Adsorption characteristics and mechanism of Cd by mealworm frass as a novel biosorbent

Adsorption characteristics and mechanism of Cd by mealworm frass as a novel biosorbent

The quest for sustainable waste management has led researchers to explore the untapped potential of mealworm frass (MF) as an effective biosorbent for heavy metal removal. Recent studies have highlighted the exceptional adsorption capacity of MF for cadmium (Cd), a highly toxic pollutant, making it a promising candidate for water remediation efforts across Europe.

Mealworm Frass Properties

Mealworms, the larvae of the Tenebrio molitor beetle, have gained significant attention as a sustainable protein source and a solution to environmental challenges. As mealworm production increases, the resulting frass (excrement) has become a readily available byproduct that can be repurposed. Analysis of the MF used in this study revealed a pH of 7.3, an electrical conductivity of 3.4 dS/m, and a carbon-to-nitrogen ratio of 12.7. The material was found to contain substantial amounts of organic matter, including cellulose, hemicellulose, and lignin, which are known to contribute to the adsorption of heavy metals.

Adsorption Capacity

The adsorption capacity of MF for Cd was examined under various environmental conditions, including initial Cd concentration, reaction time, pH, adsorbent dosage, and temperature. The results demonstrated that the adsorption of Cd by MF was predominantly influenced by the initial Cd concentration and reaction time.

The adsorption amount increased in a concentration-dependent manner up to an initial Cd concentration of 100 mg/L, at which point it reached a plateau. This behavior was well described by the Langmuir isotherm model, indicating a chemical adsorption mechanism with a maximum adsorption capacity of 48.1 mg/g. In comparison, recent studies on other woody biomass-based adsorbents have reported Cd adsorption capacities ranging from 18 to 86 mg/g, placing the MF used in this study within the intermediate range.

Adsorption Kinetics

The adsorption of Cd by MF occurred rapidly, with more than 90% of the total adsorption taking place within the first 30 minutes of the reaction. This initial rapid adsorption was attributed to the availability of active sites on the MF surface, while the slower adsorption in the later stages was likely due to internal diffusion within the adsorbent. The adsorption kinetics were best described by the pseudo-second-order (PSO) model, suggesting that the adsorption rate was controlled by chemisorption processes.

Adsorption Mechanism

To elucidate the adsorption mechanism of Cd by MF, various analytical techniques were employed, including Fourier-Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), and sequential extraction experiments.

The FTIR and XRD analyses revealed no significant changes in the surface functional groups or crystal structure of MF before and after Cd adsorption, indicating that the adsorption was predominantly surface-based. Further investigation into the adsorption mechanism revealed the involvement of several key processes:

  1. Cation Exchange: The release of K, Ca, Mg, and Na cations during Cd adsorption suggested that cation exchange played a significant role in the removal of Cd by MF.

  2. Precipitation: The decreased concentration of phosphate in the Cd-adsorbed MF solution implied that precipitation with phosphate compounds contributed to the immobilization of Cd.

  3. Complexation: The reduced concentration of dissolved organic carbon (DOC) in the Cd-adsorbed MF solution suggested that complexation with organic functional groups, such as carboxyl and hydroxyl groups, also contributed to Cd adsorption.

The sequential extraction experiment further confirmed the dominance of ion exchange and carbonate-bound Cd, accounting for 58.9% and 25.2% of the total fractionated Cd, respectively. This indicates that the Cd adsorbed by MF can be easily eluted or mobilized by environmental changes, highlighting the need for a dedicated management plan when utilizing MF as a heavy metal adsorbent.

Factors Influencing Adsorption

pH Effects

The adsorption capacity of Cd by MF was strongly influenced by the initial pH of the solution. As the pH increased from 2 to 6, the adsorption capacity increased, reaching a maximum of 64.1 mg/g at pH 6. This trend is attributed to the decreased competition between Cd ions and hydrogen ions for the available adsorption sites at higher pH levels. However, the adsorption capacity decreased at pH 7-8 due to the formation of negatively charged Cd hydroxide species, which repelled the negatively charged MF surface. At pH 9, the adsorption capacity declined sharply due to the precipitation of Cd in the alkaline environment.

Contact Time

The adsorption of Cd by MF occurred rapidly, with over 90% of the total adsorption taking place within the first 30 minutes. This rapid initial adsorption was followed by a slower, time-dependent increase in adsorption, which eventually reached equilibrium.

Adsorbent Dosage

The treatment efficiency of Cd increased with the dosage of MF, reaching 100% removal at an MF dosage of 8 g/L. However, the adsorption capacity per unit mass of MF decreased as the dosage increased, indicating that the active adsorption sites on the MF surface were not fully utilized at higher adsorbent loadings.

Adsorption Isotherms

The experimental adsorption data were fitted to the Langmuir and Freundlich isotherm models to further understand the adsorption behavior. The Langmuir model provided a better fit, with a coefficient of determination (R2) of 0.9995, suggesting that the adsorption of Cd by MF followed a monolayer chemisorption mechanism. In contrast, the Freundlich model, with an R2 of 0.8661, indicated a multilayer physical adsorption process.

The maximum adsorption capacity derived from the Langmuir model was 48.1 mg/g, which is comparable to or higher than the values reported for other woody biomass-based adsorbents. This highlights the potential of MF as an effective and cost-effective solution for Cd removal from aqueous environments.

The European Future Energy Forum (https://www.europeanfutureenergyforum.com) provides a platform for researchers and industry experts to showcase innovative technologies and strategies that are shaping the continent’s transition to a clean energy future. The findings presented in this study on the adsorption characteristics and mechanism of Cd by mealworm frass contribute to the growing body of knowledge on sustainable waste management and heavy metal remediation, which are crucial components of Europe’s broader decarbonization efforts.

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