New insight into the removal of Cd(II) from aqueous solution by diatomite

Modified Natural Diatomite with Various Additives and Its Environmental Potential

Diatomite has recently been the subject of intensive scientific research aimed at its extensive use in industry, breeding and agriculture. The only active diatomite mine is in Jawornik Ruski, in the Podkarpacie region of Poland. Chemical pollution in the environment, including that from heavy metals, poses a threat to living organisms. Reducing the mobility of heavy metals in the environment through the use of diatomite (DT) has recently gained much interest. More effective immobilisation of heavy metals in the environment with DT, mainly through the modification of its physical and chemical properties by various methods, should be applied. The aim of this research was to develop a simple and inexpensive material showing more favourable chemical and physical properties compared with unenriched DT in terms of metal immobilisation. Diatomite (DT), after calcination, was used in the study, considering three grain fractions, i.e., 0-1 mm (DT1); 0-0.5 mm (DT2) and 5-100 µm (DT3). Biochar (BC), dolomite (DL) and bentonite (BN) were used as additives. The proportion of DTs in the mixtures amounted to 75%, and of the additive, 25%. The use of unenriched DTs after calcination poses the risk of releasing heavy metals into the environment. Enrichment of the DTs with BC and DL resulted in a reduction or absence of Cd, Zn, Pb and Ni in aqueous extracts. It was found that for the specific surface area values obtained, the additive used for the DTs was of crucial importance. The reduction in DT toxicity has been proven under the influence of various additives. The mixtures of the DTs with DL and BN had the least toxicity. The obtained results have economic importance, as the production of the best quality sorbents from locally available raw materials reduces transport costs and thus the environmental impact. In addition, the production of highly efficient sorbents reduces the consumption of critical raw materials. It is estimated that the savings from producing sorbents with the parameters described in the article can be significant in comparison with popular competitive materials of other origins.

A modified diatomite additive alleviates cadmium-induced oxidative stress in Bidens pilosa L. by altering soil microbial communities

In the present study, a modified silicon adsorbent (MDSA) was used as a passivator, and we explored the mechanism by which the MDSA helps B. pilosa L. alleviate Cd-induced oxidative stress and its effect on the rhizosphere microbial community. Therefore, a field study was conducted, and MDSA was applied at four levels (control (0 mg m^-2), A1 (100 mg m^-2), A2 (200 mg m^-2), and A3 (400 mg m^-2)). The application of MDSA significantly increased the soil pH and decreased the acid-soluble Cd content, which decreased by 30.3% with A3 addition. The addition of MDSA increased the relative abundance of Sordariomycetes due to the increased invertase activity and total nitrogen (TN) and total phosphorus (TP) contents, and the increased soil pH led to increased relative abundances of Alphaproteobacteria and Thermoleophilia. Meanwhile, MDSA addition significantly decreased the Cd concentrations in leaves and stems, which decreased by 19.7 to 39.5% in stems and 24.6 to 43.2% in leaves. All MDSA additions significantly decreased the translocation factor (TF) values of Cd, which decreased by 30.5% (A1), 50.9% (A2), and 52.7% (A3). Moreover, peroxidase (POD) from the antioxidant enzyme system and glutathione (GSH) from the nonenzymatic system played vital roles in scavenging reactive oxygen intermediates (ROIs) such as H₂O₂ and ⋅O₂^- in leaves, thereby helping B. pilosa L. alleviate Cd-induced oxidative stress and promote plant growth. Hence, our study indicated that MDSA application improved the rhizosphere soil environment, reconstructed the soil microbial community, helped B. pilosa L. alleviate Cd-induced oxidative stress, and promoted plant growth.

Removal of heavy metals from wastewaters with biochar pyrolyzed from MgAl-layered double hydroxide-coated rice husk: Mechanism and application

This study reports a MgAl-LDH rice husk biochar composite (MgAl-LDH@RHB) with a regular hydrotalcite structure synthesized by a simple hydrothermal method, which was then used to remove Cd(II) and Cu(II) from water. The influencing factors on the adsorption performance were determined through batch adsorption experiments, and the adsorption characteristics and cycling capacity were evaluated with eight models and adsorption-desorption experiments. The results showed that the adsorption of Cd(II) and Cu(II) by MgAl-LDH@RHB conformed to the Langmuir-Freundlich model and PSO kinetics model, indicating single-layer chemical adsorption. In addition, the experimental maximum adsorption capacities for Cd(II) and Cu(II) were 125.34 and 104.34 mg g^-1, respectively. The adsorption of Cd(II) and Cu(II) by MgAl-LDH@RHB was dominated by surface precipitation and ion exchange. The findings reveal the mechanism for the heavy metal removal by MgAl-LDH@RHB and provide a theoretical reference for agricultural waste disposal and water pollution control.

Single and Binary Adsorption Systems of Rhodamine B and Methylene Blue onto Alkali-Activated Vietnamese Diatomite

Diatomite was slightly modified with a sodium hydroxide solution. The resulting material was characterized by using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and nitrogen adsorption-desorption isotherms. The so-treated diatomite has a high specific surface area (77.8 m2/g) and a high concentration of isolated silanol groups on the surface, and therefore, its adsorption capacity increases drastically in both the single and binary adsorption systems for rhodamine B and methylene blue. The binary system is more effective than the single system, with methylene blue being adsorbed more than rhodamine B. The adsorption process is spontaneous and fits well with the Langmuir isothermal model, and it depends on pH significantly.

Removal of Cd(II) from Micro-Polluted Water by Magnetic Core-Shell Fe3O4@Prussian Blue

A novel core-shell magnetic Prussian blue-coated Fe3O4 composites (Fe3O4@PB) were designed and synthesized by in-situ replication and controlled etching of iron oxide (Fe3O4) to eliminate Cd (II) from micro-polluted water. The core-shell structure was confirmed by TEM, and the composites were characterized by XRD and FTIR. The pore diameter distribution from BET measurement revealed the micropore-dominated structure of Fe3O4@PB. The effects of adsorbents dosage, pH, and co-existing ions were investigated. Batch results revealed that the Cd (II) adsorption was very fast initially and reached equilibrium after 4 h. A pH of 6 was favorable for Cd (II) adsorption on Fe3O4@PB. The adsorption rate reached 98.78% at an initial Cd (II) concentration of 100 μg/L. The adsorption kinetics indicated that the pseudo-first-order and Elovich models could best describe the Cd (II) adsorption onto Fe3O4@PB, indicating that the sorption of Cd (II) ions on the binding sites of Fe3O4@PB was the main rate-limiting step of adsorption. The adsorption isotherm well fitted the Freundlich model with a maximum capacity of 9.25 mg·g-1 of Cd (II). The adsorption of Cd (II) on the Fe3O4@PB was affected by co-existing ions, including Cu (II), Ni (II), and Zn (II), due to the competitive effect of the co-adsorption of Cd (II) with other co-existing ions.

Preparation of mechanically robust Fe3O4/porous carbon/diatomite composite monolith for solar steam generation

Mechanically robust Fe₃O₄/porous carbon/diatomite composite monolith was prepared from waste corrugated cardboard box and diatomite via slurrying in FeCl₃ solution, dewatering, molding, and carbonization at 600 °C. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (SEM), N₂-adsorption/desorption, Raman spectroscopy, and ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy. The water wettability, photothermal conversion property, and solar steam generation performance of the products were also evaluated. Results showed that the presence of FeCl₃ led to the formation of more pores and magnetic Fe₃O₄ crystallites, while diatomite provided good hydrophilicity for the composite. The product exhibited light absorption above 65% within the wavelength ranging from 200 to1974 nm, and its surface temperature eventually increased by 30 °C under 0.25 sun irradiation due to photothermal effect. Moreover, solar steam yield under 0.25 sun irradiation for 3600 s was improved by 67% with the presence of the monolithic composite because of the occurrence of interfacial solar steam generation and heat transfer from the composite acted as a heat island.

Naturally available diatomite and their surface modification for the removal of hazardous dye and metal ions: A review

The presence of toxic pollutants such as dyes and metal ions at higher concentrations in water is very harmful to the environment. Removal of these pollutants using diatomaceous earth or diatomite (DE) and surface-modified DE has been extensively explored due to their excellent physio-chemical properties and low cost. Therefore, naturally available DE being inexpensive, their surface modified adsorbents could be one of the potential candidates for the wastewater treatment in the future. In this context, the current review has been summarized for the removal of both pollutants i.e., dyes and metal ions by surface-modified DE using the facile adsorption process. In addition, this review is prominently focused on the various modification process of DE, their cost-effectiveness; the physio-chemical characteristics and their maximum adsorption capacity. Further, real-time scenarios of reported adsorbents were tabulated based on the cost of the process along with the adsorption capacity of these adsorbents.