| Industrialization caused a large amount of industrial wastewater containing heavy metal emissions, leading to serious waterbody heavy metal pollution. Because it is not biodegradable, the heavy metal will cause great toxicity for living body through bio-concentration. Among the current wastewater treatment technology, adsorption approach is considerable to be a promising method due to the advantage of simple operation, small footprint, non-poisonous itself and so on. But the wide application of adsorption method in water treatment is restricted due to the high cost of adsorbent material and the poor application form. Hence, the research and development of cheap and efficient adsorption materials is currently an important direction in the field of wastewater treatment.As environmental adsorption material, the exploitability of clay mineral is the abundant reserves in nature, extensive sources, low cost and especially its adsorption properties derived from the characteristics of a charged surface, channel structure and high specific surface area, etc. Even so, there still is a considerable gap for clay mineral to be used in industry by the adsorption property of itself. Therefore, improving their adsorption and selectivity by modification technology is a necessary means. By combining the structure characteristics of the mineral and pollutant species in water, polymer organic-functionalized mineral composite is prepared by a reasonable modification route with contraposing design. On this way, both the adsorption performance and application form can be improved, so being more conducive to industrial application.Several typical and representative clay mineral were selected in this article, three different kinds of organic functional modification route were designed according to their respective structure characteristic. Furthermore, the systematic adsorption experiments were carried out to evaluate the adsorption property of resultant mineral composite for heavy metal ions.Concrete design ideas and innovations are listed as follows:(1) Polyacrylic acid grafted kaolinite composite adsorbentKaolinite has TO type of layered structure, there are abandunt hydroxy on the interlayer and fracture surface of its crystal structure. By adding the Lewis acid (such as Al3+) as catalyst, a heterogeneous catalytic esterification reaction was taken on normal temperature to achieve PAA chains anchored on the kaolinite surface with ester bond, therefore PAA brush "villous" function layer appeared on the the surface of the kaolinite under mild graft.The innovation point of this method is to achieve one-step grafting of functional macromolecule on surface of mineral through "grafting to" approach, which has the advantages of mild conditions, rapid reaction, low energy consumption, simple process and keeping the shape and strength of the original substrate, meanwhile the unreacted modifier (PAA) and catalyst (Al3+) can be recovered and used again.(2) Montmorillonite crosslinked composite gel adsorbentMontmorillonite is TOT type of layered structure, bonding force between crystal lamella is weak and the surface of sheet has the permanent negative charge, the stripped crystal sheet in the water can rearrange to form a "card palace" structure. Based on this framework, a composite hydrogel with excellent mechanical property was prepared by in situ polymerization through adding acrylamide monomer into the montmorillonite aqueous solution. Moreover, the second monomer of acrylic acid was also added for copolymerization for enhancing its adsorption performance. In addition, inverse suspension polymerization technology was adopted to realize the preparation of composite gel microsphere.The innovation point of this method is that the amide group on PAAm chain can bonded on crystal sheet surface with hydrogen bonding, charge attract and coordination, so the montmorillonite plays the role of inorganic crosslinking agent for PAAm chains. By comparing with the chemical crosslinked gel, the NC gel has the mechanic features of high toughness, good compression performance and high elongation. The prepared NC gel shpere can be used in dynamic wastewater treatment as packing in fixed bed, which can overcome the defect that traditional resin is easy to fracture.(3) Novel floatable magnetic composite adsorbent based on polypyrrole coated sepioliteSepiolite is TOT type of chain layered structure. Sepiolite shows fibrous shape at the micro level, which has the biggest specific surface area in the mineral material and the unique internal channel structure. Taking advantage of the natural shape of sepiolite as template, the magnetic nano-particles were loaded on its surface to achieve magnetic modification at first, next the surface was covered with polypyrrole function layer through oxypolymerization, expecially, sodium dodecyl sulfate (SDS) was also doped in the polypyrrole matrix at the same time to form a hydrophobic surface. Therefore, a floatable magnetic anion exchange composite adsorbent was obtained.The innovation point of this method is to praprare a core-shell type composite materials by with sepiolite fiber as template, the resultant material inherits the large specific surface of the sepiolite and has strong adsorption. Moreover, the proposed composites adsorbent can realize double characteristics of magnetism and floating performance for the first time, so that the solid particles can be seperated from water surface directly by a simple magnetic field. This design can overcome the defect that adsorbent particles is difficult to be applied in the large open water because they would sink to the water bottom even if having been magnetic modificaiton.The main conclusions obtained from this research are given as follows:(1) The reults of single factor experiments for preparation conditions showed that: When the concentration of PAA is 10%, the dosage of catalyst is 2.0 g/L, the adsorption performance of prepared KLN-PAA is optimal, the optimal adsorption capacity is 27.14 mg/g (Cu2+),19.68 mg/g (Zn2+),17.84 mg/g (Ni2+); The composite gel obtained at montmorillonite content of 2%, acrylamide content of 20%, acrylic acid content of 2% can having a relatively balanced property on swelling, mechanical strength and adsorption performance, and under the optimal ratio, the composite gel sphere can be prepared on the conditions as follows:the oil-water ratio of 3:1, adding amount of dispersant (Span-60/Tween-80,5:1) of 2%, stirring speed of 200 r/min, the adsorption capacity of compound gel sheet (CG-sheet) and compound gel sphere (CG-sphere)for Cu2+can be up to 41.21 mg/g and 105.65 mg/g; Pyrrole monomer concentration is 5%, concentration of oxidant Fe3+ is 0.2 mol/L, polymerization time is 120 min, the resultant MSEP/PPy has better adsorption performance, the adsorption capacity can reach 95.33 mg/g.(2) In batch adsorption experiment, the effect of the dosage of adsorbent, interfering ions, initial pH and concentration, contact time, temperature, etc. on the removal of heavy metal ions by the three kinds of adsorbent. The results show that with the increase of the dosage of adsorbent, removal efficiency of heavy metal ions is rising. After the removal efficiency reach balance, the adsorption capacity of adsorbent begin to decline obviously. Adsorbent have different affinity for different heavy metal ions, the adsorption capacity of Cu2+, Zn2+, Ni2+ by KLN-PAA can reach 27.25,29.86 and 32.45 mg/g respectively. The selective adsorption of composite gel of NaMMT/PAAm-PAA for single or mixed heavy metal ion follow the order:Pb2+>Cu2+>Cd2+>Zn2+>Ni2+. The rise of pH is in favour of KLN-PAA and Na-MMT/PAAm-PAA to adsorb heavy metal cations, the maximum adsorption value appear at pH=6, but for the adsorption of dichromate anion with MSEP/PPy, the influence of pH is opposite, under acid condition (pH= 2), the adsorption performance is better. The coexistence ions in solution is against to the adsorption of target ion, and the influence degree of interfering ions is different, the Ca2+has the most effect on the metal cation adsorption and SO42- the highest effect for dichromate anion adsorption. The effect extent is rising with the increase of interfering ions concentration. As the extension of contact time, the adsorption capacity of adsorbent for heavy metal ions gradually rises until it reaches an equilibrium. The lower time you spend in scheming, the less time is needed for reaching the adsorption equilibrium at the lower initial concentration, but corresponding equilibrium adsorption capacity is lower. Specially, it can be found that the time for CG-sphere reaching adsorption equilibrium (2 h) is much shorter than that of CG-sheet (12 h) and the adsorption capacity is higher, which indicates that the shape of adsorbent also have significant impact on adsorption performance. Adsorption capacity of adsorbent increases with the increase of initial concentration, When the adsorbent is saturated, the adsorption capacity is no longer rising by continuing to increase the initial concentration. But the raise of temperature can improve the maximum adsorption capacity of material. What is more, the dynamic adsorption test for composite gel sphere (CG-sphere) was also tested, the results show that reducing the concentration or flow rate of imported water can be conducive to postpone the breakthrough point and the exhaustion point of the fixed bed.(3) The results of kinetics, isotherms and thermodynamics model analysis show that the adsorption process are in accordance with pseudo second order kinetics model for all the three kinds of adsorbent, and the initial concentration of heavy metal ions is higher, the initial rate of adsorption process is much faster and the equilibrium absorption capacity is also higher. The fitting plots of intra-particle diffusion models show that the adsorption process is composed of two phases, the outer surface diffusion and inner diffusion for ions entering into the of adsorbent, the finish time of the former is faster with a main contribution for adsorption capacity, the subsequent inner diffuse process has a slow rising rate of adsorption capacity and the time required to reach adsorption equilibrium is relatively long. According to the isothermal model, the adsorption data for different heavy metal ions by all the three kinds of adsorbent can be fitted well by the Langmuir adsorption isotherm model, indicating that the adsorption sites are distributed evenly on the surface material and heavy metal ions were mainly adsorbed with monolayer. Thermodynamics research results show that △G0 value of heavy metal ions adsorption process is negative and decreases with the increase of temperature, determining that the heavy metal ions adsorption by the three adsorbents is a spontaneous irreversible process, and more conducive to the adsorption performance at higher temperature. At the same time both of △H0 and △S0 are positive, suggesting that the adsorption is endothermic process and an increase in disorder at the solid-liquid interface.(4) The study on regeneration experiment show that when the concentration of desorption solution (HC1) is 0.2 mol/L, the desorption time is 6 h, the regeneration effect is optimal. The first three continuous regeneration times have less influence on the adsorption performance, but then the adsorption capacity began to drop dramatically. After six regeneration times, the adsorption capacity of KLN-PAA for Cu2+, Zn2+ and Ni2+ decreased from 27.14 to 12.67 mg/g,19.68 to 8.69 mg/g, and 17.84 to 9.59 mg/g respectively. For CG-sheet and CG-sphere, EDTA solution has better regeneration effect than HCl solution, the ideal effect can be achieved at the concentration of 0.05 mol/L, time of 1 h and 2 min, respectively. Relatively, the CG-sphere can keep more stable adsorption performance with be more thorough desorption. In the six continuous regeneration experiments, the adsorption capacity of CG-sphere had fallen by 22.62%, less than the 27.57% of CG-sheet. The four continuous desorption-regeneration experiments show that the desorption rate of MSEP/PPy is below 25%, but the adsorption performance is maintained. This phenomenon suggest that there are some other reaction happened on the adsorbent surface. XPS analysis showed that the adsorbed hexavalent Cr(VI) was reduced to three valence Cr(Ⅲ) on the MSEP/PPy surface.(5) The analysis on adsorption mechanism of the three kinds of adsorbent for heavy metal ions show that KLN-PAA can remove the metal ions by electrostatic attraction, ion exchange and coordination. First, metal ions was attracted to the adsorbent surface and exchanged with the positive ion on carboxyl group, then combined with carboxyl groups in a monodentate or bidentate coordination. There are carboxylic and amino groups in the molecular network of composite gel adsorbent, carboxylic acid groups can bind the metal ions with ion exchange and coordination, also the ione pair electrons in N atom of amino groups can coordinate with unoccupied orbital of the metal ion to remove them. The adsorbed heavy metal ions can play the role of the ionic crosslinking agent at the same time, which strengthen the stability of gel in application. For MSEP/PPy, the surface has positive charge under acid condition, which has attraction for dichromate ion, and then an exchange in the PPy matrix by the release of Cl-. What is more, some hexavalent Cr(VI) was reduced to three valence Cr(III) on the MSEP/PPy surface.The purpose of this paper is the development of novel adsorbent with the advantage of cheap, efficient, easy to be applied in industry. In addiltion, to provide the reference of organic functional modification method for clay mineral to be used as environmental adsorption material. |
PDF Full Text Request