Functionalization and Evaluation of Inorganic Adsorbents for the Removal of Cadmium in Wastewater

This study presents the feasibility of using various functionalized substrates, Fe3O4 nanoparticles (NPs) and Al2O3 spheres, for the removal of Cd from aqueous solution. To improve the materials’ affinity to Cd, we explored four different surface modifications, namely (3-Aminopropyl) triethoxysilane (APTES), L-Cysteine (Cys) and 3-(triethoxysilyl) propylsuccinic anhydride (CAS). Particles were characterized by FTIR, FIB-SEM and DLS and studied for their ability to remove metal ions. Modified NPs with APTES proved to be effective for Cd removal with efficiencies of up to 94%, and retention ratios up to 0.49 mg of Cd per g of NPs. Batch adsorption experiments investigated the influence of pH, contact time, and adsorbent dose on Cd adsorption. Additionally, the recyclability of the adsorbent and its potential phytotoxicity and animal toxicity effects were explored. The Langmuir, Freundlich, pseudo-first-order and pseudo-second-order models were applied to describe the behavior of the Cd adsorption processes. The adsorption and desorption results showed that Fe3O4 NPs modified with APTES are promising low-cost platforms with low phytotoxicity for highly efficient heavy metal removal in wastewater.

Adsorption and Desorption of Cd: Characteristics and Mechanisms in Reclaimed Soil Under the Influence of Dissolved Organic Carbon

Organic acids are widespread in the environment, where they play an important role in the adsorption, desorption, and migration of Cd in soil. This study evaluated the characteristics and mechanisms of Cd adsorption and desorption in reclaimed soil from the Panyi Mine, in the Huainan mining area (China), under the influence of humic acid (HA). The addition of HA inhibited the adsorption of Cd but enhanced its desorption in reclaimed soil. With an increasing concentration of added HA, the adsorption capacity of Cd in the reclaimed soil gradually decreased, while the desorption capacity gradually increased. That is, the adsorption inhibition/desorption promotion effect was stronger under higher concentrations of organic acid. Cd adsorption and desorption dynamics could be described as a logarithmic function of organic acid concentration. Kinetic curves for Cd adsorption and desorption in reclaimed soil under the influence of organic acid showed that both adsorption and desorption involved two stages: a fast reaction stage, followed by a slow reaction stage. The factors influencing the capacity of Cd adsorption and desorption in soil were analyzed by gray correlation analysis, and their resulting rank order was as follows: Cd concentration > HA concentration > pH > temperature. Using linear regression analysis, a multi-factor coupling functional model of soil Cd adsorption and desorption under the influence of DOC was established, in the form of Y = a + bXHA + cXT + dXPH + eXCd.

Adsorption and desorption characteristics and mechanism of Cd in reclaimed soil under the influence of dissolved organic carbon

Organic acids are widespread in the environment and play an important role in the adsorption, desorption, and migration of soil Cd. This study evaluated the characteristics and mechanism of Cd adsorption and desorption in reclaimed soil from Panyi Mine in the Huainan mining area under the influence of humic acid (HA) and citric acid (CA). The addition of HA and CA inhibited the adsorption and enhanced the desorption of Cd in reclaimed soil, with HA having a stronger effect than CA. As the concentration of added HA or CA increased, the adsorption capacity of Cd in the reclaimed soil gradually decreased, while the desorption capacity gradually increased. That is, the adsorption inhibition/desorption promotion effect was stronger for higher concentrations of organic acid. Cd adsorption and desorption could be described as a logarithmic function of organic acid concentration. The kinetic curves of Cd adsorption and desorption in reclaimed soil under the influence of organic acids show that both adsorption and desorption involved two stages: a fast reaction stage followed by a slow reaction stage.

Adsorption Characteristics of Cd and Pb on Microplastic Films Generated in Agricultural Environment

Objectives:Plastics are widely used in daily life and in various industrial fields due to their convenience and thermal insulation, and the use of plastics is continuing to increase. In agricultural environments, plastics are largely used for vinyl houses and mulching vinyl. Used plastics are degraded into small fragments through physical, chemical, and biological processes. Among these, plastics with a particle size of 5 mm or less are defined as microplastics. Since microplastics have a relatively large surface area, various pollutants including heavy metals can potentially be adsorbed and affect the agricultural ecosystem. The purpose of this study is to evaluate the Pb and Cd adsorption characteristics on microplastic films frequently generated in agricultural environments.Methods:The Pb and Cd adsorption characteristics on microplastic films were studied with 3 different plastic films. Microplastic samples were prepared by cutting PVC (polyvinyl chloride) and PE (polyethylene) films to have the sizes of < 5 mm×5 mm. The prepared microplastic samples were placed in Pb or Cd solutions having a range of concentrations and shaken for 72 h for sorption tests. The experimental results were fitted to the Langmuir, Freundlich, and Temkin isothermal adsorption models.Results and Discussion:The isothermal adsorption test results obtained fitted well to the Langmuir and Freundlich models (R²>0.9), but not the Temkin model. Regardless of the plastic type, the values of Langmuir constant (KL) and Temkin constant (AT) for Pb were larger than those for Cd, and the Freundlich constant (nF) showed a tendency of Pb>Cd>1, indicating the greater Pb sorption strength or affinity than Cd on PVC and PE.Conclusions:This study shows that the microplastics in soil adsorb heavy metals like Cd and Pb affecting the heavy metal fate in the soil environment. With an increasing attention on the environmental problems due to plastic wastes, this study provides the basis for the need of management of soil environment. Therefore, further research on microplastics in soil environment is required.

Adsorption Characteristics of Spent Coffee Grounds as an Alternative Adsorbent for Cadmium in Solution

The present study was conducted to ascertain the potential of spent coffee grounds (SCGs) as an alternative adsorbent for the removal of cadmium (Cd) from aqueous solutions. Therefore, Cd adsorption batch tests, using SCGs that had not undergone any chemical pretreatment, were conducted using Cd solutions with a wide concentration range (i.e., 0.1–120 mM) under various adsorption conditions, e.g., ion strength, pH, and solid/solution ratio. For comparison, zeolite, as a well-known common representative adsorbent, was included to determine the adsorption efficiency. The adsorption capacity of the SCGs increased with the decreasing ionic strength of the test solution from 200 to 0 mM of Ca(NO3)2 and the increasing solid:solution ratio from 1:4 to 1:400. The most significant factor influencing the adsorption capacity of the SCGs was the pH of the test solution, with increases in the adsorption capacity as the initial solution pH was increased from 2 to 10. However, the rate for Cd removal remained constant, at 71.19%, when the initial solution pH was between 4 and 8 due to the buffer capacity of SCGs. The obtained data were fitted with the Freundlich, Langmuir, and Dubinin-Radushkevich isotherm models. The Langmuir isotherm provided the best correlation for Cd adsorption onto SCGs (R2 = 0.96) and zeolite (R2 = 0.92), and the maximum Cd adsorption capacity of the SCGs was 19.32 mg g-1, which was higher than that of zeolite (13.91 mg g−1). These results mean that the SCGs can be utilized as alternative low-cost biosorbents to replace conventional adsorption materials.