in Cr(VI) removal include magnetic adsorbents,hydrogels, aerogels, and nano/microparticles. The sorption capacities of chitosan-based hybrid materials have varied from 27.25 - 357.14 mg g-1 depending on the type of adsorbent, dosage, initial Cr(VI) concentration, pH, and type of modification. Also, beneficial information through a compare-and-contrast of the effectiveness of the stated sorbent materials and their variants in themitigation of Cr(VI) is provided. Furthermore, mechanisms of Cr(VI) removal by chitosan-based sorbents accentuating the main governing mechanism, electrostatic interactions are described and discussed. Desorption and regeneration studies are presented to assess the reusability of the chitosan-based adsorptive materials utilized in the mitigation of Cr(VI) contamination. Desorptionstudies reveal that sorption of Cr(VI) onto most of the chitosan-based adsorbents are fairly reversible with desorbed percentages above 60% with the usage of an efficient stripping agent. Through the literature survey of approximately 100 recently published papers, it could be evinced that chitosan-based adsorbents have proved to be an outstanding sorbent even if some challengesremain. Keywords: Adsorption, chitosan, Cr(VI), hybrid materials, mechanis

Biochars as Potential Adsorbers of CH4, CO2 and H2S

Methane gas, as one of the major biogases, is a potential source of renewable energy for power production. Biochar can be readily used to purify biogas contaminants such as H2S and CO2. This study assessed the adsorption of CH4, H2S, and CO2 onto four different types of biochars. The adsorption dynamics of biochars were investigated in a fixed-bed column, by determining the breakthrough curves and adsorption capacities of biochars. The physicochemical properties of biochars were considered to justify the adsorption performance. The results showed that CH4 was not adsorbed well by the subjected biochars whereas CO2 and H2S were successfully captured. The H2S and CO2 breakthrough capacity were related to both the surface adsorption and chemical reaction. The adsorption capacity was in the following order: perilla > soybean stover > Korean oak > Japanese oak biochars. The simultaneous adsorption also leads to a competition of sorption sites. Biochars are a promising material for the biogas purification industry

Hydrometallurgical processes for heavy metals recovery from industrial sludges

Hydrometallurgical approaches have been successfully employed for metal separation and recovery from various types of waste materials. Therefore, hydrometallurgy is a promising technology for metal recovery and the removal of potentially toxic heavy metals found in industrial sludge. However, a comprehensive review that focuses on the heavy metal recovery from industrial sludge using hydrometallurgical approaches has not been conducted in the recent past. The present review discusses the capacity of hydrometallurgical techniques in recovering heavy metals sourced from different types of industrial sludges, highlighting recent scientific findings. Hydrometallurgical approaches primarily consist of three process stages: metal dissolution, concentration and purification, and metal recovery. The chemical characteristics of industrial sludge, including the type, concentration and speciation of heavy metals, directly impact selection of the best recovery method. Solvent extraction, ion-exchange, and adsorption are the major techniques employed in concentration and purification, whereas electrodeposition and precipitation are the main methods used in metals recovery. Future research should focus on the development of more efficient and environmentally-friendly methods for metal dissolution from industrial sludges contaminated with multiple metals, while increasing selectivity and energy use efficiency in the concentration and purification, and recovery steps

Recent technologies for nutrient removal and recovery from wastewaters: A review

Water scarcity and its pollution has become a concern in recent times. The disposal of nutrient-rich (nitrogen and phosphorous) wastewater is also one of the main cause of water pollution through eutrophication, reduced dissolved oxygen that poses threat to aquatic ecosystems. As a result, nutrient removal has become a mandate apart from the removal of organics. However, the removal of nutrients from sewage is a challenging task. Conversely, conventional biological treatment processes provide little relief in nutrient removal. The treated effluents from conventional biological processes do not achieve the stringent nutrient removal disposal standard limits and become primary cause of pollution in the receiving water bodies. This has stressed upon the need for eco-friendly, low-energy and cost-efficient nutrient removal treatment technologies. Various biological treatment combinations or variants are in use for the efficient removal of nutrients. The biological processes in itself or in combination with chemical processes are preferred over technologies based solely on physico-chemical processes for its treatment performance at lower cost. This review summarizes the existing treatment processes and their possible up-gradation with the aim to accomplish the marked effluent standards for the nutrients. The concept of conventional systems and advanced systems for nutrients (nitrogen and phosphorous) removal which are already developed or under development are deeply discussed. Further, the challenges of each treatment systems are abridged. Finally, the possible suggestions for the modification/retrofitting of existing treatment systems for achieving stringent disposal standards are pointed out

Biochars as Potential Adsorbers of CH4, CO2 and H2S

Methane gas, as one of the major biogases, is a potential source of renewable energy for power production. Biochar can be readily used to purify biogas contaminants such as H2S and CO2. This study assessed the adsorption of CH4, H2S, and CO2 onto four different types of biochars. The adsorption dynamics of biochars were investigated in a fixed-bed column, by determining the breakthrough curves and adsorption capacities of biochars. The physicochemical properties of biochars were considered to justify the adsorption performance. The results showed that CH4 was not adsorbed well by the subjected biochars whereas CO2 and H2S were successfully captured. The H2S and CO2 breakthrough capacity were related to both the surface adsorption and chemical reaction. The adsorption capacity was in the following order: perilla > soybean stover > Korean oak > Japanese oak biochars. The simultaneous adsorption also leads to a competition of sorption sites. Biochars are a promising material for the biogas purification industry

Metal Release From Serpentine Soils in Sri Lanka

Ultramafic rocks and their related soils (i.e., serpentine soils) are non-anthropogenic sources of metal contamination. Elevated concentrations of metals released from these soils into the surrounding areas and groundwater have ecological-, agricultural-, and human health-related consequences. Here we report the geochemistry of four different serpentine soil localities in Sri Lanka by coupling interpretations garnered from physicochemical properties and chemical extractions. Both Ni and Mn demonstrate appreciable release in water from the Ussangoda soils compared to the other three localities, with Ni and Mn metal release increasing with increasing ionic strengths at all sites. Sequential extraction experiments, utilized to identify “elemental pools,” indicate that Mn is mainly associated with oxides/(oxy)hydroxides, whereas Ni and Cr are bound in silicates and spinels. Nickel was the most bioavailable metal compared to Mn and Cr in all four soils, with the highest value observed in the Ussangoda soil at 168 ± 6.40 mg kg−1 via the 0.01-M CaCl2 extraction. Although Mn is dominantly bound in oxides/(oxy)hydroxides, Mn is widely dispersed with concentrations reaching as high as 391 mg kg−1 (Yudhaganawa) in the organic fraction and 49 mg kg−1 (Ussangoda) in the exchangeable fraction. Despite Cr being primarily retained in the residual fraction, the second largest pool of Cr was in the organic matter fraction (693 mg kg−1 in the Yudhaganawa soil). Overall, our results support that serpentine soils in Sri Lanka offer a highly labile source of metals to the critical zone

Pyrolysis temperature and steam activation effects on sorption of phosphate on pine sawdust biochars in aqueous solutions

Biochar can be used as an adsorbent for phosphate removal in aquatic environments to treat eutrophication problems. Designing biochars that have large phosphate adsorption capacity through altering pyrolysis conditions and applying activation techniques will improve phosphate removal efficiency. In this study, four pine sawdust biochars were produced at 300 and 550 °C with and without steam activation. Batch sorption experiments including isotherm and kinetic studies were conducted to understand how phosphate removal capabilities and adsorption mechanisms of biochars were affected by pyrolysis temperature and steam activation. Our results showed that the steam activation and pyrolysis temperature did not affect phosphate adsorption by the biochars. The four biochars removed <4% of phosphate from the aqueous solution, which were not affected by the pH of the solution and biochar application rate. The repulsion forces between biochar surfaces and phosphate ions were likely the cause of the low adsorption

Pyrolysis temperature and steam activation effects on sorption of phosphate on pine sawdust biochars in aqueous solutions

10 Halama

Cr(VI) Formation Related to Cr(III)-Muscovite and Birnessite Interactions in Ultramafic Environments

Chromium is abundantly and primarily present as Cr­(III) in ultramafic rocks and serpentine soils. Chromium­(III) oxidation involving chromite (FeCr₂O₄) via interactions with birnessite has been shown to be a major pathway of Cr­(VI) production in serpentine soils. Alternatively, Cr­(III)-bearing silicates with less Cr­(III) may provide higher Cr­(VI) production rates compared to relatively insoluble chromite. Of the potential Cr­(III)-bearing silicates, Cr­(III)-muscovite (i.e., fuchsite) commonly occurs in metamorphosed ultramafic rocks and dissolution rates may be comparable to other common Cr­(III)-bearing phyllosilicates and clays. Here, we examine the formation of Cr­(VI) related to Cr­(III)-muscovite and birnessite (i.e., acid birnessite) interactions with and without humic matter (HM) via batch experiments. Experimentally, the fastest rate of Cr­(VI) production involving Cr­(III)-muscovite was 3.8 × 10^–1 μM h^–1 (pH 3 without HM). Kinetically, Cr­(III)-muscovite provides a major pathway for Cr­(VI) formation and Cr­(VI) production rates may exceed those involving chromite depending on pH, available mineral surface areas in solution, and the abundance of Cr­(III) present. However, when HM is introduced to the system, Cr­(VI) production rates decrease by as much as 80%. This highlights that HM strongly decreases but may not completely suppress the formation and mobilization of Cr­(VI). A Sri Lankan serpentine soil was utilized to provide context with regards to the experimental results. Despite Cr­(VI) in the soil solids and Cr­(VI) formation being favorable from Cr­(III)-bearing minerals, no detectable Cr­(VI) was released into soil solutions potentially due to the abundance of HM. Overall, the dynamic interactions of Cr­(III)-bearing silicates and birnessite provide a kinetically favorable route of Cr­(VI) formation which is tempered by humic matter

Sorption process of municipal solid waste biochar-montmorillonite composite for ciprofloxacin removal in aqueous media

This study evaluates a novel adsorbent for ciprofloxacin (CPX) removal from water using a composite derived from municipal solid waste biochar (MSW-BC) and montmorillonite (MMT). The composite adsorbent and pristine materials were characterized using powder X-Ray Diffraction (PXRD), Fourier-Transform Infrared (FTIR) spectroscopy, and Scanning Electron Microscope (SEM) before and after the adsorption. Batch experiments were conducted to study the mechanisms involved in the adsorption process. Ciprofloxacin sorption mechanisms were interpreted in terms of its pH-dependency and the distribution coefficients. The SEM images confirmed the successful binding of MMT onto the MSW-BC through flaky structure along with a porous morphology. Encapsulation of MMT onto MSW-BC was exhibited through changes in the basal spacing of MMT via PXRD analysis. Results from FTIR spectra indicated the presence of functional groups for both pristine materials and the composite that were involved in the adsorption reaction. The Hill isotherm model and pseudo-second-order and Elovich kinetic models fitted the batch sorption data, which explained the surface heterogeneity of the composite and cooperative adsorption mechanisms. Changes made to the MSW-BC through the introduction of MMT, enhanced the active sites on the composite adsorbent, thereby improving its interaction with ionizable CPX molecules giving high sorption efficiency