Environmental Applications of Diatomite Minerals in Removing Heavy

Jun 11, 2019 - There are a few reviews on removing heavy metals from water using ..... (10) (11)Du's team(76,77) prepared manganese-modified diatomite...
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Cite This: Ind. Eng. Chem. Res. 2019, 58, 11638−11652

Environmental Applications of Diatomite Minerals in Removing Heavy Metals from Water Yan Zhao,† Guangyan Tian,†,‡ Xinhui Duan,*,†,‡ Xiuhong Liang,†,‡ Junping Meng,†,‡ and Jinsheng Liang*,†,‡ †

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(Key Laboratory of Special Functional Materials for Ecological Environment and Information, Ministry of Education, Hebei University of Technology, Tianjin 300130, People’s Republic of China ‡ Institute of Power Source and Ecomaterials Science, Hebei University of Technology, Tianjin 300130, People’s Republic of China

ABSTRACT: Heavy metals released into the environment could accumulate in living organisms, thus causing various diseases and disorders. Diatomite, an eco-environmental functional material, has been proved to be a very promising and effective adsorbent in heavy metal-contaminated water treatment because of its high porosity, low density, high specific surface area, and surface silanol groups. This review (i) describes the characteristics, advantages, and limitations of diatomite; (ii) presents the capability of heavy metals adsorption onto diatomite-based functional materials; and (iii) discusses main adsorption mechanisms involved (ion-exchange, electrostatic interaction, chelation, and complexation) as well. Future studies about enhancing adsorption capacity and regeneration of diatomite-based functional materials are suggested.

1. INTRODUCTION Nowadays, water pollution has caused extensive concern with rapid development of industry and economy. For example, water polluted by heavy metals (e.g., As, Zn, Cu, Pb, Hg, Cr, Ni, and Cd) has brought serious problems including pollution of soil, plants, and animals.1−8 Many industries, such as batteries manufacturing,9 electrolysis, metal electroplating,10 metallurgy, mineral extraction, and ceramics need heavy metals to improve the performance of products. Meanwhile, a considerable amount of wastewater with heavy metals is generated during their production. In the 1950s in Japan, people eating cadmium rice were infected by neuralgia and ostealgia as the rice was irrigated by metallurgy wastewater. Most of the heavy metals are toxic, carcinogenic, nonbiodegradable, nonthermodegradable, and stable.11,12 Entering into water, heavy metals can be adsorbed by aquatic plants, animals, and crops directly after irrigation, eventually adsorbed by human beings through the food chains. Heavy metals are prone to bioaccumulation in food chains, which results in a long-term toxic effect. More seriously, water polluted by heavy metals might be accessible to drinking water someday.13 Therefore, removing heavy metals before discharge is necessary. © 2019 American Chemical Society

In recent years, several physical, chemical, and biological methods have been developed for removal of heavy metals from water.11,14−16 For example, chemical precipitation is the widely used, and the pH is the major parameter that significantly improves the removal efficiency of heavy metals. Membrane technology is a significant way to remove metal ions from water due to nonphase change, easy fabrication and high removal efficiency.11 Ultrafiltration, reverse osmosis, nanofiltration, and electrodialysis are common membrane processes. And the biological method has advantages of being eco-friendly, low-cost, and efficient at low level of contamination. However, chemical precipitation may cause secondary pollution as it requires a large amount of chemical reagent (lime, limestone) to reduce heavy metals to an acceptable level. Membrane technology needs relatively larger capital investment in devices. Large-scale application of the biological method is limited by parameters such as pH, temperature, and the content of oxygen Received: Revised: Accepted: Published: 11638

April 10, 2019 June 4, 2019 June 11, 2019 June 11, 2019 DOI: 10.1021/acs.iecr.9b01941 Ind. Eng. Chem. Res. 2019, 58, 11638−11652

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Industrial & Engineering Chemistry Research

Figure 1. Shapes of diatomite: (a) tub, (b) disk, (c) column, (d) oval.

2. STRUCTURE AND PROPERTIES OF DIATOMITE ECO-ENVIRONMENT MATERIAL Diatomite, also known as diatomaceous earth or kieselguhr, is a kind of siliceous sedimentary rock resulting from fossilizing of diatom frustules,42 which consists of porous siliceous algae skeletons with Si−O tetrahedral interconnection nets.43 It mainly contains amorphous hydrated or opaline silica (SiO2·nH2O) accompanied by amounts of clay minerals, silica sand, carbonate minerals, iron oxides, and organic matter.44 Diatomite has high porosity, low density, and high specific surface area because of the large amount of diatom frustule at microscale and pores at nanoscale. It is generally classified into two classes: centric (radial symmetry) and pennate (bilateral symmetry). As shown in Figure 1, there are a large number of pores distributed in diatomite and the size of the pores is approximately nanometer.45 Therefore, it is not surprising that diatomite has excellent adsorption performance. In addition, diatomite is also nontoxic with strong acid resistance and low thermal conductivity, and it is available at low cost. Consequently, diatomite has been used in various applications as adsorbents,38 filters,46−49 fillers,49 catalysts supports,50 and electrode material for energy conversion and storage.51 Diatomite is also classified as an eco-environmental functional material because of its excellent purification and compatibility with environment. The adsorption capacity of diatomite is strongly related to the net negative charge resulting from hydroxyl groups of silica. The surface and pores of diatomite have plenty of hydroxyl species in predominance (isolated hydroxyl groups, H-bonded hydroxyl groups and physically adsorbed water).52 Both the isolated and H-bonded silanols on the surface of diatomite are bonded with physically adsorbed water at room temperature (Figure 2). The hydroxyl silicon can dissociate into Si−O− and H+, resulting in a negatively charged surface (eq 1).

for a microorganism. Among the numerous techniques, adsorption is the most widely utilized because of its advantages of cost-effectiveness, easy operation, and high efficiency at trace quantities,17−19 but its problems such as regeneration, poor selectivity, and secondary pollution still need further improvement. In general, adsorbents commonly used are metals oxides,20,21 activated carbon,22−25 zeolites,13,26,27 polymers,28−30 biomaterials,31,32 clays,33−39 and others.40 Most commercial systems currently use activated carbon (AC) as adsorbent to purify metal-contaminated water because of its excellent adsorption capability. However, its widespread use is restricted because of the high cost and chelating agents required.12 Thus, many low-cost adsorbents including clays and waste materials from industry and agriculture have been proposed. Natural clay minerals are well-known and familiar to humankind since the earliest days of civilization. In the two most recent decades, clays have been highlighted for low cost, selectivity, and high efficiency in removal of heavy metals from water.33−39 Among clays, diatomite is recognized as an eco-environmental functional material in environmental remediation application for ion exchange because of its 3D structure, high permeability, low density, high specific surface area, and large amount of silanol groups. There are a few reviews on removing heavy metals from water using diatomite, and they have focused on feasibility applications and adsorption capacity of raw and modified diatomite.16,41 In our work, the mechanisms and specific modified method have been concluded and proposed for individual heavy metal species. The specific aim of this paper is to review and summarize the published studies about modified methods and removal mechanisms of heavy metals on diatomite-based adsorbent as a kind of environmental functional material. According to the properties (high porosity, low density, high specific surface area and surface silanol groups) of diatomite materials, the characteristics, advantages, and limitations are summarized and discussed. At the same time, various mechanisms involved are presented in the review. Furthermore, future development and uncertainties that exist in the preparation and application of diatomite-based functional material are identified.

≡S−OH ↔ ≡ S−O− + H+

(1)

As a result, diatomite has negative electrophoretic mobility, cation adsorption and exchange properties (e.g., metal cations or dyes). Diatomite is electropositive due to protonation when 11639

DOI: 10.1021/acs.iecr.9b01941 Ind. Eng. Chem. Res. 2019, 58, 11638−11652

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were mentioned by Sheng et al.61,62 They reported that adsorption of Pb(II) was strongly dependent on ionic strength (outer-sphere complexation or ion exchange) at low pH, whereas on inner-sphere complexation or precipitation at high pH. They found that the adsorption of Pb(II) dropped from 88 to 86% after five runs, which proved that diatomite could be recycled and reused. By this token, the diatomite has excellent compatibility to environment due to its high recycle efficiency. Thus, it has a great potential on application for cost-effective disposal of Pb(II)-polluted wastewater. The main composition of raw diatomite is SiO2·nH2O and the content can reach up to 80%. Isolated silanol groups (−SiOH), free dual silanol group (-−i(OH)2), and −Si−O-Si bridges with oxygen atoms are separated on the surface of silica (Figure 3).63 Also there is a great amount of silanol groups (−OH) on the surface of diatomite.

Figure 2. Hydroxyl structure on the diatomite surface: (a) isolated, (b) bonded with physically adsorbed water.

the pH is below pHPZC (pH where the net charge is zero). Therefore, diatomite may possess excellent adsorption capacity on cations and anions by changing the pH of solution.

3. ADSORPTION OF HEAVY METALS ON DIATOMITE-BASED FUNCTIONAL MATERIALS Diatomite is composed of amorphous silica and small proportions of alumina, iron, calcium, magnesium, sodium, potassium, titanium, and others.53 It is commonly used as an adsorbent for removing heavy metals during water treatment because of its unique physical and chemical properties.54 Generally, interactions between adsorbate and adsorbent occur at the surface water interface. In view of the limited amount of electronegative hydroxyl groups on the surface of diatomite, surface modification is necessary to remove variant heavy metals from aqueous water. Several methodologies of modification on diatomite are available such as calcination, acidification, and inorganic and organic functionalization. 3.1. Adsorption of Heavy Metals on Raw Diatomite. The application of diatomite in purifying heavy metalcontaminated water has been widely studied in recent years. For further application, it is fundamental to study the property and adsorption capacity of raw diatomite. Irani et al.55 found that the maximum adsorption capacity of raw diatomite from Iran for Pb(II) was 25 mg/g, using adsorbent/solution of 1/500. The adsorption data fitted to the Langmuir model by isothermal analysis. Š ljivić et al.56 used diatomite from Serbia for the removal of Cu(II) from aqueous solution. They found that the uptake of Cu(II) was near 100% (0.047 mmol/g) at pH >7, but decreased at lower pH. And the immobilization of Cu(II) on diatomite was favorable, feasible, and spontaneous, which is suitable for large-scale application. Caliskan et al.57 reported that the adsorption capacity of diatomite for Zn(II) was mainly related to the surface charge of diatomite. Higher surface negative charge would contribute to metal cations adsorption. Besides, the adsorption capacity of diatomite for Zn(II) was affected by pH. The removal of Zn(II) was mainly accomplished via adsorption at pH 1200 °C), cristobalite phase appeared, diatomaceous amorphous silica disappeared and diatom frustule mesoporous structure deconstructed. The study indicated that calcination at optimum temperature would contribute to grafting modifying agent for exposing more isolated silanols on the surface of diatomite. Other studies about the effect of acid (HCl, H2SO4, and aqua regia) and thermochemical (calcinated and treated with H2SO4) treatment on chemical composition and structure of diatomite was done by Mohamed and Alyosef.44,65 Both of them found that the fractional population of silica increased to >92 wt % from around 80 wt %, whereas alumina and alkaline compounds were removed after acid treatment. The structure of diatomite did not change a lot, but it does have positive influences on the pore structure after treatment. Moreover, Alyosel65 found that the specific surface area of diatomite, diatomite-aqua regia, diatomite-H2SO4, and diatomite-HCl were 4, 88, 116, and 124 m2/g, respectively. And he concluded that the increasing specific surface area was attributed to the distribution of particle size being transformed by acid corrosion of the particles. Ma et al.66 reported the surface active sites and surface adsorptive behaviors of diatomite modified by calcination and acid. They used disk diatomite containing a large number of pores as material and the morphologies were shown in Figure 5. They found that the amount of opening micro/nanopores increased, the morphologies of diatomite stayed the same and the specific surface area increased after calcination or acid modification.

≡SiOH F ≡ SiO− + H+

(4)

≡SiOH 2+ F ≡ SiOH + H+

(5)

≡FeOH F ≡ FeO− + H+

(6)

≡FeOH 2+ F ≡ FeOH + H+

(7)

≡AlOH F ≡ AlO− + H+

(8)

≡AlOH 2+ F ≡ AlOH + H+

(9)

On the basis of the results mentioned above, it is sure that calcination and acid modification would increase the specific surface area, the number of open pores, and hydroxyl groups of diatomite by removing adsorbed, coordination water and carbonate. So, it would enhance the adsorption capacity of diatomite for cations. Nevertheless, investigation on the effects of the quantity of hydroxyl groups, specific surface area, and pore distribution of diatomite on adsorption application is still a blank. 3.3. Adsorption of Heavy Metals on Inorganic Functionalization Diatomite. Although diatomite and calcinated and acid-modified diatomite possess excellent adsorptive performance on heavy metals, they are still less competitive compared with zeolite, kaolinite, and montmorillonite.39 Therefore, further improvement of the adsorption capacity of diatomite, such as changing surface characteristics is necessary. Thorough investigations on the use of surface manganese modified diatomite for the removal of heavy metals from aqueous solution have been carried out by researchers.9,57,67−72 Caliskan et al.57 successfully modified diatomite (from Turkey) by treatment with NaOH and MnCl2 solution. The batch experiments indicated that the surface of diatomite was charged by SiOH2+ and SiO− ionizations and the charges was depended on the pH. They found that active sites provided by MnO2 for adsorbing Zn(II) had higher negative charge than SiO2 (the main component of diatomite).70 They considered that the low adsorption rate was limited by intraparticle diffusion. Al-Degs et al.9,71,72 used the same method as Caliskan to modify diatomite and studied the amount of coated manganese. They found that the amount of manganese oxides loaded was depended on the nature of the surface, solution acidity and

Figure 5. SEM images of (a) diatomite, (b) calcinated diatomite, (c) calcinated-acidified diatomite. 11641

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Figure 6. Images of Mn/diatomite: (a) SEM, (b) EDX element mapping.

duration of treatment, and the manganese oxides was found to be δ-birnessite. The specific surface area of diatomite increased from 33 to 80 m2/g after modification by Sears’ method. They considered that the increasing specific surface area was attributed to more hydroxyl groups covering on the surface of modified diatomite. Al-Degs et al.71 also found that monolayer adsorption occurred in adsorption of Pb(II) on diatomite modified before and after, while multilayer adsorption occurred in Cu(II) and Cd(II). The adsorption capacity of manganese oxides modified diatomite was higher than diatomite, and it was mainly because of (1) increasing specific surface area of modified diatomite; (2) the higher negative charge of modified diatomite than the main component of diatomite (SiO2).73 They confirmed that the desorption of Mn(II) from modified diatomite was really low (0.02−0.04 ppm for 22 h). Further adsorption mechanism of modified diatomite for Pb(II) was studied by Li et al.70 They prepared MnO2/ diatomite by two steps: (1) forming carbon layers; (2) growing MnO2 to replace the carbon layer on the surface of diatomite. They found that a thin layer of MnO2 was formed uniformly on the surface of diatomite (Figure 6) and the specific surface area of diatomite was increased by forming metal oxide nanocrystals after modification.74 The adsorption capacity of MnO2/ diatomite for Pb(II) (56.843 mg/g) was higher than diatomite (8.5058 mg/g), and Pb(II) was removed by exchanging with H+ of hydroxyl groups (eqs 10 and 11). The Langmuir isotherm fitted well on adsorption of Pb(II) on diatomite, while Freundlich isotherm on MnO2/diatomite. They considered that MnO2 on the surface of diatomite formed more heterogeneous sites, which was consistent with Jiang.75 Oxide aggregation and ion diffusion on the surface of diatomite showed fast kinetics. −Mn − OH + Pb2 + → − Mn − O − Pb + H+

Figure 7. SEM images of (a, b) flowerlike MnO2 diatomite, (c) wirelike MnO2 diatomite, (d) sheetlike MnO2 diatomite.

specific surface area of flower, wire and sheet like MnO2 diatomite were 66.5, 144.2, and 49.5 m2/g, and much higher than diatomite (25 m2/g). Thus, wire like MnO2 diatomite possessing a large number of hydroxyl groups had the highest adsorption capacity on Cr(VI). The Cr(VI) and As(V) were adsorbed on MnO2 modified diatomite by interacting with the hydroxyl groups on the surface, forming Cr−O−Mn and As−O−Mn bonding. They considered that hydroxyl groups played an important role in Cr(VI) adsorption. In short, manganese-modified diatomite does improve adsorption capability on heavy metals in aqueous solution, especially for Pb(II). This is attributed to more hydroxyl groups on the surface and manganese oxides possessing higher negative charge. Electrostatic interaction and ion exchange are

(10)

−Mn − OH + Pb2 + → ( − Mn − O)2 − Pb + 2H+ (11) 76,77

Du’s team prepared manganese-modified diatomite with different morphologies (flower-, wire-, and sheetlike (Figure 7)) to remove Cr(VI) and As(V) from water. They found that the 11642

DOI: 10.1021/acs.iecr.9b01941 Ind. Eng. Chem. Res. 2019, 58, 11638−11652

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Industrial & Engineering Chemistry Research

Figure 8. SEM images of (a) diatomite, (b) diatomite-Fe, (c) magnification of diatomite-Fe.

pH 8.5. In addition, the removal of As(III) and As(V) was physical adsorption via electrostatic attraction in early stage, while chemisorption via covalent bonding in later stage. Chang et al.83 treated diatomite (from Jilin, China) in FeSO4·7H2O and KMnO4 solution (samples referred as FMBO-diatomite). After modification, the pore structure of diatomite still remained, the specific surface area increased by 36%, and a strong surface complex formed between Fe−Mn binary oxide and diatomite (Figure 10). They found that oxidizing

the main adsorption mechanism. And researches demonstrate that the adsorption type of modified diatomite is chemisorption or nonexchangeable.78 It is clear from the results that manganese oxide modified diatomite has a considerable potential for removal of Pb(II) from water. Diatomite modified by iron oxide for arsenic removal from water was another hot topic.79−82 Knoerr et al.80 obtained diatomite-Fe by treating diatomite in Fe(SO4)·7H2O solution to remove trace quantities of As(III) from water. The Fe compound on diatomite was mainly ferric oxyhydroxide (lepidocrocite (α-FeOOH) and goethite (γ-FeOOH)) with fibrillary shape (Figure 8). They considered that the increasing specific surface area (from 10 to 60 m2/g) was attributed to ferric oxide crystals (intergrain porosity) generated by oxyhydroxide phases. They found that the diatomite-Fe was achieved by two steps: (1) forming ferric oxyhydroxide; (2) oxidizing at air atmosphere (eq 12). The main arsenic species in water are H3AsO3 (pH ranging from 7.5 to 8.0) and H2AsO- 3 (pH ranging from 8.0 to 10.0). The authors inferred the adsorption process as eqs 13 and 14. 4Fe

2+

−

III

+ 5OH + O2 → 3Fe OOH + Fe

3+

Figure 10. Schematic bonding for Fe−Mn binary oxide with diatomite.

from As(III) to As(V) by manganese dioxide in Fe−Mn binary oxide promoted the adsorption efficiency. They also concluded that electrostatic attraction and specific adsorption were the main mechanisms for As(III) removal by FMBO-diatomite. As a result, As(V) was adsorbed by forming bidentatebinuclear bridging complexes between arsenate and iron oxyhydroxide, not depending on ion exchange but via surface complexation by forming an inner-surface complex through a process of ligand exchange involving −OH or −OH2 groups on As(III). The high specific surface area and highly hydrated structure of iron oxyhydroxide phases both facilitated the adsorption of As(V) and As(III). Moreover, it was reported that the iron modified diatomite was stable relatively in neutral and basic solution, which suggests that changing pH value to basic range could repeat the modification process and the adsorbent could be recycled. It gives a prospect future of ironhydroxide-modified diatomite in removal of arsenic from water. Knoerr et al.84 also reported a novel method to modified diatomite and application in Pb(II) removal from aqueous solution. They prepared deviline (CaCu4(SO4)2OH6·3H2O)/ diatomite by treatment in copper(II) sulfate solution and found that cupric species of deviline bonded strongly with hydrogen bonds on the surface silanol groups of diatomite. The adsorption capacity of modified diatomite for Pb(II) was 130 mg/g, which was five times as much as diatomite approximately. They found that Pb(II) was removed by OH atoms in octahedral geometry of deviline and formed caledonite by ligand-exchange (from Cu−OH to Cu−Pb). Furthermore, caledonite/diatomite was fairly stable, releasing no Pb(II) or Cu(II) in solution. High adsorption capacity and stability of deviline/diatomite give useful insights into Pb(II) adsorbing onto the diatomite. A significant contribution to the employment of modified diatomite for removal of Cu(II), Pb(II), Zn(II), and Cd(II)

+ H 2O (12)

≡FeOH + H3AsO3 → ≡ FeH 2AsO3 + H 2O ≡FeOH +

H 2AsO−3

→≡

FeHAsO−3

+ H 2O

(13) (14)

Du et al. also prepared α-Fe2O3-modified diatomite by precipitation−deposition and calcination. They found that nanowires α-Fe2O3 were deposited on the surface of diatomite and the pores of diatomite were still clear (Figure 9). Different 82

Figure 9. (a, b) SEM images of α-Fe2O3-modified diatomite.

from Knoerr’s work,80 Du found that the specific surface area of diatomite decreased to 30 from 140 m2/g because of the blocking of pores by α-Fe2O3 after the modification. However, the adsorption capacity of modified diatomite for arsenic enhanced. They found that the As(III) and As(V) generated the As−O−Fe bonding with α-Fe2O3 at pH 3.5 and exchanged with OH− of Fe(OH)3 on the surface of the α-Fe2O3 at 11643

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Figure 11. Removal mechanisms of (a) Cr(VI) and (b) Ni(II) on modified diatomite.

from water has been made by Liu et al.85 The diatomite was treated with Na2CO3 and CaCl2 solution. They found that formation of microspores CaCO3 was the main reason for increasing specific surface area of diatomite. As a result, the adsorption capacity of modified diatomite was higher than diatomite. They concluded that two inspects enhanced the adsorption capacity: (1) large specific surface area of modified diatomite; (2) formation of precipitation combining CaCO3 with metal ions. In addition, they reported that the adsorption capacity of modified diatomite on metal ions was as follows: Pb(II) > Cu(II) > Zn(II) > Cd(II), the orders was related to charges and radius of metal ions. 3.4. Adsorption of Heavy Metals on Organic Functionalization Diatomite. To further improve the adsorption capacity, organic functionalization is also considered to modify diatomite. Researchers have investigated grafting organics onto the surface of diatomite to increase the number of active sites for adsorption of heavy metals. Caner et al.86 investigated the chitosan-modified diatomite to remove Hg(II) speciation from aqueous solution. They prepared chitosan/diatomite by coating the diatomite with chitosan gel. They found that the specific surface area and total pore volume of diatomite increased by opening the closed pores (treatment in oxalic acid). The Hg(II) maximum monolayer adsorption capacity was 68.1 mg/g on raw diatomite and 116.2 mg/g on chitosan/diatomite with efficiency higher than 70%. And the maximum uptake of Hg(II) occurred at pH 5. At pH