Fixed-bed Experiments Research Articles

Implementation of Ion Exchange Processes for Carbon Dioxide Mineralization Using Industrial Waste Streams

Sequestration of CO2 within stable mineral carbonates (e.g., CaCO3) represents an attractive emission reduction strategy because it offers a leakage-free alternative to geological storage of CO2 in an environmentally benign form. However, the pH of aqueous streams equilibrated with gaseous streams containing CO2 (pH < 4) are typically lower than that which is required for carbonate precipitation (pH > 8). Traditionally, alkalinity is provided by a stoichiometric reagent (e.g., NaOH) which renders these processes environmentally hazardous and economically unfeasible. This work investigates the use of regenerable ion-exchange materials to induce alkalinity in CO2-saturated aqueous solutions such that the pH shift required for mineralization occurs without the need for stoichiometric reagents. Na+-H+ exchange isotherms (at [H+] = 10−8–10−1 M) and rates were measured for 13X and 4A zeolites and TP-207 and TP-260 organic exchange resins in batch equilibrium and fixed-bed exchange experiments, respectively. At solutions equilibrated with CO2 at 1.0 atm (pH = 3.9), H+ exchange capacities for the materials were similar (1.7–2.4 mmol H+/g material) and resulted in pH increases from 3.9 to greater than 8.0. Multi-component mixtures using Ca2+ and Mg2+ cations (at 10−3–10−1 M) in CO2-saturated water were used to probe competitive ion exchange. The presence of divalent cations in solution inhibited H+ exchange, reducing capacities to as low as 0.2 mmol H+/g for both resins and zeolites. Dynamic H+ exchange capacities in fixed-bed ion exchange columns were similar to equilibrium values for resins (∼1.5 mmol/g) and zeolites (∼0.8 mmol/g) using inlet solutions that were equilibrated with gaseous streams of CO2 at 1.0 atm. However, exchange kinetics were limited by intraparticle diffusion as indicated by the increased rate parameters with increasing inlet flow rates (20–160 cm3 min−1). Experimental calcite precipitation from mixing the alkaline CO32−-rich water solution obtained from the ion-exchange column with a simulated liquid waste stream solution achieved thermodynamic maximum yields. The results from these studies indicate that ion exchange processes can be used as an alternative to the addition of stoichiometric bases to induce alkalinity for the precipitation of CaCO3, thereby opening a pathway toward sustainable and economic mineralization processes.

Mechanism on catalytic cracking tar with CaO-based catalysts for hydrogen-rich gas by DFT and experiments

As the model compound of tar, the catalytic cracking performances of toluene with CaO-based catalysts were studied by DFT simulation and fixed bed experiment. The DFT results showed the adsorption energy of toluene on (1 0 0) surfaces increased as FC < CaO < NC < NFC at 650 °C. The optimal reaction path for toluene to dissociate H was obtained in the process of toluene cracking using NC and NFC catalysts. The NC catalyst doped with Fe enhanced the catalytic activity of Ni, and the rate determining step reaction activation barrier to generate H2 was decreased from 1.50 eV to 0.58 eV. And the activation barrier for CH4 formation was higher than that for H2, which promoted the cracking reaction to produce H2. The experimental results showed that the catalytic toluene cracking effect of NFC was the best among the four catalysts at 650 °C, with 98.11% conversion rate and 93.11% H2 yield. Compared with NC, H2 yield was increased by 27.92%, CH4 yield was decreased by 50.43%. Thermogravimetric results showed that NFC was less likely to be inactivated due to carbon deposition than NC. NFC will be a kind of compound catalyst for tar cracking with high efficiency.

Adsorptive Separation of Aromatic Compounds from Alkanes by π-π Interactions in a Carbazole-Based Conjugated Microporous Polymer.

Separation of aromatic/alkane mixtures of similar size and properties is critical for the chemical industry as conventional thermal separation is a high-cost and an energy-intensive process. Adsorptive separation based on porous materials is a prospective and economical technology as well as a suitable alternative to the energy-inefficient heat-driven separation process. With this in mind, we design and synthesize a novel microporous polymer (termed CMP-S-1) with a conjugated aromatic skeleton as a porous adsorbent for aromatic/alkane separation. CMP-S-1 possesses high aromatic adsorption selectivity in two representative separation systems (benzene vs cyclohexane and 3-methylthiophene vs n-octane) based on a vapor adsorption experiment and an ideal adsorbed solution theory simulation. The instant adsorption rate, adsorption energy calculations, and liquid fixed-bed breakthrough experiments give convincing demonstrations on the preferential selective adsorption of aromatic compounds over alkanes in CMP-S-1. The strong π-π interaction between aromatics and the naphthalene ring is considered as the main reason for the strong affinity of aromatic compounds in the CMP-S-1 skeleton. The remarkable aromatic/alkane separation performance of CMP-S-1 verifies the important influence of the π-conjugation interaction in the conjugated porous polymer for the low-energy consumption adsorption separation process.

Effective removal of basic dye onto sustainable chitosan beads: Batch and fixed-bed column adsorption, beads stability and mechanism

This study aims to develop sustainable low-cost chitosan-based beads by a simple dropping method, with instantaneous formation by reticulation with tripolyphosphate anion. The beads were characterized and applied to basic blue 7 (BB7) uptake. The adsorption process was optimized through variation of several parameters in batch experiments such as bead loading, pH, dye concentration and temperature. At a BB7 concentration of 400 mg/L and 60 min, the best BB7 adsorption capacity (1410 mg/g) and partition coefficient (8.22 mg/g/μM) were achieved by chitosan beads, which were quite superior to others described in the literature. The obtained data best fitted to mixed- and pseudo-first order kinetic models and to Redlich-Peterson isothermal model. A fixed-bed column experiment was conducted and the adsorption capacity and partition coefficient at 100% breakthrough were 12.06 mg/g and 0.06 mg/g/μM. The bead storage study indicated that its adsorption capacity was not affected for a two-week period and reached 325 mg/g. Also, the presence of basic brown 4 (BB4) dye did not harm basic blue 7 in the simultaneous adsorption process and a removal capacity of 232 and 259 mg/g was reached for BB4 and BB7 dye, respectively. The presence of NaCl reduced the percentage of BB7 removal from 91% to 73%. The removal mechanism based on XPS, FTIR and BET analyses suggests that hydrogen bonds are the main adsorption mechanism followed by cation-exchange. This study gave rise to encouraging results and chitosan/tripolyphosphate beads proved to be potential adsorbents for industrial effluent treatments.

Extended Rate Constant Distribution Model for Sorption in Heterogeneous Systems: 3. From Batch to Fixed-Bed Application and Predictive Modeling

Here, we have demonstrated that the recently developed extended rate constant distribution (RCD) model for sorption on heterogeneous sorbents can be applied to predict the column behavior of the sorbents using parameters determined from the sorption kinetics in batch. The suggested approach has been validated on the batch and fixed-bed experimental data of Cu(II), Cd(II), and Zn(II) ion sorption on a polyethyleneimine (PEI) cryogel. Although the average sorption rate constants decreased in the order Cu(II) > Zn(II) > Cd(II), “fast” and “slow” sorption centers with different affinities have been identified for all investigated ions. This explained why, depending on the experimental conditions (flow rate and metal ion concentrations), one or another ion from the mixture can adsorb preferentially on PEI in fixed-bed applications, while at long equilibration time, the ratio between adsorbed metal ions was determined by the sorbent affinity. We have also shown that sorption rate constants for the PEI cryogel determined from the batch data were ∼0.6 log units lower than those determined from fixed-bed experiments that proved higher sorption efficiency for this type of sorbent under dynamic conditions. The column efficiency coefficient introduced to the RCD model for the fixed-bed application allowed us to reveal intrinsic and operational defects in soft monolith sorbents materials, which originated from imperfection of fabrication or destruction of the porous structure at high flow rates, respectively.

Experimental and numerical investigations on the separation performance of [Cu(INA)2] adsorbent for CH4 recovery by VPSA from oxygen-bearing coal mine methane

In this work, [Cu(INA)2] metal–organic framework (MOF) adsorbent has been used to recover methane from coal mine methane (CMM) using vacuum pressure swing adsorption process (VPSA). The adsorption capacity and separation performance were investigated at one bar and different temperatures (288 K, 298 K and 308 K) for methane, nitrogen and oxygen. The breakthrough curves of the single component and gas mixture were experimentally obtained through a dynamic fixed-bed experiment. Based on the LDF model, the mass transfer coefficient was investigated according to the experimental breakthrough curves. VPSA process was also developed using adsorption equilibrium and kinetics parameters to achieve the maximum recovery of methane. The VPSA operational parameters were optimized involving feed gas flow rate, adsorption time, adsorption pressure and evacuation pressure. The maximum 90% recovery of methane with 50% purity can be achieved.

Comparison of organic materials for the passive treatment of synthetic neutral mine drainage contaminated by nickel: Short- and medium-term batch experiments

This paper addresses the comparison of various low-cost organic materials for the passive treatment of circum-neutral pH contaminated mine water. First, the effectiveness of five organic materials to remove Ni from a contaminated neutral drainage (CND) was compared in short-term batch experiments (24 h) at various pH values. Second, based on results of the short-term experiments, two substrates (brown algae and sawdust) were eliminated and three (horticultural peat, compost, wood ash) were compared along with a new substrate (field-collected surface peat) in medium-term (56 days) batch experiments to treat CND. In these experiments, calcite was added to peat samples and all substrates performed equally well, sequestering over 97% of the Ni. Chemical extractions revealed that Ni was more strongly bound to the horticultural peat-calcite (HD-peat-calcite) residue than to the field-collected peat-calcite (LT-peat-calcite) residue. Compost, because of its higher density, was identified as the most promising candidate for sorption-based fixed-bed column experiments. Nevertheless, wood ash should not be discarded as its alkaline properties favor nickel removal.

High-efficiency removal of low-concentration Hg(II) from aqueous solution by bentonite nanocomposite: Batch and fixed-bed column adsorption study

ABSTRACT A nanocomposite based on bentonite was synthesized. Its adsorption properties for Hg(II) were evaluated by batch and fixed-bed column adsorption experiments. The results showed that the nanocomposite was an efficient adsorbent for the treatment of wastewater containing low-concentration Hg(II). It removed more than 92% of Hg(II) from aqueous solution with a pH ranged from 4.00 to 12.00. Its adsorption capacity for Hg(II) increased linearly with the increase of mercury concentration. The nanocomposite had excellent reusability. HCl solution can be used to elute Hg(II) ions loaded. The developed Bohart–Adams model can be used for the design of adsorption column.

Equilibrium, Kinetic and Thermodynamic Studies for the Adsorption of Gemfibrozil onto Graphitized Carbon Black (GCB)

Adsorption of gemfibrozil (GF) from aqueous solution by highly/lowly graphitized carbon black (GCB-H/GCB-L) was investigated by batch and fixed-bed experiments. Results showed that the adsorption of GF onto GCB greatly depended on solution pH and graphitization of carbon. The pseudo-second-order kinetic model was found to be more appropriate in describing the adsorption processes of two GCBs. The adsorption equilibriums were well fitted with the Langmuir isotherm model, and GCB-H exhibited a higher adsorption capacity of 47.68 mg g−1 at 298 K, compared with GCB-L of 4.33 mg g−1. GCB-H achieved 69.76% GF removals in simulated water after 6 h, indicating its potential practicability. The GCBs were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR), and the results indicated that the [Formula: see text]-[Formula: see text] interaction was strongly involved in the adsorption. Thermodynamic studies further suggested that the adsorption of GF on GCB-H was exothermic, which could happen spontaneously at any temperature mainly through [Formula: see text]-[Formula: see text] interaction for physical adsorption on the surface of the plane.

Immobilization of nanosilver onto glycine modified lignin hydrogel composites for highly efficient p-nitrophenol hydrogenation

A new type of nanosilver immobilized catalyst based on glycine modified lignin hydrogels with excellent catalytic performance towards p-nitrophenol reduction have been prepared. Glycine was used to provide amino and carboxy groups for complexing silver ions, simultaneously which was also beneficial for in-situ reduction and suppression of silver nanoparticles. As-prepared composites were characterized by X-ray diffraction, scanning electron microscope with EDS, high-resolution transmission electron microscope, X-ray photoelectron spectroscopy, Fourier Transform Infrared Spectrometer and Thermal Gravimetric Analyzer. The developed network with enriched anchoring sites could provide the stability during the catalytic process and enough traction to disperse and stabilize the silver nanoparticles. Batch experiments of 4-NP hydrogenation were carried out to correlate the catalytic performance. The concentration of 4-NP of 5 mmol/L could be finished with only 101 s and the rate of k was 0.0151 s−1. Moreover, fixed-bed experiments were creatively applied for the tests and the catalyst can maintain high efficiency about 300 min with the flowing rate of 3 mL/min. After being reused ten cycles, the catalyst retained the conversion rate beyond 98%; and the structure has no obvious deterioration and the leaching of nanosilver was neglectful, which could be attributed to the crucial role of introduced glycine component.

Influence of auxiliary cylinders on the dynamics of overland flow upstream of the main cylinder based on particle image velocimetry

The position of auxiliary cylinders in flow experiments noticeably affects soil erosion and pollutant transportation processes by altering the dynamic flow field in front of the main cylinder. However, few studies have been conducted for overland flow due to the shallow flow depth and limited observation technology. In this study, we conducted fixed-bed flume experiments and developed a high-resolution and high-frequency particle image velocimetry system to investigate the two-dimensional dynamic field of overflow using a symmetrical surface (i.e., the vertical plane passing through the axis of the main cylinder in the flow direction) upstream of the main cylinder as the observation area. Two auxiliary cylinders in each position were placed upstream, parallel, and downstream (i.e., pre-column condition, parallel condition, and post-column condition, respectively). Results showed that: For a given flow discharge, the ranking of the cylinder configurations in terms of the stability of the hydrodynamic conditions (velocity, turbulent energy, shear force, and Reynolds stress) was pre-column condition < parallel condition < post-column condition. The number of horseshoe vortices and the rotation intensity of the main horseshoe vortex were similar for the three configurations. The position of the main horseshoe vortex was closest to the main cylinder in the pre-column condition, causing substantial soil erosion around the main cylinder. For different flow rates, the water depth and turbulent intensity showed monotonic increasing trends as the flow discharge increased, whereas the shear force and Reynolds stress exhibited a single peak. As the Reynolds number of the cylinder increased, the flow separation point moved upstream, and the rotation intensity of the main horseshoe vortex was negatively correlated with the water depth. Therefore, a shallower water depth corresponded to significantly greater erosion damage by the horseshoe vortex. Overall, the configuration of the auxiliary cylinders downstream of the main cylinder resulted in the optimum water flow control. The findings of this study provide theoretical support for designing vegetation management strategies that minimize soil erosion and protect water resources.

Memory effect induced the enhancement of uranium (VI) immobilization on low-cost MgAl-double oxide: Mechanism insight and resources recovery

It remains challenging to develop high-performance technologies for uranium (U(VI)) removal/recovery from wastewater/seawater. In this study, MgAl-double oxide (MgAl-LDO-500) was fabricated by calcining MgAl-layered double hydroxide (MgAl-LDH) at 500 ℃ in air. It showed excellent performance in U(VI) removal with an equilibrium time of 15 min and the maximal adsorption capacity of 1098.90 mg g−1. MgAl-LDO-500 also showed good adaptability in a wide range of pH (from 3 to 10), coexisting ions and different water matrices for U(VI) immobilization. It was found that the anion form of U(VI) intercalated into the layer of MgAl-LDO-500 and caused recombination of layered structures. A series of characterizations (XRD, SEM, FTIR, XPS) proved that memory effect and surface complexation were the key mechanism for the enhancement of U(VI) immobilization on MgAl-LDO-500. Due to the remarkable memory effect, the performance of MgAl-LDO-500 for U(VI) immobilization was superior to MgAl-LDH and other high-cost materials. Besides, the fixed-bed column experiments illustrated that the removal rate achieved 99 % before 1500 BV at initial U(VI) concentration of 20 μg L−1, and the breakthrough volumes (BVs) were 4500 BVs. These results confirm that MgAl-LDO-500 is a promising material for extracting U(VI) from seawater and wastewater.

Effective and simultaneous removal of organic/inorganic arsenic using polymer-based hydrated iron oxide adsorbent: Capacity evaluation and mechanism

In this study, resin-based hydrated iron oxide (HFOR) composites were prepared and used as a functional adsorbent for the simultaneous removal of p-Arsanilic acid (p-ASA) and arsenate (As (V)). The effects of solution pH and coexisting substances on the adsorption of different arsenic species were also investigated. Results showed that the coexisting substances slightly affected the adsorption process of two arsenic species. Analysis of the adsorption behavior, isotherm equilibrium, and adsorption kinetics, as well as that results of the X-ray photoelectron spectroscopy, zeta potential, and other analytical methods revealed that the satisfactory adsorption performance of HFOR can be attributed to the electrostatic interactions induced by the positively charged groups and the coordination of the hydrated iron oxide nanoparticles, which exhibited excellent specific adsorption for both arsenic species. Moreover, HFOR showed high acid and alkali resistance and reusability, as well as a constant co-removal performance for different arsenic species in five consecutive operating cycles (55 mg As/g of As(V) and 18 mg/g of p-ASA). Results of continuous running fixed-bed column experiments confirmed that HFOR enabled excellent simultaneous adsorption for p-ASA and As(V).

Gas products generation mechanism during co-pyrolysis of styrene-butadiene rubber and natural rubber

In this study, reaction molecular dynamics were combined with experiments to gain in-depth understanding of the gaseous pyrolysis products generation mechanism and optimal paths during natural rubber (NR), styrene-butadiene rubber (SBR) and mixed rubbers (NR-SBR) for effective recovery of waste rubber. The results show that the pyrolysis temperature of NR increases gradually with SBR addition. The monomers produced during the initial stage of SBR pyrolysis are mainly 1,3-butadiene and styrene, in which the energy barriers of the formed H and CH2=CH in styrene are higher than those in 1,3-butadiene, and during further pyrolysis the main gas products are H2 and CH4. During co-pyrolysis of NR-SBR, the reaction paths show that increasing H2 yield and decreasing CH4 yield take place easily as SBR content rises. By contrast to pyrolysis of NR, the path of generating CH2=CH in SBR is more difficult while that of CH2=CH abstracting H occurred easily, leading to first enhancement in produced CH2=CH2 followed by a decline. Fixed bed experiments and gas chromatography (GC) analysis identify the main gas products of the three rubbers (NR, SBR, NR-SBR)as H2, CH4 and CH2=CH2 and the change of yield caused by the increase of SBR content are consistent with the simulation results.

Simulations of breakthrough curves for fixed-bed column adsorption of cobalt (II) ions on spent tealeaves.

The objective of this study is to model the breakthrough adsorption curves of Co (II) ions using spent tealeaves in fixed-bed column experiments. Spent leaves of green tea (GT), peppermint tea (PM) and chamomile (CM) were packed in glass columns with a diameter of 2 cm and height of 15 cm, and used as filters for the removal of the pollutant. Aqueous solutions of cobalt (II) ions (100 mg/L) at pH 6 were prepared and pumped against gravity through the columns at a uniform flow rate of 5 mL/min. Breakthrough curves were fitted for the residual concentration data using the Thomas, Yoon-Nelson, and Clark models, with added empirical terms to delineate the lower tail of the breakthrough curve. These mathematical models were successfully linearized using the natural logarithm for parameter estimation. The results reveal that the Co (II) adsorption fits all three models for all the adsorbents. The Thomas model indicated that the calculated adsorption capacities followed the trend: PM > GT > CM with values of 59.7, 25.2, and 24.9 mg/g respectively. Moreover, CM showed the highest adsorption rates with all the mathematical models, whereas Yoon-Nelson theory provided evidence that PM has the longest 50% adsorption breakthrough among the adsorbents. Lastly, morphological and textural studies indicate that all spent leaves are good candidates as adsorbents due to their high surface heterogeneity. This study proposes the use of spent tealeaves as Co (II) adsorbents because they are inexpensive and environmentally beneficial.

Acidification Chicken Feather as Sorbent for Selectively Achicken feather as sorbent for selectively adsorbing of Cr(VI) ions in aqueous solution

Chicken feathers (CFs) were acidized by nitric acid, and the structure and morphology of acid treated feathers (NF) were characterized by FT-IR, XRD and SEM. The adsorption performance of Cr(VI) ions by NF was explored by batch and fixed bed experiments. Some of the amino groups of keratin were cationic after being treated with nitric acid, indicating that NF is a potential adsorbent for eliminating Cr(VI) ions from water. For Cu(II), Mn(II) and Co(II) ions, NF did not show any adsorption, while Cr(VI) could be adsorbed by NF. And NF still shows higher selectivity to Cr (VI) ions even in the presence of Cl−, SO42-, or NO3− ions. Langmuir and Freundlich isotherm models were used to fit the equilibrium sorption isotherms of Cr(VI) ions. In addition, breakthrough and desorption curves were obtained by using fixed bed columns packed with NF at different desorption flow rates and NaOH concentrations. The results showed that 0.05 mol·L-1 of an aqueous NaOH solution is an appropriate desorption agent, with 5 times the theoretical desorption amount and a flow rate lower than 12.3 mL⋅min-1. An aqueous solution of HNO3 with a mass fraction of 0.34 % is a feasible regeneration regent. The fixed bed adsorption capacity of NF was 39.04 mg⋅g-1 (0.751 mmol⋅g-1). CFs can serve as a low-cost alternative for adsorbing acid and selectively enriching toxic Cr(VI) ions from water.

Nanoconfined hydrous titanium oxides with excellent acid stability for selective and efficient removal of As(V) from acidic wastewater

Efficient removal of arsenic from acidic industrial effluents is still a challenge because of the high arsenic concentrations and low pH. Herein, a millimeter-sized nanocomposite Ti-201, with excellent acid resistance in acidic arsenic-contaminated water, was synthesized by dispersing the nanosized titanium oxides inside the networking pores of a commercial anion exchanger (D201). Negligible titanium dissolution was observed from Ti-201 in the solution of acidic pHs (above 2). The resulting nanocomposite exhibited superior adsorption capacity and selectivity for As(V) removal from acidic solution, even in the presence of coexisting Cl−/NO3−/SO42− ions at higher concentrations. The adsorption kinetics and thermodynamics of the nanocomposite were also well investigated to elucidate the adsorption behavior. FTIR and XPS study suggested the impregnated titanium oxide nanoparticles played a dominant role during the adsorption process, and As(V) was mainly removed via ligand exchange reactions between arsenic species and hydroxyl groups on the nanoparticles. Besides, fixed-bed column experiments further proved its applicability for As(V) removal in synthetic acid effluents with a working capacity of 1800 BV. The exhausted adsorbent was also amenable to be regenerated by NaOH-NaCl solution for repeated use without any significant capacity loss. All the results demonstrated that the prepared Ti-201 nanocomposite was a promising alternative adsorbent for efficient As(V) removal from acidic contaminated water.

Polyethylenimine-functionalized polyacrylonitrile anion exchange fiber as a novel adsorbent for rapid removal of nitrate from wastewater

The development of an adsorbent with high adsorption ability and favorable cyclic regeneration performance for the removal of nitrate residues from wastewater is a task of vital importance. To this end, polyacrylonitrile fiber (PANF) was modified with polyethyleneimine (PEI), and alkyl groups were then introduced around the active amine groups to prepare three polymer-based anion exchange fibers (PAN-PEI-3C, PAN-PEI-5C, and PAN-PEI-8C). The novel fibers were characterized using techniques such as scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The adsorption isotherms of the fibers were best fitted by the Langmuir model, and PAN-PEI-5C exhibited a higher adsorption amount of nitrate (31.32 mg/g) than the other adsorbents. The equilibrium was reached expeditiously (within 10 min), and both pseudo-first-order and pseudo-second-order models could well describe the adsorption kinetics. More attractively, the saturated PAN-PEI-5C could be eluted using a low-concentration (0.3 M) NaCl solution, without any sharp loss of adsorption amount for five consecutive cycles in the presence of dissolved organic matter (DOM). Furthermore, PAN-PEI-5C could effectively adsorb low-concentration nitrate from real secondary effluents in a fixed-bed column experiment. Our work provides a promising and low-cost material for the removal of nitrate residues in practical applications.

Comparison of Adsorption Properties of Activated Carbons with Different Crops Residues as Precursors for Gold Cyanide Recovery: An Iranian Gold Industry Guide

Adsorption of gold cyanide on three types of Activated Carbons (ACs) has been investigated in batch and column adsorption conditions. Applied ACs have been derived from different crops precursors i.e., coconut shell (CAC), peach stone (PAC), and walnut shell (WAC). As peach stone and walnut shells are abundant agricultural residues in Iran, the activated carbons produced from these precursors are economically preferable for the gold recovery process. The ACs were characterized using FTIR, SEM, BET, and Wet-Bed Compaction Hardness analyses. Batch equilibrium adsorption data were analyzed using the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models. Freundlich isotherm was in better agreement with the equilibrium data and the maximum adsorption capacity (40.8 mg/g) was tabulated for CAC. Adsorption kinetics was also modeled using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The PAC recorded the highest removal rate in the light of better agreement of the pseudo-second-order model. Fixed-bed column experiments were carried out at different initial gold concentrations (30, 50 mg/L) to determine the characteristics of the breakthrough curves. The maximum bed capacity (37.55 mg/g) was obtained for CAC. Three dynamic adsorption models including, Thomas, Yoon–Nelson, and Adam–Bohart,were applied to describe the breakthrough curves. Both Thomas and Yoon–Nelson models were in appropriate agreement with the experimental column adsorption data. This research introduces peach stone as a promising precursor considering availability, technical features, and economical aspects for the production of AC for gold recovery from cyanide leach solutions. Results are also conducive for gold mining industries to select the appropriate activated carbon.

Black liquor as biomass feedstock to prepare zero-valent iron embedded biochar with red mud for Cr(VI) removal: Mechanisms insights and engineering practicality

Black liquor (BL) is an agro-industrial residue with high number of lignocellulosic components which could be recognized as a biomass feedstock. In this work, BL coupled with red mud (RM), were applied to prepare cost-effective zero-valent iron (ZVI) embedded in biochar. The oligomers in BL acted as reductants for RM to generate ZVI, while the organic components could be converted into biochar during pyrolysis. The RM/BL demonstrated excellent performance in the removal of Cr(VI) (349.5 mg/g), as the mechanisms were reduction and adsorption. The fixed-bed column study was conducted and 1.7 L simulated wastewater could be treated by 1.0 g RM/BL. After reaction, 95.5% ± 0.8% and 82.5%±3.2% Cr-loaded adsorbents could be recovered by an external magnet for batch and fixed-bed experiments, respectively. All these results shed light on valorizing these two widespread agro-industrial byproducts, and bridged the knowledge gap between magnetic bio-adsorbent preparation and its industrial practicality on wastewater purification.