Assistance Of Microwave Irradiation Research Articles

Rapid fractionation of corn stover by microwave-assisted protic ionic liquid [TEA][HSO4] for fermentative acetone–butanol–ethanol production

BackgroundThe use of ionic liquids (ILs) to fractionate lignocelluloses for various bio-based chemicals productions is in the ascendant. On this basis, the protic ILs consisting of triethylammonium hydrogen sulfate ([TEA][HSO4]) possessed great promise due to the low price, low pollution, and high efficiency. In this study, the microwave-assistant [TEA][HSO4] fractionation process was established for corn stover fractionation, so as to facilitate the monomeric sugars production and supported the downstream acetone–butanol–ethanol (ABE) fermentation.ResultsThe assistance of microwave irradiation could obviously shorten the fractionation period of corn stover. Under the optimized condition (190 W for 3 min), high xylan removal (93.17 ± 0.63%) and delignification rate (72.90 ± 0.81%) were realized. The mechanisms for the promotion effect of the microwave to the protic ILs fractionation process were ascribed to the synergistic effect of the IL and microwaves to the depolymerization of lignocellulose through the ionic conduction, which can be clarified by the characterization of the pulps and the isolated lignin specimens. Downstream valorization of the fractionated pulps into ABE productions was also investigated. The [TEA][HSO4] free corn stover hydrolysate was capable of producing 12.58 g L−1 of ABE from overall 38.20 g L−1 of monomeric sugars without detoxification and additional nutrients supplementation.ConclusionsThe assistance of microwave irradiation could significantly promote the corn stover fractionation by [TEA][HSO4]. Mass balance indicated that 8.1 g of ABE and 16.61 g of technical lignin can be generated from 100 g of raw corn stover based on the novel fractionation strategy.

Development of phytoactive essential oils-composed organogels against oral pathogens

Gingivitis, one of the most common periodontal diseases, is the inflammation of the gingival tissues that causes swelling, tenderness, redness and bleeding. The treatment should contain anti-inflammatory, antibacterial, and antioxidant agents to decrease the risk of dental plaque formation and periodontal tissue damage in the inflammatory process. In line with the antibacterial, antifungal, and antioxidant effects of the geranium (Pelargonium graveolens L'Hér.) herb, common sage (Salvia officinalis L.) herb, and mastic tree (Pistacia lentiscus L.) leaf, in this work we propose their use as active pharmaceutical ingredients of periodontal gels. The organogel formulations were produced either by microwave-assisted irradiation method or by the conventional approach (microwave-free), and further characterized for their physicochemical properties, antimicrobial efficacy and cell viability. Among them, the gel formulation containing 4% geranium essential oil obtained by the conventional method was identified as the best candidate to be developed as an effective and safe alternative suitable for oromucosal applications.

Microwave-assisted ultrafine silver nanoparticle synthesis using Mitragyna speciosa for antimalarial applications

Abstract This study investigates the antiplasmodial activity of ultrafine silver nanoparticles (AgNPs, 2–5 nm) synthesized using a green approach involving the Mitragyna speciosa extract and emphasizing the microwave-assisted irradiation technique. Various synthesis parameters were optimized, resulting in the successful production of spherical AgNPs, which exhibited a characteristic surface plasmon resonance peak at around 440 nm. The synthesized AgNPs demonstrated high stability, indicated by a zeta potential value of −28 mV. The antimalarial efficacy of the microwave-assisted AgNPs against the P. falciparum strain was evaluated, demonstrating a half-maximum inhibitory concentration (IC50) value of 1.56 µg·mL−1. Further enhancement in the antimalarial performance was observed when the AgNPs were conjugated with chloroquine (CQ), a traditional antimalarial drug, achieving an impressive IC50 value of 24 ng·mL−1. Additionally, all formulations exhibited low toxicity, with a cytotoxic concentration (CC50) exceeding 800 µg·mL−1 in Vero cells. Complementing these experimental findings, specific computational studies offered insights into the interactions between silver atoms and bioactive compounds in M. speciosa, as well as shedding light on the dynamics of CQ functionalization. These experimental and computational findings emphasize the potential of a sustainable, low-toxicity, and cost-effective AgNP synthesis process, showcasing significant promise in advancing green nanotechnology for the development of effective antimalarial medications.

Fast synthesis of Hcy-modified Au nanoclusters under microwave-assisted irradiation and their response to Cu (II) ions

In this work, Cu (II) ion was chosen as an analyte because it is an important trace element in the human body to maintain organism health. AuNCs (gold nanoclusters) with a mean diameter of ∼2 nm were prepared by a microwave-assisted method, using homocysteine (Hcy) as a reducer and stabilizer. The optimal sample showed a red emission peaking at 624 nm with an emission quantum yield of 17.1%, which was assigned as a ligand to metal nanoparticle core charge transfer (LMNCT). This red emission was effectively quenched by Cu (II) but immune to competing metal cations and anions, showing good selectivity. The quenching mechanism was found as the Cu (II) quenching on the LMNCT procedure to AuNCs. The primary advantage of Hcy-modified AuNCs was the easy and quick synthetic procedure, along with acceptable sensing performance, including good selectivity, linear response, and wide working region.

Enantioselective MW-US-assisted Synthesis, DFT Simulation and Molecular Docking of Spiro Pyrrolidine-2,3'-Thieno [2,3-d]Pyridazin-Hydrazide as Green Agricultural Product

Effecient green tools of β-(3,4-dichlorophenyl-acrylic acid, thioglycolic acid and β-(2-oxopyrrolidin-1-yl) ethanol under ultrasonic and microwave-assisted irradiation (MW-US) as found to be a simple approach for preparing interesting Spiro derivatives. A novel class Spiro-pyrrolidine-2,3'-thieno[2,3-d]pyridazine derivatives (2–6) were efficiently synthesized via one-pot multicomponent reactions. Ethoxy acetyl Spiro pyrrolidine-2,3'-thieno[2,3-d]pyridazin-hydrazide (5) assessed as ecofriendly microbicidal and insecticidal activities to establish a structure-activity relationship. Elemental analysis and spectroscopic data were used to describe the structures of all synthesized products. The Spiro-heterocyclic compounds are exclusive non-planar structures and branded for binding to biomolecules. The microbicidal and insecticidal activities of the compounds were investigated on ATCC coded test microorganisms upon the applied dose and time was studied through two kinetic study. The interaction with these antibacterial and insect’s third instar larvae was medicinal and economically valuable. The preliminary testing demonstrated flourishing microbicidal capability to prevent the growth of threatening pathogenic bacteria as well as blossoming insecticidal efficacy. Moreover, Molecular docking and density functional theory simulations can be used to validate the experimental results. Also, we utilized the docking approach with secondary bacterial infection indicated that Ethoxy acetyl Spiro hydrazide (5) and Spiro arylidene hybrid (6) have exerted the strongest docking binding value against the active sites of 6LU7.

Synthesis, characterization, and gas sensing application of crumpled CdO sheets prepared in alkaline media

CdO crumpled sheets have been synthesized by microwave-assisted irradiation in alkaline solutions of NH4OH and NaOH, respectively, and in presence of a surfactant. The morphology of CdO samples have been investigated using Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). X-ray diffraction analysis (XRD) has been used to determine the cadmium oxy/hydroxide crystalline phases of CdO formed during synthesis and successive annealing at 300 °C. Crystallite sizes of CdO nanostructures obtained from NH4OH and NaOH reagent-assisted synthesis are 44 nm and 57 nm, respectively. The specific surface area (SSA), lattice parameter, X-ray density, dislocation density, macrostrain, and texture coefficient (TC), have also been determined. The optical characteristics of CdO samples were investigated using UV reflectance (DRS) and K-M plot. NO2 gas detecting capabilities of CdO nanostructures have been investigated at low working temperature of 150 °C. When compared to its NaOH counterpart, the CdO sample prepared by NH4OH reagent-assisted synthesis demonstrated a superior sensitivity to NO2, being demonstrated even at the low concentration of 0.14 ppm, with excellent selectivity.

Effect of calcium peroxide assisted microwave irradiation pretreatment on humus formation and microbial community in straw and dairy manure composting

In this study, the effects of four pretreatment methods on the crystallinity of maize straw were compared, and the CaO2 assisted microwave pretreatment was selected for straw and dairy manure composting. The humification and microbial community were investigated. Results showed that the pretreatment increased the initial water-soluble carbon, which favored the microbial activity, and the CO2 release increased by 15.71%. Pretreatment promoted the lignocellulose degradation, with total degradation ratio of 37.06%. The final humic acid content was 11.39 g/kg higher than the control. Spearman correlation analysis indicated that polyphenols and amino acids were significantly related to humus formation. In addition, pretreatment rendered the Firmicutes the most dominant phylum, and increased the metabolic intensity of reducing sugar metabolism, aromatic amino acid biosynthesis and carbon fixation pathways. Redundancy analysis revealed that the dominant genus of Firmicutes was significantly positively correlated with humus, while that of Actinobacteriota was correlated with CO2 release.

Sun irradiated high efficient photocatalyst ZnO nanoparticles obtained by assisted microwave irradiation

In this research microwave assisted zinc oxide synthesis applied to methylene blue dye mineralization is reported. This technique strongly influences the presence of certain intrinsic defects that is similar to those obtained by techniques where a rich oxygen atmosphere is used. These defects play a strong role in photocatalysis, showing a short time (70 min) at 95.3 % efficiency in methylene blue degradation under direct sunlight irradiation. Through X-ray diffraction, crystallinity, and nanoparticle size (5 nm) were obtained, meanwhile the morphology and elemental composition was observed by Scanning Electron Microscopy and Energy-Dispersive X-ray respectively. Additionally, Ultraviolet–Visible and Photoluminescence methods complete the analysis letting calculate the band gap (3.5 eV) and the presence/contribution of intrinsic defects. Such defects are identified as direct and strong contributors to carrier’s concentration separation and alternatively provoking low recombination levels, which is a characteristic desired in organic materials mineralization.

Rapid synthesis of hierarchical silicoaluminophosphate molecular sieves using carbon-silicon composites from rice husk ash for deoxygenation of stearic acids

To improve the low temperature fluidity of bio-diesel, a Ni/SAPO catalyst is prepared using hierarchical silicoaluminophosphate (i.e., SAPO-11/SAPO-31) molecular sieves as supports. A rapid, cost-effective, and eco-friendly approach is proposed using carbon-silicon composites from rice husk ash (RHA). The carbon-silicon composite is obtained via an anaerobic calcination of RHA, which acts as the silicon source and mesopore filler, simultaneously. As-synthesized SAPO-11/SAPO-31 intergrowth molecular sieve has a high mesopore volume due to the mesopore filler function of carbon in RHA. A lower crystallization temperature and shorter duration are achieved with the assistance of microwave irradiation. Ni/SAPO catalysts exhibit a high conversion of stearic acids (concentration, 0.5–5 wt%) to form n-/isoparaffins. These results are important to develop a potential efficient catalyst for the upgrade of bio-oil and the novel synthesis method provides reference to the synthesis of other types of SAPO molecular sieves.

Non-soluble chalcones and their potential application as corrosion coatings on carbon steel exposed to 1 M HCl solutions

Chalcones are secondary metabolites of great interest in chemistry due to their broad biological activities. In recent years, the versatility of this class of molecules has been demonstrated, especially in several synthetic analogs, which have shown behavior as corrosion inhibitors. In this article, a series of chalcone analogs were synthesized using the Claisen-Schmidt methodology in both conventional and microwave-assisted irradiation methodologies. The obtained compounds are non-soluble, which widens the window of chalcones with the possibility of being used as anticorrosive. These were analyzed by electrochemical impedance spectroscopy, potentiodynamic polarization, weight loss measurements and scanning electron microscopy. The results suggest that the chalcone compounds such as the trans-4-(N,N-diphenylamino)chalcone (CH11), (E)-1-phenyl-3-(thiophen-2-yl)prop-2-en-1-one (CH15) and (E)-1-phenyl-3-(thiophen-3-yl)prop-2-en-1-one (CH16) have anticorrosive activity for carbon steel in HCl solutions. The electrochemical techniques showed that increasing the mass of the three chalcone on carbon steel increases the efficiency of the inhibition. A mass amount of 1.4 mg of CH15 or CH16 was shown to have the maximum inhibition from corrosion, but for CH11 the maximum inhibition was 3.0 mg on carbon steel. The percentage inhibition that reached these three compounds was about 95 % at 20 °C. Theoretical calculations by Density Functional Theory (DFT) were performed to explain the measured corrosion decrease assisted by compounds CH11, CH15, and CH16.

One-pot multi-step synthesis of gamma-valerolactone from furfuryl alcohol: Microwave vs continuous flow reaction studies

Mesoporous materials Al-SBA-15 have been synthesized and functionalized with different Al/Zr molar ratio and 0.5% Au and Ru, respectively, by a mechanochemical approach. Textural and structural properties of synthesized materials have been analysed by TEM (Transmission Electron Microscopy), SEM (Scanning Electron Microscopy), XPS (X-ray photoelectron spectroscopy), XRD (X-ray Diffraction) and N2 adsorption/desorption technique, among others. The catalytic activity of synthesized materials has been evaluated in the one-pot conversion of furfuryl alcohol (FAL) employing 2-propanol as H-donor solvent to obtain γ-valerolactone (GVL), both under microwave-assisted irradiation (CEM Discover® 2.0) and continuous flow conditions (Phoenix Flow ReactorTM, ThalesNano). All materials displayed conversions up to 99%, with a GVL selectivity of ca. 20–41% after one-hour reaction time. The study of the reusability of the materials showed a good GVL production during 3 reaction cycles employing both microwave-assisted and flow reaction conditions.

Gradient Si- and Ti-doped Fe2O3 hierarchical homojunction photoanode for efficient solar water splitting: Effect of facile microwave-assisted growth of Si-FeOOH on Ti-FeOOH nanocorals

The construction of a homojunction is an effective approach for addressing issues such as slow charge separation and charge-transfer kinetics in photoanodes. In the present work, we designed a gradient Si- and Ti-doped Fe2O3 homojunction photoanode to improve the photoelectrochemical (PEC) performance of a Ti-doped Fe2O3 photoanode. Ti-FeOOH nanocorals were synthesized using a hydrothermal process, and Si-FeOOH was grown on Ti-FeOOH nanocorals using a rapid and facile microwave-assisted (MW) technique. By varying the MW irradiation time, the thickness of the Si/Ti:Fe2O3 photoanode was adjusted and an optimized 3-Si/Ti:Fe2O3 photoelectrode was achieved with a significantly enhanced photocurrent density (1.37 mA cm−2 at 1.23 V vs. RHE) and a cathodic shift of the onset potential (150 mV) compared with that of bare Ti-Fe2O3. This enhanced PEC performance can be ascribed to homojunction formation and Si gradient doping. The Si dopant increased the donor concentration and the formation of a homojunction improved the intrinsic built-in electric field, thereby promoting charge separation and charge transfer. Furthermore, the as-formed homojunction passivated the surface-trapping states, consequently improving the charge transfer efficiency (60% at 1.23 VRHE) at the photoanode/electrolyte interface. These findings could pave the way for the microwave-assisted fabrication of diverse efficient homojunction photoanodes for PEC water splitting applications.

Luminescent carbon dots obtained from different precursors and methods and their applications as sensors for metal ions

Carbon dots (CDs) represent a rising class of nanomaterials with numerous optical and physicochemical properties that make them useful for a variety of applications. In this work, three different methods, conventional hydrothermal carbonization (CHC), microwave-assisted irradiation and heating reflux reaction technique, were used as different bottom-up approaches to obtain different CDs. The carbon nanoparticles were prepared using three different carbon precursors (maleic, fumaric and adipic acids) and one nitrogen source (ethylenediamine). The CDs were characterized using different techniques including Fourier transform infrared spectroscopy, Transmission electron microscopy, Ultraviolet-visible, Fluorescence and Zeta potential measurements. Both, the experimental method used to obtain the CDs and the different precursors, presented a considerable influence on the quantum yield values (QY), and also on the fluorescence emission curves. The molar proportion between the carbon and nitrogen source was varied to improve the QY, and one of the samples prepared by CHC method reached QY = 25.4%. The carbon nanoparticles obtained by CHC were used as nanoprobes for metal ions and exhibited great selectivity and sensitivity for Fe3+ ions, reaching a limit of detection of 35 nM, which is lower than most reported values for CD-based PL method used as probes for Fe3+ ions.

Microwave-Assisted Synthesis of Sulfide Solid Electrolytes for All-Solid-State Sodium Batteries

Sulfide-based solid electrolytes are one of the potential candidates for all-solid-state batteries due to their high ionic conductivity and low synthesis temperature compared to their oxide counterparts. However, the preparation methods involve the requirement of an inert atmosphere for ball milling, heating, and quenching facilities that severely limit their implementation, which necessitates a resurgence of interest in alternate synthesis approaches. Herein, a simple, accelerated, and energy-efficient method for the synthesis of highly crystalline cubic sodium thiophosphate solid electrolyte (Na3PS4) is developed using the microwave-assisted irradiation technique. Along with impedance studies, 23Na solid-state NMR spin-lattice relaxation experiments are performed to obtain insights into Na-ion mobility in Na3PS4 solid electrolytes. The electrochemical properties and the interfacial stability of the electrolyte with the metallic sodium anode are thoroughly investigated and presented in this work. Further, a prototype full-cell constructed using a Na3V2(PO4)3 cathode, a Na3PS4 solid electrolyte, and a modified sodium anode showed promising electrochemical properties. Although numerous studies focus on solid electrolyte interface modification and design, the simple and energy-efficient approach for sodium-ion solid electrolyte synthesis presented here will provide a meaningful advance to the accelerated synthesis of sulfide electrolyte-based all-solid-state batteries.

Microwave-Assisted Synthesis of 5′-O-methacryloylcytidine Using the Immobilized Lipase Novozym 435

Nucleobase building blocks have been demonstrated to be strong candidates when it comes to DNA/RNA-like materials by benefiting from hydrogen bond interactions as physical properties. Modifying at the 5′ position is the simplest way to develop nucleobase-based structures by transesterification using the lipase Novozym 435. Herein, we describe the optimization of the lipase-catalyzed synthesis of the monomer 5′-O-methacryloylcytidine with the assistance of microwave irradiation. Variable reaction parameters, such as enzyme concentration, molar ratio of the substrate, reaction temperature and reaction time, were investigated to find the optimum reaction condition in terms of obtaining the highest yield.

Development of Dual-Nanoarchitecture Photoanodes Based on Microwave-Assisted ZnS(en)0.5 Nanoplates for Photoelectrochemical Hydrogen Generation

In this study, a microwave-assisted (MW) synthesis approach is used to develop an inorganic–organic ZnS(en)0.5 nanoplate (NP) on a Zn foil substrate. Morphological studies revealed that the growth of the ZnS(en)0.5 NP on Zn foil increased with prolonged MW irradiation time. Additionally, X-ray diffraction analysis, fourier transform infrared spectroscopy, and transmission electron microscopy unveiled the structural properties of the inorganic–organic hybrid ZnS(en)0.5 NP electrode to investigate the morphological evolution and the growth mechanism. Furthermore, the fabricated inorganic–organic ZnS(en)0.5 NP was exposed to Cd2+-ion exchange at different temperatures (140, 160, and 180 °C for 6 h) to improve its light absorption and photoelectrochemical properties. The Zn1–xCdxS porous nanoplate (PNP)/ZnO nanorod (NR)160 photoanode (160 °C for 6 h) exhibited a high photocurrent density of 4.81 mA·cm–2 at 0.5 V vs RHE. The optimized photoanode also yielded a hydrogen evolution rate of 89.76 μmol·cm–2 under 3 h of solar light illumination. Thus, the formation of the Zn1–xCdxS porous structure and the growth of the ZnO NR during Cd2+-ion exchange enhanced the photocurrent density and, consequently, prolonged the recombination lifespan of the Zn1–xCdxS PNP/ZnO NR160 photoanode.

Surfactant assisted microwave irradiation pretreatment of corncob: Effect on hydrogen production capacity, energy consumption and physiochemical structure

The combination pretreatment strategy is an effective way to intensify photo-fermentative biohydrogen production (PFHP) process. In this study, the synergistic effects of microwave irradiation and surfactants on the hydrogen production performance, energy analysis and structural characteristics was evaluated. Results revealed that hydrogen production performance was improved after microwave irradiation pretreatment (MIP) and surfactants assisted microwave irradiation pretreatment (SMIP). SMIP group had a higher cumulative hydrogen yield (CHY) of 367.87 ± 6.481 mL compared with control group (223.26 ± 4.329 mL) and MIP group (303.66 ± 3.366 mL), which was an increase of 36.01% and 64.77%, respectively. Energy evaluation analysis showed that the energy ratio of SMIP (0.49) was higher than that of MIP (0.37) in the PFHP system, therefore, SMIP can save more energy. After SMIP, the corncob lignocellulose structure was greatly damaged, which was verified by SEM, FTIR, XRD and XPS analyses.

Size Distribution of Hexagonal Prismatic-Shaped ZnO Nanorods Synthesized by Microwave-Assisted Irradiation of Precursors

Size Distribution of Hexagonal Prismatic-Shaped ZnO Nanorods Synthesized by Microwave-Assisted Irradiation of Precursors

Rapid microwave-assisted biosynthesis of platinum nanoparticles and evaluation of their antioxidant properties and cytotoxic effects against MCF-7 and A549 cell lines.

In this study, platinum nanoparticles (Pt NPs) were synthesized by a green method using an aqueous extract of Eucalyptus camaldulensis with assistance of microwave irradiation (850W) and their physicochemical characteristics were studied by UV-visible spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses. Antioxidant activities, hemocompatibility, and cytotoxic effects of the prepared Pt NPs were then evaluated. The attained results showed that the newly formed Pt NPs possess a size range between 7.4 and 11.2nm. These spherical-shaped NPs were slightly aggregated and held various functional groups on their surface. The antioxidant activity of Pt nanostructures was comparable to that of butylated hydroxyl anisole at concentrations higher than 320µg/mL. At the same concentration of 640μg/mL, the scavenging activities were 3.36 ± 0.9% (hexachloroplatinic acid) and 52.13 ± 0.43% (Pt NPs). The results of hemolytic assay revealed satisfactory hemocompatibility of the Pt NPs even at the concentration as high as 4mg/mL (hemolysis percent equal to 3.5 ± 1.3%). The cytotoxicity studies revealed that MCF-7, A549, and 3T3 cell lines treated with hexachloroplatinic acid and cisplatin for 24h and 48h showed a higher percentage of cell death compared with the Pt NPs. After 24h, for A549, 3T3, and MCF-7 cells exposed to Pt NPs, the cell viability was measured to be 80 ± 3.2%, 96 ± 1%, and 89 ± 2.6%, respectively, at concentration of 640µg/mL. Further investigations are required to elucidate the mechanisms behind the biological activities of as-synthesized Pt NPs.

Removal of heavy metal ions from an aqueous solution by CS/PVA/PVP composite hydrogel synthesized using microwaved-assisted irradiation

A simple protocol of chitosan/polyvinyl alcohol/polyvinylpyrrolidone (CS/PVA/PVP) hydrogel synthesis using microwave-assisted irradiation to initiate and accelerate network formation was explored. The optimum ratio of CS:PVA:PVP was 0.3:0.6:0.3 g with epichlorohydrin as a cross-linking agent under 600 watts of irradiation for 3 min. The gel swelling degree and gel fraction of the hydrogel were approximately 1627.4% and 42.6%, respectively. The individual metal ion removal efficiency was the highest for Pb(II), followed by Cu(II), Ni(II), and Cd(II) as adsorbed by the CS/PVA/PVP hydrogel. Varying the pH of the solution containing metal ions in the range of 4.0 – 8.0 did not significantly affect the adsorption capacity. However, the order of adsorption capacity of mixed metal ions differed from that of individual ions. The maximum capacities of Pb(II), Cu(II), Ni(II), and Cd(II) ions were 80.02, 33.07, 15.13, and 8.36 mg g−1, respectively. The desorption and reusability study of the CS/PVA/PVP hydrogel revealed that 0.1 M EDTA was a suitable candidate to regenerate the hydrogel as more than 60% of metal ion removal efficiency was still achieved after five adsorption cycles. In addition, the reaction mechanism of the hydrogel network formation derived from infrared spectrometry indicated that network formations were developed through the reaction between the crosslinking agent and hydroxyl and amino functional groups of the starting materials. Adsorption mechanism based on the results obtained from x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) was also presented. This study showed that the proposed composite hydrogel could be further developed and used as a platform to efficiently adsorb metal ions.