Synthesis of a bioadsorbent from jute cellulose, and application for aqueous Cd (II) removal

Mediated biosynthesis of CdS QDs by EPS from Desulfovibrio desulfuricans sub sp. under carbon source-induced reinforcement

This paper describes a unique molecular mechanism for the EPS-mediated synthesis of CdS QDs by sulfate-reducing bacteria (SRB) under carbon source-induced reinforcement. Under the induced by carbon sources (HCOONa, CH3COONa and C6H12O6), there was a significant increase in EPS production of SRB, particularly in protein, and the capacity of Cd(II) adsorption was further enhanced. CdS QDs were extracellularly synthesized by adding S2- after Cd(II) adsorption. The results showed that CdS QDs were wrapped or adhered by EPS, and the most significant increase in Arg and Lys among basic amino acids in EPS after HCOONa-induced was 133.34% and 63.89%, respectively. This may serve as a biological template for QD synthesis, producing protein gels with a large number of microcavities and controlling the nucleation of CdS QDs. The highest yield of HCOONa-CdS was achieved after induction, with 23.59 g/g biomass per unit strain, which was 447.34% higher than that before induction and was at a high level in previous studies. The synthesized CdS QDs were uniform in size distribution and had higher luminescence activity and a larger specific surface area than those synthesized by the chemical synthesis route, provides a new idea for EPS treatment of heavy metal wastewater and metal biorecovery.

Efficiency and mechanism in preparation and heavy metal cation/anion adsorption of amphoteric adsorbents modified from various plant straws

Cellulose can be modified for the loading of functional groups such as amino groups, sulfydryl groups, and carboxyl groups. Cellulose-modified adsorbents generally have specific adsorption capacities for either heavy metal anions or cations, and possess the advantages of wide raw material source, high modification efficiency, high adsorbent recyclability, and great convenience in recovery of the adsorbed heavy metals. At present, preparation of amphoteric heavy metal adsorbents from lignocellulose has attracted great attention. However, the difference in efficiency of preparing heavy metal adsorbents by modification of various plant straw materials and mechanism for the difference remain to be further explored. In this study, three plant straws, including Eichhornia crassipes (EC), sugarcane bagasse (SB) and metasequoia sawdust (MS), were sequentially modified by tetraethylene-pentamine (TEPA) and biscarboxymethyl trithiocarbonate (BCTTC) to obtain amphoteric cellulosic adsorbents (EC-TB, SB-TB and MS-TB, respectively), which can simultaneously adsorb heavy metal cations or anions. The heavy metal adsorption properties and mechanism before and after modification were compared. Pb(II) and Cr(VI) removal rates by the three adsorbents were 2.2-4.3 folds and 3.0-13.0 folds of those before modification, respectively, following the order of MS-TB > EC-TB > SB-TB. In the five-cycle adsorption-regeneration test, the Pb(II) and Cr(VI) removal rate by MS-TB decreased by 58.1 % and 21.5 %, respectively. Among the three plant straws, MS possessed the most abundant hydroxyl groups and the largest specific surface area (SSA), and accordingly MS-TB had the highest load of adsorption functional groups [(C)NH, (S)CS and (HO)CO] and also the largest SSA among the three adsorbents, which contribute to its highest modification and adsorption efficiency. This study is of great significance for screening suitable raw plant materials to prepare amphoteric heavy metal adsorbents with superior adsorption performance.

Exploring of toxic Pb(II) removal by low-cost bio-adsorbent of camphor leaf forestry waste after camphor oil extraction

Camphor leaf (CL) was widely used to extract camphor oil and thus led to abundant forestry waste. In order to reduce pollution, the waste CL was used to prepare bio-adsorbent for Pb(II) removal after alkali treatment and functional modification. The effects of solution pH, initial Pb(II) concentration, contact time and solution temperature were investigated on adsorption process to evaluate the potential application in heavy metal ions' removal. It was found that the massive hydroxyl groups released and plenty of micro-pores formed after the alkali treatment of CL bio-adsorbent, which obviously increased the Pb(II) adsorption. And the adsorption performance promoted continually after further functional modification by ionized 1,2,3,4-butanetetracarboxylic acid (BTCA). The increase of pH was favourable for the adsorption even though the precipitation effect was deducted. Linear fitting method was more suitable to describe the adsorption process than nonlinear fitting method, including adsorption isotherms and adsorption kinetics research. The adsorption thermodynamics was better to be described by nonlinear fitting method due to its lower root mean square error (RMSE) value and higher R² value. Among which, the adsorption isotherm and adsorption kinetics were fitted well to Langmuir model and pseudo-second-order model, respectively. The adsorption thermodynamics was exothermic in nature and the process was spontaneous at low solution temperature. The adsorption mechanism was revealed as the combination of dominant chemical adsorption and assistant physical adsorption.

Graft modification of lignin-based cellulose via enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization and free-radical coupling

In this study, a green approach combining enzyme-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization and free-radical coupling was developed for the modification of jute fiber, which is a typical lignin-based cellulose. Jute fiber surface was covered by rich amount of lignin, which offered great opportunities for further functional modification. The controlled polymerization of vinyl monomers, acrylamide (AM) or butyl acrylate (BA), was carried out by horseradish peroxidase (HRP)-initiated RAFT to form well-defined polymers with well-controlled molecular weights and structures. Enzymatic grafting by HRP occurred between the free radicals of well-defined polymers and free radicals of lignin on jute. Gel permeation chromatography (GPC) analysis indicated the alkyl chain length of polymers prepared via HRP-initiated RAFT polymerization was well-controlled. Other results of flourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed that well-controlled alkyl chains prepared via enzymatic catalysis were grafted on the exposed lignin of jute. The study explores a new and eco-friendly modification method for lignin-based materials with the controlled graft chain structure via two different catalysis with HRP.

Chitosan-grafted tetrapolymer using two monomers: pH-responsive high-performance removals of Cu(II), Cd(II), Pb(II), dichromate, and biphosphate and analyses of adsorbed microstructures

For circumventing the cumbersome and expensive multifunctional and multipolymer adsorbents for high-performance removals of hazardous water-contaminant(s), chitosan-g-[2-acrylamido-2-methyl-1-propanoic acid (AMPS)-co-2-(3-acrylamidopropanamido)-2-methylpropane-1-sulfonic acid (APAMPS)-co-2-(N-(3-amino-3-oxopropyl)acrylamido)-2-methylpropane-1-sulfonic acid (NAOPAMPS)-co-acrylamide (AM)] (i.e., chitosan-g-tetrapolymer), a multifunctional scalable and reusable hydrogel, was synthesized by grafting of chitosan and in situ attachments of N-H functionalized NAOPAMPS and APAMPS hydrophilic acrylamido-monomers during free-radical solution-polymerization of the two ex situ added AMPS and AM monomers in water. The response surface methodology was employed to synthesize one hydrogel envisaging the optimum balance between swelling and stability for the superadsorption of Cu(II), Cd(II), Pb(II), Cr₂O₇^2-, and HPO₄^2-. The in situ attachments of NAOPAMPS and APAMPS, grafting of chitosan into tetrapolymer, structures and properties, pH-responsive abilities, superadsorption mechanism, and reusability were understood via in depth microstructural analyses of adsorbed and/or unadsorbed chitosan-g-tetrapolymer(s) through ¹H/¹³C NMR, FTIR, XPS, TGA, XRD, DLS, and pH_PZC. The maximum adsorption capacities of Cd(II), Cu(II), Pb(II), Cr₂O₇^2-, and HPO₄^2- were 1374.41, 1521.08, 1554.08, 47.76, and 32.76 mg g^-1, respectively.

Efficient adsorption of Cd2+ from aqueous solution using metakaolin geopolymers

In this study, geopolymers were prepared using metakaolin (MK) raw material under different alkali activator moduli (SiO₂/Na₂O = 0.8, 1.2, 1.6, 2.0 M ratio) in order to analyze their capacity and mechanism for adsorbing cadmium (Cd²⁺) from solution. Instrumental analyses including X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy(XPS), Fourier transform infrared (FTIR), and Brunauer-Emmett-Teller (BET) were performed to examine the mineralogical features of the MK and geopolymers before and after Cd²⁺ adsorption. The effect of initial pH, temperature, contact time, and initial concentration on Cd adsorption performance was studied to obtain the equilibrium isotherm. Kinetic data of the geopolymers fitted the pseudo-second-order kinetic model well. Moreover, the adsorption equilibrium data of Cd²⁺ adsorbed by the geopolymers fitted the Langmuir model better than the Freundlich model. The result shows that chemisorption dominates Cd²⁺ adsorption by geopolymers and that the adsorption capacity differs when prepared using different alkali-activated modulus agents. The geopolymer prepared using an alkali activator modulus of 0.8 M (molar ratio) exhibited the best Cd²⁺ adsorption performance with a maximum adsorption capacity of 70.3 mg g^-1. The removal rate of Cd²⁺ by geopolymer still remained above 85% after five round of recycling.

Adsorption of Cd2+ on GO/PAA hydrogel and preliminary recycle to GO/PAA-CdS as efficient photocatalyst

In this work, a GO (graphene oxide)/PAA (poly acrylic acid) hydrogel was prepared by graft polymerization between GO and AA. It was employed as highly efficient adsorbent for Cd²⁺ removal from wastewater. The GO/PAA-Cd²⁺ composite after the adsorption process was recycled through in-situ precipitation to obtain GO/PAA-CdS composites. During the synthesis process, the amounts of GO and AA were optimized to enable the hydrogel with maximum adsorption of Cd²⁺ (316.4 mg/g at 25 °C). The structure and chemical composites of GO/PAA hydrogel were investigated through FTIR spectra, Raman spectra, and TGA. The adsorption kinetics and isotherms of Cd²⁺ on GO/PAA were analyzed. The synthesized products served as an efficient adsorbent for Cd²⁺ and a suitable matrix for the CdS quantum dots formation which was confirmed by various characterizations, including XPS, SEM-EDS and HRTEM. The roles of GO and PAA in the successive adsorption-photocatalyst process were proved to be complementary: PAA improved the adsorption of Cd²⁺ while GO enhanced the photocatalyst efficiency. The photodegradation rate of MB (30 mg/L) was over 90% within 2 h.