Efficient Cr(VI) removal from wastewater by D-(+)-xylose based adsorbent: Key roles of three-dimensional porous structures and oxygen groups

Efficient selective adsorption of Cr(VI) by S-doped porous carbon prepared from industrial lignin: Waste increment and wastewater treatment

Industrial lignin is a waste product of the paper industry, which contains a large amount of oxygen group structure, and can be used to treat industrial wastewater containing Cr(VI). However, lignin has very low reactivity, so how to enhance its adsorption performance is a major challenge at present. In this study, a two-stage hydrothermal and activation strategy was used to activate the lignin activity and doping S element to prepare high-performance S-doped lignin-based polyporous carbon (S-LPC). The results show that the surface of S-LPC is rich in S and O groups and has a well-developed pore structure, which is very beneficial to Cr(VI) uptake -reduction and mass transfer on the material. In the wastewater, the utmost adsorption potential of Cr(VI) by S-LPC achieved 882.83 mg/g. After 7 cycles of regeneration, the adsorption of S-LPC decreased by only approximately 18 %. Ion competition experiments showed that S-LPC has excellent specificity for Cr(VI) adsorption. In factory wastewater, the adsorption performance of S-LPC for Cr(VI) remained above 95 %, which shows the excellent performance of S-LPC in practical applications. The results are of great significance for green chemical utilization of waste lignin, treatment of industrial wastewater and sustainable development.

Silica-magnesium coupling in lignin-based biochar: A promising remediation for composite heavy metal pollution in environment

Lignin hydrothermal silica-carbon material served as a backbone for MgCl2 activation to prepare lignin-based silicon/magnesia biochar (ALB/Si-Mg) for Cd2+, Pb2+, Cu2+, and Zn2+ removal from water and soil environment. Characterization studies revealed a 1017.71-fold increase in the specific surface area of ALB/Si-Mg compared to the original lignin biochar (ALB), producing abundant oxygen functional groups (OC-O, Si-O, Mg-O), and mineral matter (Mg2SiO4 and MgO). Crucially, batch adsorption experiments demonstrated that the adsorption capacity of ALB/Si-Mg for Cd2+, Pb2+, Cu2+, and Zn2+ was 848.17, 665.07, 151.84, and 245.78 mg/g, which were 29.09-140.45 times of the ALB. Soil remediation experiments showed that applying ALB/Si-Mg increased soil effective silicon (109.04%-450.2%) and soil exchangeable magnesium (276.41%-878.66%), enhanced plant photosynthesis, and notably reduced the bioavailability of heavy metals in soil as well as the content of heavy metals in Pakchoi, thereby promoting Pakchoi growth and development. The presence of oxygen-containing functional groups on ALB/Si-Mg, along with Mg2SiO4 and MgO nanoparticles, enhanced the adsorption capacity for heavy metals through the promotion of heavy metal precipitation, ion exchange, and complexation mechanisms. This study establishes the groundwork for the coupling of silica and magnesium elements in biochar and the remediation of composite heavy metal environmental pollution.

Environmental applications of lignin-based hydrogels for Cu remediation in water and soil: adsorption mechanisms and passivation effects

Heavy metal pollution, particularly the excessive release of copper (Cu), is an urgent environmental concern. In this study, sodium lignosulfonate/carboxymethyl sa-son seed gum (SL-Cg-g-PAA) designed for remediation of Cu-contaminated water and soil was successfully synthesized through a free radical polymerization method using lignin as a raw material. This hydrogel exhibits remarkable Cu adsorption capability when applied to water, with a maximum adsorption capacity reaching 172.41 mg/g. Important adsorption mechanisms include surface complexation and electrostatic attraction between Cu(Ⅱ) and oxygen-containing functional groups (-OH, -COOH), as well as cation exchange involving -COONa and -SO3Na. Furthermore, SL/Cg-g-PAA effectively mitigated the bioavailability of heavy metals within soil matrices, as evidenced by a notable 14.1% reduction in DTPA extracted state Cu (DTPA-Cu) content in the S4 treatment (0.7% SL/Cg-g-PAA) compared to the control group. Concurrently, the Cu content in both the leaves and roots of pakchoi exhibited substantial decreases of 55.19% and 36.49%, respectively. These effects can be attributed to the precipitation and complexation reactions facilitated by the hydrogel. In summary, this composite hydrogel is highly promising for effective remediation of heavy metal pollution in water and soil, with a particular capability for the immobilization of Cu(Ⅱ) and reduction of its adverse effects on ecosystems.

Two-stage hydrothermal oxygenation for efficient removal of Cr(VI) by starch-based polyporous carbon: Wastewater application and removal mechanism

Cr(VI) is of concern because of its high mobility and toxicity. In this work, a two-stage hydrothermal strategy was used to activate the O sites of starch, and by inserting K-ion into the pores, starch-based polyporous carbon (S-PC) adsorption sites was synthesized for removal of Cr(VI). Physicochemical characterization revealed that the O content of the S-PC reached 20.66 % after activation, indicating that S-PC has excellent potential for adsorption of Cr(VI). The S-PC removal rate for 100 mg/L Cr(VI) was 96.29 %, and the adsorption capacity was 883.86 mg/g. Moreover, S-PC showed excellent resistance to interference, and an equal concentration of hetero-ions reduced the activity by less than 5 %. After 8 cycles of factory wastewater treatment, the S-PC maintained 81.15 % of its original activity, which indicated the possibility of practical application. Characterization and model analyses showed that the removal of Cr(VI) from wastewater by the S-PC was due to CC, δ-OH, ν-OH, and C-O-C groups, and the synergistic effect of adsorption and reduction was the key to the performance. This study provides a good solution for treatment of Cr(VI) plant wastewater and provides a technical reference for the use of biological macromolecules such as starch in the treatment of heavy metals.

Competitive heavy metal adsorption on pinecone shells: Mathematical modelling of fixed-bed column and surface interaction insights

Pinecone shells are assessed as a cost-effective biosorbent for the removal of metal ions Pb(II), Cu(II), Cd(II), Ni(II), and Cr(VI) in a fixed-bed column. Influent concentration, bed height, and flowrate are studied to improve efficiency. The breakthrough data is well fitted by the Sips adsorption model, suggesting a surface complexation mechanism, with maximum adsorption capacities of 11.1 mg/g for Cu(II) and 66 mg/g for Pb(II). In multimetal solutions, the uptake sequence at breakthrough and saturation is Pb(II) > Cu(II) > Cd(II). Characterization via FTIR and XRD reveals carboxyl and hydroxyl functional groups interacting with metal ions. Ca(II) does not compete with Pb(II), Cu(II), and Cd(II) adsorption, highlighting the ability of pinecone to adsorb heavy metals via surface complexation. Its application in the treatment of industrial effluents containing Cu(II), Ni(II), and Cr(VI) is explored. The study investigates bed media regeneration via eluting adsorbed metal ions with hydrochloric acid solutions. The potential of pinecone shells as an efficient biosorbent for removing toxic metal ions from industrial wastewater is emphasized. These findings enhance our understanding of the adsorption mechanism and underscore the fixed-bed column system's applicability in real-world scenarios, addressing environmental concerns related to heavy metal contamination of industrial effluents.

Ultrafast adsorption of hexavalent chromium from aqueous effluents using covalent triazine frameworks

We have synthesized low-cost high performance covalent triazine framework (CTF) through Schiff base reaction of melamine and terephthalaldehyde with different proportions of the reactants. The synthesized adsorbents showed excellent capacity for adsorption of Cr (VI) at acidic pH while almost negligible adsorption at higher pH. The adsorbent displays excellent reusability, with a little decrease in adsorption capacity with the increasing number of cycles. Moreover, Cr (VI) the adsorption is unaffected by the presence of 50-500 times higher concentration of alkali metal and halide ions in solution, while sulphate ions demonstrate shielding behavior decreasing the adsorption capacity. Mechanistic studies indicate electrostatic attractions, ion exchange and reduction being responsible for the adsorption mediated by abundant nitrogen sites that also imbibes the adsorbent with high capacity. The adsorbent was also utilized to recover chromium from an industrial electroplating effluent, which demonstrates applicability of material for practical applications.

Enhancement of Cr(VI) adsorption on lignin-based carbon materials by a two-step hydrothermal strategy: Performance and mechanism

Cr(VI) is a carcinogenic heavy metal that forms an oxygen-containing anion, which is difficult to remove from water by adsorbents. Here, industrial alkali lignin was transformed into a Cr(VI) adsorbent (N-LC) by using a two-step hydrothermal strategy. The characterization results of the adsorbent showed that O and N were uniformly distributed on the surface of the adsorbent, resulting in a favorable morphology and structure. The Cr(VI) adsorption of N-LC was 13.50 times that of alkali lignin, and the maximum was 326.10 mg g-1, which confirmed the superiority of the two-step hydrothermal strategy. After 7 cycles, the adsorption of N-LC stabilized at approximately 62.18 %. In addition, in the presence of coexisting ions, N-LC showed a selective adsorption efficiency of 85.47 % for Cr(VI), which is sufficient to support its application to actual wastewaters. Model calculations and characterization showed that N and O groups were the main active factors in N-LC, and CO, -OH and pyridinic-N were the main active sites. This study provides a simple and efficient method for the treatment of heavy metals and the utilization of waste lignin, which is expected to be widely applied in the environmental, energy and chemical industries.

Cellulose-based slow-release nitrogen fertilizers: Synthesis, properties, and effects on pakchoi growth

The application of most slow-release fertilizers is limited by complex preparation processes and short slow-release periods. In this study, carbon spheres (CSs) were prepared by a hydrothermal method using cellulose as the raw material. Using CSs as the fertilizer carrier, three new carbon-based slow-release nitrogen fertilizers were prepared using direct mixing (SRF-M), water-soluble immersion adsorption (SRFS), and co-pyrolysis (SRFP) methods, respectively. Examination of the CSs revealed regular and ordered surface morphology, enrichment of functional groups on the surfaces, and good thermal stability. Elemental analysis showed that SRF-M was rich in nitrogen (total nitrogen content of 19.66 %). Soil-leaching tests showed that the total cumulative nitrogen release of SRF-M and SRF-S was 55.78 % and 62.98 %, respectively, which greatly slowed down the release of nitrogen. Pot experiment results revealed that SRF-M significantly promoted the growth of pakchoi and improved crop quality. Thus, SRF-M was more effective in practical applications than the other two slow-release fertilizers. Mechanistic studies showed that CN, -COOR, pyridine-N and pyrrolic-N participated in nitrogen release. This study thus provides a simple, effective, and economical method for the preparation of slow-release fertilizers, providing new directions for further research and the develop of new slow-release fertilizers.

Three-dimensional lignin-based polyporous carbon@polypyrrole for efficient removal of reactive blue 19: A synergistic effect of the N and O groups

Reactive blue 19 is one of the abundant carcinogens commonly used in industrial applications. This study transformed industrial lignin into a lignin-based polyporous carbon@polypyrrole (LPC@PPy) by a hydrothermal-activation-in situ polymerization strategy for removal of reactive blue 19. The hydrothermal reaction and polypyrrole polymerization provide abundant O and N groups, and the pore-making process promotes the even distribution of O and N groups in the 3D pore of LPC@PPy, which is favorable for the adsorption of reactive blue 19. The adsorption capacity of LPC@PPy for reactive blue 19 is 537.52 mg g-1, which is 2.04 times the performance of LPC (only hydrothermal and activation process, only have O groups) and 3.36 times that of LC (direct lignin activation, lack of O and N groups). After 8 cycles, LPC@PPy still maintained a high adsorption capacity of 92.14 % for reactive blue 19. In addition, this study found that N and O groups in the material played an important role in adsorption, mainly pyridinic-N, C-OH, -COOR, -C-O- and CC. This work provides a new strategy for the removal of reactive blue 19 and determines the groups that mainly interact with reactive blue 19, which provides a new reference for adsorption, catalysis and related fields.

Performance and mechanism of starch-based porous carbon capture of Cr(VI) from water

Cr(VI) pollution has seriously affected the survival of biological organisms and humans, so reducing the harm of Cr(VI) pollution is a significant scientific goal. Natural starch exhibits a low adsorption capacity for Cr(VI); thus, physical or chemical modification is needed to improve the adsorption and regeneration performance of starch. In this study, a novel starch-based porous carbon (SPC) was prepared to remove Cr(VI) from water by using soluble starch as a raw material. The characterization results show that the SPC shows a ratio surface area of 1325.39 m²/g. Kinetics suggest that the adsorption of Cr(VI) on SPC is dominated by chemisorption. The isotherm data demonstrated that the adsorption of Cr(VI) by SPC adhered to the Freundlich model. SPC exhibits a multimolecular layer adsorption structure, and the highest amount of adsorbed Cr(VI) in SPC was 777.89 mg/g (25 °C). Ion competition experiments show that SPC exhibits significant selectivity for Cr(VI) adsorption. In addition, the adsorption cycle experiment shows that SPC maintains a 63 % removal rate after 7 cycles. In this study, starch was transformed into high-quality adsorbent materials by hydrothermal and activation strategies, offering a new innovation for the optimization of starch-based adsorbents.

Adsorption of lead ions by activated carbon doped sodium alginate/sodium polyacrylate hydrogel beads and their in-situ recycle as sustainable photocatalysts

In this study, sodium alginate (SA), sodium polyacrylate (PAAS) and powdered activated carbon (PAC) were cross-linked by calcium ions [(Ca(II)] to form SA/PAAS/PAC (SPP) hydrogel beads. The hydrogel-lead sulfide (SPP-PbS) nanocomposites were successfully synthesized by in-situ vulcanization after the lead ions [(Pb(II)] adsorption. SPP showed an optimal swelling ratio (600% at the pH value of 5.0) and superior thermal stability (206 °C of heat-resistance index). The adsorption data of Pb(II) was compatible with the Langmuir model, and the maximum adsorption capacity of SPP was 391.65 mg/g after optimizing the mass ratio of SA to PAAS (3:1). The addition of PAC not only enhanced the adsorption capacity and stability, but also promoted photodegradation. The significant dispersive capacity of PAC and PAAS resulted in PbS nanoparticles with particle sizes of around 20 nm. SPP-PbS showed good photocatalysis and reusability. The degradation rate of RhB (200 mL, 10 mg/L) was 94% within 2 h and maintained above 80% after 5 cycles. The treatment efficiency of SPP was more than 80% in actual surface water. The results of quenching experiments and electron spin resonance (ESR) experiments revealed that the superoxide radicals (O₂^-) and holes (h⁺) were the main active species in the photocatalytic process.

Preparation of N-doped cellulose-based hydrothermal carbon using a two-step hydrothermal induction assembly method for the efficient removal of Cr(VI) from wastewater

Cr(VI) pollution is a growing problem that causes the deterioration of the environment and human health. We report the development of an effective adsorbent for the removal of Cr(VI) from wastewater. N-doped cellulose-based hydrothermal carbon (N-CHC) was prepared via a two-step hydrothermal method. The morphology and structural properties of N-CHC were investigated by various techniques. N-CHC has many O and N groups, which are suitable for Cr(VI) adsorption and reduction. Intermittent adsorption experiments showed that N-CHC had an adsorption capacity of 151.05 mg/g for Cr(VI) at pH 2, indicating excellent adsorption performance. Kinetic and thermodynamic analyses indicates that the adsorption of Cr(VI) on N-CHC follows a monolayer uniform adsorption process, which is a spontaneous endothermic process dominated by chemical interaction and limited by diffusion within particles. In a multi-ion system (Pb²⁺, Cd²⁺, Mn⁷⁺, Cl^-, and SO₄^2-), the selectivity of N-CHC toward Cr(VI) was 82.62%. In addition, N-CHC demonstrated excellent reuse performance over seven adsorption-desorption cycles; the Cr(VI) removal rate of N-CHC in 5-20 mg/L wastewater was >99.87%, confirming the potential of N-CHC for large-scale applications. CN/C-OR, pyridinic-N, and pyrrolic-N were found to play a critical role in the adsorption process. This study provides a new technology for Cr(VI) pollution control that could be utilized in large-scale production and other environmental applications.

Synthesis of honeycomb lignin-based biochar and its high-efficiency adsorption of norfloxacin

In this study, honeycomb lignin-based biochar (HLB) was prepared by hydrothermal activation using industrial lignin as raw material to remove norfloxacin from water. Batch adsorption test results showed that HLB has a strong ability to remove norfloxacin at a wide pH. The maximum adsorption capacity was 529.85 mg/g at 298 K, which is 1.52-fold to 201.46-fold higher than that of other reported materials. HLB showed good selectivity and recycling ability for the adsorption of norfloxacin, the removal rate of NOR reached 99.5% in the presence of competitive ions and maintained at least 98% removal rate after 12 adsorption cycles. The removal rate of norfloxacin in different water reached more than 99% within 8 mins. Pore filling, electrostatic interaction, π-π interaction, and hydrogen bond contributed significantly to the removal of norfloxacin. Among them, the highly aromatized structure of HLB and the abundant oxygen-containing functional groups (OH, CO, etc.) promoted π-π interaction.

Cellulose-based aerogel beads for efficient adsorption-reduction-sequestration of Cr(VI)

The reduction and sequestration of toxic Cr(VI) via a one-step process in an aqueous solution is critical to eliminate its environmental risk. In this study, amine functionalized cellulose-based aerogel beads (CGP) was developed for simultaneous and efficient adsorption- reduction- sequestration of Cr(VI). CGP showed a maximum Cr(VI) adsorption capacity of 386.40 mg/g at 25 °C due to its strong electrostatic attraction towards Cr(VI). The simultaneous Cr(VI) adsorption- reduction- sequestration performance of CGP over a wide Cr(VI) concentration range was examined. The mechanism was investigated in-depth via the analysis of adsorption kinetics, XPS spectra, and FTIR spectra. Moreover, the Cr immobilization stability of CGP after adsorption was evaluated in simulated neutral, acidic, and alkaline conditions. The effect of pH, temperature, ionic strength and the presence of interfering ions on CGP adsorption performance were investigated by batch adsorption experiments. Fixed-bed column adsorption study was performed to explore the application potential of CGP beads in a wastewater treatment process.