Concerns of Heavy Metal
Heavy metals are toxic or low-concentration toxic metal chemical elements with density generally greater than 5.0g/cm3 and atomic number above 24, such as Cu(II), Hg(II), Pb(II), Cd(II), Cr(VI) and other 45 kinds, excluding radioactive elements.
To some extent, heavy metals are continuously enriched in living organisms through food, drinking water, and air.
As trace elements, some heavy metals (e.g., copper, selenium, zinc) are essential for maintaining human metabolism. However, at higher concentrations, they can cause toxicity.
Heavy metals are commonly found in various industrial wastewater. Plating and surface treatment processes lead to the production of large amounts of wastewater containing heavy metals.
In addition, wastewater from industries such as leather, tanning, textiles, pigments and dyes, paints, wood processing, petroleum refining industries, and photographic film production contain large amounts of heavy metals.
Due to the enrichment, toxicity, and biodegradability, heavy metal pollution has become a serious environmental problem, and the persistence of heavy metals in polluted water bodies has caused many health problems for humans and animals.
In order to reduce the uncontrolled discharge of these harmful heavy metals in polluted water bodies, the prevention and control of heavy metals in polluted water bodies have been a research hotspot and difficulty in the international environmental protection community.
Therefore, the pressing research on heavy metal pollution control technologies and related basic theoretical issues for these heavy metal pollution problems has become an urgent need to be solved.
Heavy Metal Wastewater Treatment Method
Removal of toxic heavy metals from polluted water bodies is imperative for health and environmental protection.
To this end, chemical precipitation, chelate precipitation (flocculation), ion exchange, electrochemical, flotation, advanced oxidation, and membrane separation methods have been developed to address heavy metals, but no universal and effective treatment methods have been found so far.
There are some defects in the current methods of treating heavy metal wastewater.
- The chemical precipitation method is difficult to meet the standard for treating wastewater containing complexing agents, and it is easy to lead to secondary pollution.
- The membrane separation is easy to pollute membrane material with low permeate flux.
- The ion exchange method is only suitable for low concentrations of heavy metal wastewater and the resin exchange capacity is limited.
- The chelating flocculant used in the chelating flocculation process is not likely to be recycled and the cost is relatively high.
- The flotation method has a large initial investment, with high maintenance, and operation costs.
- The electrochemical process has a large investment in treatment and high power consumption, which limits its popular application.
In comparison with other treatment methods, the adsorption process is suitable for various types of heavy metal wastewater, in particular for the deep treatment of low-concentration affluent, due to its high efficiency, relatively low cost, and simple operation.
Basically, adsorption equipment is an economical, effective, and the most valuable application of heavy metal wastewater treatment.
The key to adsorption technology is the preparation of environmentally friendly, cheap, and efficient adsorbents.
The use of natural minerals to treat pollution and repair the environment is based on making full use of the laws of nature, reflecting the characteristics of natural self-purification.
This article mainly discusses advances in clay minerals and their derivatives for treating heavy metal effluent.
How To Get Adsorbent?
An array of efficient adsorbents including natural and modified clay minerals have been investigated and developed for the removal of heavy metals from contaminated waters.
Clay minerals are small particles that exist naturally on the earth’s surface. It is mainly composed of water, alumina, silica, and weathered rocks.
Clay mineral materials also contain exchangeable cations, including Na+, Ca2+, and K+, making them highly efficient adsorbents.
They are mostly negatively charged, due to the substitution of Si4+ and Al3+ by other cations, and are widely used in removing heavy metal cations from wastewater because of a large surface area contact and high cation exchange capacity.
Adsorption Properties Of Various Modified Clays?
Heavy metals are adsorbed by clay minerals and their derivatives by a complex series of mechanisms including ion exchange, surface complexing, direct binding of heavy metal cations to clay surfaces, or a combination of these physical and chemical processes.
Modified clays typically exhibit increased surface area and porosity compared to their pristine counterparts. This allows for a larger adsorption capacity and improved accessibility for target molecules.
According to their internal layer composition, clays can be divided into two types, amorphous and crystalline.
The crystal structure of crystalline clay can be divided into 1:1 lamellar (kaolinite), 1:1 tubular (halloysite), 2:1 lamellar (montmorillonite, vermiculite), and 2:1 lamellar chain (Attapulgite, sea foam) and so on. Each type of clay has different inherent adsorption characteristics.
Let’s explore the specific substances that the modified clay is known to adsorb. This could include heavy metals, organic pollutants, dyes, pharmaceuticals, or other contaminants.
Heavy Metal Removal By 1:1 Type Clay
Halloysite nanotubes (HNTs) are aluminosilicate clays with nanotubes and hollow microstructures.
In addition, HNTs have active hydroxyl groups on the surface, which can be modified by some organic compounds to improve the selectivity of adsorption on heavy metal ions.
In order to improve the adsorption capacity and solid-liquid separation performance of halloysite nanotubes (HNTs), Fe3O4 nanoparticles are used to modify the surface of HNTs and they are modified with silane coupling agents.
Using HNTs, Fe3O4 nanoparticles and aniline-methyl-triethoxysilane (KH-42) as the main raw materials, the clay mineral cation exchange sites are first activated with hydrochloric acid and Fe(III), then Fe3O4 is immobilized on the clay surface by in situ co-precipitation method, and finally, the silane is grafted onto the clay surface by condensation reaction.
A new adsorbent elosite nanotube/Fe3O4 composite (KH-42), denoted as m-elosite nanotube/Fe3O4, was successfully synthesized.
This new adsorbent showed the highest adsorption capacity for Cr(VI); the Cr(VI) removal rate reached 100% when the initial concentration of Cr(VI) was <40 mg/L.
In addition, the maximum removal of Sb(V) in the single-solute system increased from 67.0% to 98.9% in the two-solute system, indicating that the presence of Cr(VI) enhanced the removal of Sb(V) by the m-elosite nanotube/Fe3O4 adsorbent.
FTIR and XPS measurements confirmed the formation of inner spheres between Cr(VI) and functional groups of m-elosite nanotube/Fe3O4 complexes, and the study showed that m-elosite nanotubes/Fe3O4 have promising applications in the co-blending of heavy metal ions such as Sb(V) and Cr(VI) for the treatment of wastewater.
Heavy Metal Removal With 2:1 Laminated Clay
Synthesis of functionalized modified montmorillonite/carbon nanocomposites.
Montmorillonite is a unique montmorillonite clay that is widely distributed in the natural environment.
Due to its large specific surface area and swellable layered structure, modified montmorillonite is utilized to remove heavy metal pollutants with good adsorption properties.
It has been shown that the preparation of montmorillonite/carbon nanocomposites by hydrothermal carbonization using d-glucose as the carbon nanoparticle precursor and bentonite as the filler provides a basis for grafting functional groups such as -COOH, -OH, and -NH2.
The montmorillonite/carbon nanocomposite -COOH was prepared by mixing H2O2 solution with montmorillonite/carbon nanocomposite by vigorous stirring.
The montmorillonite/carbon nanocomposite was added to NaOH solution and transferred to a high-temperature hydrothermal device at 150°C to react to obtain montmorillonite/montmorillonite/carbon nanocomposite -OH.
The montmorillonite/carbon nanocomposite was mixed with an ethane diamine solution.
According to the above method, three different organic functional groups (-COOH, -OH, and NH2) were initially introduced on the surface of the montmorillonite/carbon nanocomposites.
After the modification of the montmorillonite/carbon nanocomposites, the adsorption capacity of the functionalized montmorillonite/carbon nanocomposites for Pb(II) was significantly increased in the following order: montmorillonite/carbon nanocomposites -COOH>montmorillonite/carbon nanocomposites -OH>montmorillonite/carbon nanocomposites -NH2>montmorillonite/carbon nanocomposites.
In addition, the adsorption capacity of the reaction system for Pb(II) increased as the pH increased from 2 to 5. The complexation of Pb(II) with different functional groups (-NH2, -COOH, -OH groups) showed different adsorption effects on the three adsorbents.
Synthesis And Adsorption Properties Of Modified Bentonite
Bentonite is an aluminium-layered silicate sorbent based on montmorillonite.
The presence of common impurities such as mica, quartz, feldspar, calcite, organic felts and carbonates have a negative impact on the cation exchange capacity (CEC) and thermal stability of bentonite.
In addition, in acidic environments, the release of H+ ions from the edges of the structure can lead to the adsorption of Cd(II), Zn(II), and Pb(II) from contaminated water to the edges of the structure.
However, purification of bentonite is necessary to obtain excellent physical properties, such as thermal stability and mechanical properties.
In this process, bentonite is usually purified by precipitation and acid treatment prior to surface modification.
It has been shown that bentonite can be used as a filler in polymer matrices due to the strong interaction between the Si-O-Si groups of bentonite and the functional groups in the polymer matrix, including OH, COOH, NH2 and n-acetylglucosamine groups.
A series of chitosan-polyvinyl alcohol/bentonite composites with different bentonite contents were prepared using cross-linking and interpenetrating polymer network techniques.
A new adsorbent, chitosan-polyvinyl alcohol/bentonite, was synthesized by combining bentonite with a chitosan-polyvinyl alcohol polymer matrix.
The prepared chitosan-polyvinyl alcohol/bentonite nanocomposite has a mesoporous structure, good adsorption capacity, and selectivity for Hg(II) ions.
The equilibrium adsorption capacity of chitosan-polyvinyl alcohol/bentonite for Hg(II) was much higher than that of Pb(II), Cd(II), and Cu(II), indicating that the synthesized chitosan-polyvinyl alcohol/bentonite has exceptional selective adsorption capacity for Hg(II).
The adsorption capacities of chitosan-polyvinyl alcohol/bentonite for Hg(II) were 360.73, 392.19, 455.12, and 460.18 mg/g at 50, 30, 10 and 0% bentonite content, respectively.
Under the same conditions, the adsorption capacity of the pretreated bentonite for Hg(II) was 11.20 mg/g.
If the bentonite particles were simply dispersed in the chitosan-polyvinyl alcohol polyvinyl alcohol polymer matrix, the adsorption capacity of chitosan-poly(vinyl alcohol)/bentonite should be equal to the total adsorption capacity of the chitosan-poly(vinyl alcohol) polymer and the phengite, i.e. 235.69, 325.46 and 415.28 mg/g for chitosan-poly(vinyl alcohol)/bentonite with bentonite contents of 50, 30 and 10% respectively.
The experimental data are much higher than the calculated values, indicating that chitosan-poly(vinyl alcohol)/bentonite is not a simple mixture.
In addition, bentonite was involved in the preparation of chitosan-poly(vinyl alcohol)/bentonite, which improved the adsorption capacity of Hg(II) to a certain extent.
Synthesis And Adsorption Properties Of Modified Vermiculite
Vermiculite is a common clay mineral found in layered silicates.
The presence of exchangeable cations such as K+, Na+, Ca2+, Mg2+, etc. in the interlayer space compensates for the lack of positive charge in the parallel 2:1 layers.
In this way, the two layers are combined in a configuration commonly referred to as a 2:1 layered silicate.
Many reports indicate that inorganic or organically modified vermiculite has a greater capacity to adsorb heavy metal ions than raw vermiculite, as these additives provide more active sites or bind more strongly to heavy metals.
In addition, acid treatment increases the specific surface area of vermiculite, removes mineral impurities by partially dissolving the outer layers and forms additional silica hydroxyl groups (Si-OH) without destroying the original lamellar structure.
The reactivity of the Si-OH groups allows chemical modifications of the vermiculite surface to be easily accomplished and these modifications can enhance the affinity of the vermiculite for different compounds.
Therefore, an effective method for the functionalization of vermiculite with Pb(II) removal can be developed by significantly increasing the specific surface area and Si-OH groups of vermiculite through acid activation and further modifying the vermiculite surface by introducing more functional amine groups through a series of organic reactions.
Therefore, the original vermiculite was first acid-treated. The acid-treated vermiculite was then organically modified by adding 3.0 g of vermiculite, 1.0 mL of water, and 3.0 mL of 3-methacryloxypropyltriethoxysilane in 100 mL of toluene and ultrasonication for 30 min to produce propyltriethoxysilane modified acid vermiculite.
The intermediate product polyacrylamide/vermiculite was prepared by dispersing the modified acid vermiculite in a three-necked flask and adding a certain amount of toluene solution and stirring for 10 min, followed by the reaction of acrylamide monomer and 2,2-azoisobutyronitrile initiator.
The intermediate product was dispersed in a flask containing distilled water and the pH of the mixture was adjusted with sodium hydroxide or hydrochloric acid solution.
Stirring at a certain temperature, formaldehyde and triethylenetetramine are added.
The resulting adsorbent material is g-polyacrylamide/vermiculite.
The adsorption efficiency of the modified vermiculite for Pb(II) was significantly higher than that of the original vermiculite at different pH values.
Furthermore, the selectivity of g-polyacrylamide/vermiculite for Pb(II) ions was better than that of Zn(II), Cd(II) and Cu(II) ions; the g-polyacrylamide/vermiculite adsorption isotherm curves were in good agreement with Langmuir adsorption isotherm curves.
The kinetic data were in good agreement with the proposed second-order kinetic data; the strong adsorption capacity of g-polyacrylamide/vermiculite may be due to the presence of Pb(II) and – NH2 group and the presence of strong covalent bonds between them.
It shows that the g-polyacrylamide/vermiculite adsorbent is promising for efficient adsorption of Pb(II).
Removal Of Heavy Metals Using 2:1 Laminated Chain Clay
With its unique fibrous crystal structure, attapulgite has a variety of excellent support properties from a performance point of view as an excellent colloid, catalyst, adsorbent material, and physico-chemical filler.
It has the advantages of a large specific surface area, good compatibility with microorganisms, and a strong ability to adsorb heavy metal ions.
When the molecular diameter of the substance adsorbed by the attapulgite is smaller than the pore diameter of the attapulgite, the adsorption mechanism is internal surface adsorption.
A study of the crystal structure of attapulgite confirms that its basic building blocks are composed of silicon-oxygen tetrahedral double chains parallel to the C-axis, with the individual chains linked by oxygen atoms, and the free oxygen atoms of the silicon-oxygen tetrahedra pointing, i.e. the angular tops of the silicon-oxygen tetrahedra, arranged in groups of four, alternating up and down.
The result is an interlocking arrangement of chain units and pore channels with a cross-section of approximately 0.38 nm * 0.63 nm. Thus, the pore channels of concavite are honeycomb-shaped in cross-section and have a large specific surface area.
When the molecular diameter of the substance adsorbed is larger than the pore diameter of the attapulgite, it cannot enter its pores and the adsorption mechanism is external surface adsorption. The adsorption is colloidal and ion-exchange due to the structural and surface charge of the gravel.
A new chitosan-polyvinyl alcohol/attapulgite nanocomposite adsorbent was prepared by glutaraldehyde cross-linking method. The nanocomposite showed excellent performance in the treatment of wastewater containing low concentrations of Cu(II) ions.
The adsorption capacity and mechanism of the chitosan-polyvinyl alcohol/attapulgite nanocomposite adsorbent for Cu(II) ions were strongly influenced by the pH of the solution.
The whole adsorption process fitted well with the proposed first-order kinetic model, but the initial 7 min adsorption process fitted better with the proposed first-order kinetic equation.
The adsorption process of Cu(II) ions on the nanocomposites was heat-absorbing, and this process could be better explained by the Freundlich model.
The preparation of chitosan poly(vinyl alcohol)/constantan nanocomposite was similar to that of chitosan poly(vinyl alcohol)/constantan.
The raw materials were cross-linked from concanavalin, poly(vinyl alcohol) and chitosan.
The mixture of concanavalin, poly(vinyl alcohol) and chitosan was subjected to strong stirring to obtain the desired homogeneous suspension time.
The appropriate amount of glutaraldehyde solution was added to obtain the gel.
After several freeze-thaw cycles, chitosan-polyvinyl alcohol/attapulgite nanocomposites can be obtained.
Contrast Analysis of Adsorption Properties of Various Clay Adsorbent Materials
| Absorbent | Adsorbent/Modified Clay | Qm/Removal rate | Adsorption mechanism |
| Sb(V) | m-elosite nanotube/Fe3O4 complexes | 98.9% | Formation of anionic complexes |
| Pb(II) | montmorillonite/carbon nanocomposite -COOH, -OH, and NH2 | 247.85 mg/g | Complexation of Pb(II) with functional groups |
| Hg(II) | chitosan-polyvinyl alcohol/bentonite composites | 460.18 mg/g | Void structure adsorption |
| Pb(II) | g-polyacrylamide/vermiculite composite | 219.4 mg/g | Formation of covalent bonds between NH2 and Pb(II) |
| Cu(II) | chitosan-polyvinyl alcohol/attapulgite nanocomposite | 99.61% | Chelation of NH2 groups with Cu(II) |
How to Improve The Adsorption Capacity
Although clay minerals and modified clay mineral adsorbent materials have achieved good results in small-scale laboratory application studies, there are still some difficulties in actual industrial and large-scale applications.
The development of efficient and practical heavy metal adsorption materials shall be carried out in the following areas:
- To enhance the design of structural and performance aspects of clay mineral and its derivative adsorbent materials, and design the synthesized materials with excellent structure, high specific surface area, and abundant surface groups to improve the adsorption performance of heavy metals.
- Optimization of the conditions for the preparation of clay minerals and their derivatives adsorbent materials and the adsorption conditions for the adsorption of heavy metal ions by using the orthogonal experimental method, response surface method, and neural network method, so as to determine the optimal parameters.
- Strengthen the basic theoretical research on the adsorption process of clay minerals and their derivative adsorbent materials, and explore the thermodynamic and kinetic characteristics of adsorption of heavy metals by clay minerals and their derivative adsorbent materials, as well as the mechanism of adsorption, so as to provide the theoretical basis and technical support for the design, synthesis, improvement, and application of new high-efficiency heavy metal adsorbent materials.
- Design and synthesize clay minerals and their derivative adsorbent materials with excellent selective and specific adsorption for different heavy metal ions to achieve selective adsorption and separation of different heavy metal ions.
- Through the in-depth research and development of clay minerals and their derivatives adsorbent materials, it is necessary to solve the common problems of adsorption method that the adsorbent materials cannot be reused, the adsorbed heavy metals are difficult to be recovered and have a short service life, and the adsorption process lacks designability and controllability, so as to promote the application of adsorption method in the treatment of heavy metal wastewater, and then realize the efficient treatment of heavy metal wastewater.
Conclusion:
Clay minerals have been used as excellent adsorbents due to the presence of different types of active sites on their surfaces, such as ion exchange sites, Lewis acid sites, and Bronsted sites.
Modified natural and synthetic clays such as kaolin, bentonite, montmorillonite, silver mica, and attapulgite are the most widely used clays for the preparation of high-performance nanocomposites.
The combination of modified clays with other fillers will be the subject of future research and a hot topic.
In the future, low-cost adsorbents, such as modified natural clays, show great promise.
The use of modified natural clays in industrial wastewater treatment is of great importance.
The future involves large-scale applications of natural and modified clays, which require significant technical resources.
In the area of chemically modified clays, although some attempts have been made, there are still opportunities for new chemical reactions and the development of new modified clays.
Hopefully the content will help you to secure the best solution on heavy metal removal. Welcome to send us your inquiries for further discussion.
