New insights into the interactions between Pb(II) and fruit waste biosorbent

Adsorption dynamics of Cd2+(aq) on microwave-synthetized pristine biochar from cocoa pod husk: Green, experimental, and DFT approaches

Biochar obtained via microwave-assisted pyrolysis (MAP) at 720 W and 15 min from cocoa pod husk (CPH) is an efficient adsorbent of Cd2+(aq). Biochar of residual biomass of CPH (BCCPH) possesses favorable physicochemical and morphological properties, featuring a modest surface area yet a suitable porous structure. Adsorption, predominantly governed by physisorption, is influenced by the oxygen-containing active sites (-COOR, -C(R)O, and -CH2OR; R = H, alkyl). CdCO3 formation occurs during adsorption. Experimental data were well-fitted into various kinetic models for a broad understanding of the sorption process. Langmuir model indicates a maximum adsorption capacity of 14.694 mg/g. The thermodynamic study confirms the spontaneous and endothermic sorption. Studies at the molecular level have revealed that the Cd2+ ion tends to bind to surface aromatic carbon atoms. This sustainable approach produces BCCPH via MAP as a solution for waste transformation into water-cleaning materials.

Multilayer adsorption of lead (Pb) and fulvic acid by Chlorella pyrenoidosa: Mechanism and impact of environmental factors

When the multilayer adsorption of lead (Pb) and fulvic acid (FA) occurs on algal surface, the adsorption capacity of Pb on the algae will increase dramatically, thus increasing the environmental risk of Pb. However, the corresponding mechanism and the influence of environmental factors on the multilayer adsorption remain unclear. Here, microscopic observation methods and batch adsorption experiments were exactly designed to investigate the adsorption behavior of multilayer adsorption of Pb and FA on algal surface. The results of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed that carboxyl groups were the major functional groups responsible for the binding of Pb ions in multilayer adsorption, and its number was more than that in monolayer adsorption. The solution pH, with an optimal pH of 7, was a critical factor influencing the occurrence of multilayer adsorption because it influences the protonation of the involved functional groups and determines the concentration of Pb²⁺ and Pb-FA in the solution. Increasing the temperature was beneficial for multilayer adsorption, with ΔH for Pb and FA varied from +17.12 to +47.68 kJ/mol and +16.19 to +57.74 kJ/mol, respectively. The multilayer adsorption of Pb and FA onto algal surface also followed the pseudo-second order kinetic model, but was extremely slower than the monolayer adsorption of Pb and FA by 30 times and 15 orders of magnitude, respectively. Therefore, the adsorption of Pb and FA in the ternary system had a different adsorption behavior than that in the binary system, which verified the presence of multilayer adsorption of Pb and FA and further support the multilayer adsorption mechanism. This work is important to provide data support for water ecological risk prevention and control of heavy metals.

Facile solvent-free radical polymerization to prepare itaconate-functionalized hydrochar for efficient sorption of methylene blue and Pb(II)

An itaconate-functionalized hydrochar (IFHC) was prepared from one-step solvent-free radical copolymerization of bamboo hydrochar, itaconic acid, ammonium persulphate and sodium hydroxide in solvent-free environment, and was employed to absorb methylene blue (MB) and Pb(II) from wastewater. Characterizations show IFHC has rich carboxylate and tends to adsorb cationic contaminants. The largest adsorbed quantities of MB and Pb(II) by IFHC are up to 1036 and 291.8 mg·g^-1 at 298 K respectively as per the Langmuir isotherm. Sorption of MB and Pb(II) onto IFHC can be expressed well by Langmuir isotherm and pseudo-2nd-order kinetics equations. The high sorption performance depends on the rich carboxylate, which can adsorb MB/Pb(II) through an electrostatic interaction/inner-surface complexation mechanism. The sorptive capacity of regenerated IFHC decreased below 10% after 5 desorption-resorption cycles. Thus, the solvent-free free radical copolymerization is an environmentally-friendly strategy to synthesize novel efficient sorbents that can clean cationic contaminants from wastewater.

Synthesis of Cellulose-Poly(Acrylic Acid) Using Sugarcane Bagasse Extracted Cellulose Fibres for the Removal of Heavy Metal Ions

In this study, sugarcane bagasse (SCB) was treated with sodium hydroxide and bleached to separate the non-cellulose components to obtain cellulose (CE) fibres. Cross-linked cellulose-poly(sodium acrylic acid) hydrogel (CE-PAANa) was successfully synthesised via simple free-radical graft-polymerisation to remove heavy metal ions. The structure and morphology of the hydrogel display an open interconnected porous structure on the surface of the hydrogel. Various factors influencing batch adsorption capacity, including pH, contact time, and solution concentration, were investigated. The results showed that the adsorption kinetics were in good agreement with the pseudo-second-order kinetic model and that the adsorption isotherms followed the Langmuir model. The maximum adsorption capacities calculated by the Langmuir model are 106.3, 333.3, and 163.9 mg/g for Cu(II), Pb(II), and Cd(II), respectively. Furthermore, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectrometry (EDS) results demonstrated that cationic exchange and electrostatic interaction were the main heavy metal ions adsorption mechanisms. These results demonstrate that CE-PAANa graft copolymer sorbents from cellulose-rich SCB can potentially be used for the removal of heavy metal ions.

The role of biomass elemental composition and ion-exchange in metal sorption by algae

The use of macroalgae, microalgae and cyanobacteria for metal sorption has been widely reported. Still, there are no studies allowing a direct comparison of the performance of these biomasses, especially while evaluating metal competition. The simultaneous sorption of Co²⁺, Cu²⁺, Ni²⁺ and Zn²⁺ present in a multi-elemental solution by six macroalgae, two microalgae and three cyanobacteria was evaluated. Brown macroalgae were shown to be the most promising biosorbent, with Undaria pinnatifida having a total metal sorption capacity of 0.6 mmol g^-1. Overall, macroalgae performed better than microalgae, followed by cyanobacteria. Carboxyl groups were identified as being the main functional groups involved in metal sorption, and all biomass samples were found to be selective to Cu²⁺. This was linked not only to its higher complexation constant value with relevant functional groups when compared to the remaining metals, but also the Irving-Williams series. The release of K⁺ and Ca²⁺ to the aqueous solution during the metal sorption was followed. The obtained results suggest they are readily exchanged with metals in the solution, indicating the occurrence of an ion-exchange mechanism in metal sorption by most biomass. Red macroalgae are an exception to the reported trends, suggesting that their metal sorption mechanism may differ from the other biomass types.