Author: Bo Wang1
1Active Minerals International, LLC
Proceeding for the WFC13, Oct 6-9, 2022, San Diego, CA, USA
A high-performance adsorbent was developed based on natural attapulgite for mercury and PFAS adsorption. The surface of attapulgite was functionalized to enhance mercury and PFAS adsorption. Test results indicated that this new attapulgite adsorbent was highly effective in removing mercury and PFAS in aqueous solutions. The surface mercury affinity group and PFAS affinity group combined with intrinsic high surface area and nano pore structure contributes to high mercury and PFAS adsorption. The new attapulgite mercury and PFAS adsorbent could be used for various mercury and PFAS removal applications. For example, it could be combined with a filter aid in a pressure filter for continuous mercury and PFAS removal process.
Mercury contaminants can be found in oil, liquified natural gas and wastewater. Such toxic pollutants in industrial and municipal wastewaters are harmful to human health and the environment . Mercury in refinery hydrocarbon streams can cause catalyst poisoning, corrosion, and safety issues. Common commercially available mercury removal technologies include activated carbon adsorption , sulfur-impregnated activated carbon adsorption , separation by microemulsion liquid membranes , ion exchange  and colloid precipitation . The slow kinetics, poor selectivity for mercury and low mercury loading capacity of these technologies make the mercury removal process less efficient and expensive due to the high cost of disposing large volumes of waste.
Per- and polyfluoroalkyl substances (PFAS) are a group of man-made chemicals used in industry and consumer products and are known to be relatively stable chemicals with long-term persistence [7 – 8]. As emerging contaminates, PFAS are attracting increasing attention worldwide as PFAS pollutants in industrial and municipal wastewaters can find their way into groundwater, water bodies and other the water environments, posing health risk to humans and wildlife [9 – 10]. Remediation of PFAS contaminated water tends to be very challenging. Activated carbon adsorption is the most common commercially available PFAS removal technology [11 – 12]. Other treatment technologies include anion exchange and high-pressure membranes such as nanofiltration and reverse osmosis [13 – 14]. Activated carbon is a less effective while anion exchange resin and high-pressure membranes technologies are typically more expensive.
Attapulgite is a natural occurring magnesium aluminosilicate clay mineral with a chain crystal lattice that is structurally different from other clays such as montmorillonite or bentonite. Namely, the tetrahedral sheets of attapulgite are divided into ribbons by inversion because adjacent bands of tetrahedra within one tetrahedral sheet point in opposite directions rather than in one direction thus creating a structure of ribbons of 2:1 layers joined at their edges, and the octahedral sheets are continuous in two dimensions only with a general chemical formula of (Mg,Al)2Si4O10(OH)·4(H2O) . This structure leads to the formation of channels that extend throughout the longitudinal direction of the one-dimensional (1D) nanorods [16 – 19]. The intrinsic pores and channels in the attapulgite structure coupled with high surface area makes attapulgite suitable for adsorption applications [20 – 22]. In this study, we report the performance of the economically attractive and highly effective surface functionalized attapulgite adsorbent for mercury and PFAS adsorption in aqueous solutions.
2.1 Preparation of surface functionalized attapulgite adsorbents
A natural attapulgite from Climax in Georgia was used as the feed material. Fig.1a and Fig. 1b show the typical scanning electron micrographs of the one-dimensional (1D) nanorod-shaped natural attapulgite particles. As illustrated in Scheme 1, surface of this attapulgite material was functionalized with mercury affinity group or PFAS affinity group using a patent pending proprietary process.
Fig.1a. SEM image of natural attapulgite at x25K magnification
Fig.1b. SEM image of natural attapulgite at x200K magnification
Scheme 1. Schematic illustration of attapulgite surface functionalization process
2.2 Sorbent characterization
Attapulgite particle size was measured using a laser particle size analyzer. Surface area was measured by the nitrogen adsorption method of the BET (Brunauer– Emmett–Teller) method. Pore volume and pore size distribution was determined by mercury porosimetry.
2.3 Adsorption test
Influent mercury solutions were prepared by spiking 1000 ppm mercury standard solution into deionized (DI) water to make mercury concentrations around 100 ppb to 10 ppm. Mercury concentrations were measured using Inductively Coupled Plasma (ICP) before and after the adsorption reaction. For the mercury adsorption test, adsorbent was mixed with mercury solution at 0.125 to 8 g/L in a glass flask for 5 to 30 minutes on a magnetic stirrer plate at room temperature. Mercury removal efficiency was calculated as follows:
(𝐼𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛−𝑅𝑒𝑠𝑖𝑑𝑢𝑎𝑙 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛)×100 / 𝐼𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
Influent PFAS solutions were prepared by spiking 20 parts per billion (ppb) PFAS standard solution containing 35 PFAS substances into deionized (DI) water to make PFAS concentrations around 80 parts per trillion (ppt). Actual PFAS concentrations were measured using liquid chromatography-mass spectrometry (LC-MS) according to standard EPA Method 533 . For the PFAS adsorption test, 300 mg of adsorbent was mixed with 250 ml of the prepared PFA solution on a shaker for 18 hours at room temperature. After the adsorption test, the adsorbent was separated from the liquid using EPA standard solid-phase extraction (SPE) method SW846 . PFAS concentration of the filtrate was measured using the LC-MC based on EPA Method 533. PFAS removal efficiency was calculated using the same equation for mercury removal efficiency.
RESULTS, INTERPRETATIONS AND DISCUSSIONS
3.1 Mercury adsorbent
Fig. 2 shows the pore size distribution of natural attapulgite and surface functionalized attapulgite. The bimodal distribution in both samples indicates small intrinsic pores around 15 nm inside attapulgite nanorods, and large inter pores around 5 microns between attapulgite nanorods.
Fig. 2 Pore size distribution of natural attapulgite and surface functionalized attapulgite
As shown in Table 1, surface functionalization process slightly increases pore volume, porosity and particle size. There is minimum impact by surface functionalization process on attapulgite surface area.
Table 1. Particle Size of Surface Functionalized Attapulgite.
|Pore Volume (mL/g)||Porosity (%)||Surface Area (m2/g)|
Fig. 3 demonstrates that high mercury removal can be achieved at low adsorbent usage of surface functionalized attapulgite. For comparison, higher adsorbent usage is need for commercial activated carbon mercury adsorbent to achieve equivalent removal. For example, about 95% removal efficiency is achieved at 0.5 g/L adsorbent loading for the surface functionalized attapulgite. For comparison, 8 g/L loading of commercial activated carbon mercury adsorbent is needed to achieve similar 92% removal efficiency, i.e., 16 times higher than surface functionalized attapulgite.
Fig. 4 shows that surface functionalized attapulgite adsorbs significantly more amount of mercury than commercial activated carbon mercury adsorbent. This indicates surface functionalized attapulgite is more cost-effective: it not only reduces adsorbent material costs but also reduces spent material disposal cost.
Fig. 3 Mercury removal efficiency versus adsorbent loading for surface functionalized attapulgite and commercial activated carbon mercury adsorbent.
Fig. 4 Mercury adsorption isotherm for surface functionalized attapulgite and commercial activated carbon mercury adsorbent
Fig. 5 shows that surface functionalized attapulgite is fast acting to reduce mercury adsorption process time compared to commercial activated carbon mercury adsorbent.
Fig. 5 Impact of contact time on mercury removal efficiency.
3.2 PFAS adsorbent
Table 2 shows that 92% total PFAS removal efficiency in 35 PFAS starting solution is achieved with surface functionalized attapulgite PFAS adsorbent. As shown in Fig. 6, over 90%, 95% and 97% removal efficiency is achieved for 30, 29 and 17 PFAS substances, respectively. Fig. 7 shows that over 96% removal efficiency is achieved for the most common PFAS substances such as perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorohexanesulfonic acid (PFHxS) and perfluorononanoic acid (PFNA).
Table 2 PFAS adsorption using surface functionalized attapulgite as adsorbent
|Fluorinated Alkyl Substances (PFAS)||Influent (ppt)||Effluent (ppt)||
Removal Efficiency (%)
|Perfluorobutanoic acid (PFBA)||79.88||58.7||26.5|
|Perfluoropentanoic acid (PFPeA)||78.34||35.1||55.2|
|Perfluorohexanoic acid (PFHxA)||72.44||13.4||81.5|
|Perfluoroheptanoic acid (PFHpA)||83.94||5.1||93.9|
|Perfluorooctanoic acid (PFOA)||80.08||2.53||96.8|
|Perfluorononanoic acid (PFNA)||86.08||2||97.7|
|Perfluorodecanoic acid (PFDA)||76.68||3.33||95.7|
|Perfluoroundecanoic acid (PFUnA)||87.06||3.01||96.5|
|Perfluorododecanoic acid (PFDoA)||83.34||2.92||96.5|
|Perfluorotridecanoic acid (PFTriA)||72.72||1.91||97.4|
|Perfluorotetradecanoic acid (PFTeA)||78.98||2.04||97.4|
|Perfluorobutanesulfonic acid (PFBS)||80.54||2.35||97.1|
|Perfluoropentanesulfonic acid (PFPeS)||89.84||1.05||98.8|
|Perfluorohexanesulfonic acid (PFHxS)||77.7||1.42||98.2|
|Perfluorooctanesulfonic acid (PFOS)||73.46||1.77||97.6|
|Perfluorononanesulfonic acid (PFNS)||67.46||2.23||96.7|
|Perfluorodecanesulfonic acid (PFDS)||75.54||0.955||98.7|
|Perfluorododecanesulfonic acid (PFDoS)||59.64||0.515||99.1|
|N-methylperfluorooctanesulfonamidoacetic acid (NMeFOSAA)||76.96||1.37||98.2|
|N-ethylperfluorooctanesulfonamidoacetic acid (NEtFOSAA)||74.24||1.59||97.9|
|Total PFAS concentration (ng/L)||208.08||92.4|
Fig. 6 Number of PFAS substances with removal efficiency over 90%, 95% and 97%
Fig. 7 Total PFAS removal efficiency and removal efficiency for most common PFAS substances.
Natural attapulgite can be surface functionalized with mercury affinity group or PFAS affinity group for mercury and PFAS removal applications. Compared to other mercury removal adsorbent, surface functionalized attapulgite mercury adsorbent can achieve high mercury removal with lower adsorbent usage. Surface functionalized attapulgite mercury adsorbent is also fast acting to reduce mercury adsorption process time. Surface functionalized attapulgite PFAS adsorbent can effectively remove majority of PFAS substances in mixed PFAS solutions. High removal efficiency on the most common PFAS substances such as PFOA, PFOS, PFHxS, PFNA can be achieved with surface functionalized attapulgite PFAS adsorbent.
This work is supported by Active Minerals International, LLC.
 Beckers, F., Rinklebe, J., Cycling of mercury in the environment: Sources, fate, and human health implications: A review. Crit. Rev. Environ. Sci. Technol., 2017, 47, 693–794.
 Perrich, J.R. Activated carbon adsorption for wastewater treatment. Boca Raton, Fla; CRC Press: Chicago, 2018.
 Wang, J.; Deng, B.L.; Wang, X.R.; Zheng, J.Z. Adsorption of aqueous Hg (II) by sulfur-impregnated activated carbon. Environ. Eng. Sci., 2009, 26, 1693-1699.  Urgun-Demirtas, M.; Benda, P.L.; Gillenwater, P.S.; Negri, M.C.; Xiong, H. Snyder, S.W. Achieving very low mercury levels in refinery wastewater by membrane filtration. J. Hazard. Mater., 2012, 215-216, 98-107.
 Ghodbane, I.; Hamdaoui, O. Removal of mercury(II) from aqueous media using eucalyptus bark: Kinetic and equilibrium studies. J. Hazard. Mater., 2008, 160(2- 3), 301-309.
 Okoronkwo, N.E.; Igwe, J.C.; Okoronkwo, I.J. Environmental impacts of mercury and its detoxification from aqueous solutions. Afr. J. Biotechnol., 2007, 6, 335- 340.
 Prevedouros, K., I.T. Cousins, R.C. Buck, and S.H. Korzeniowski. Sources, fate and transport of perfluorocarboxylates. Environmental Science and Technology, 2006, 40: 32–44.
 Wang, Z., I.T. Cousins, M. Scheringer, R.C. Buck, and K. Hungerbu¨hler. Global emission inventories for C4–C14 perfluoroalkyl carboxylic acid (PFCA) homologues from 1951 to 2030, part I: Production and emissions from quantifiable sources. Environment International, 2014, 70: 62–75.
 Houde, M., A.O. De Silva, D.C.G. Muir, and R.J. Letcher. Monitoring of perfluorinated compounds in aquatic biota: An updated review. Environmental Science and Technology, 2011, 45: 7962–7973.
 Post, G.B., P.D. Cohn, and K.R. Cooper. Perfluorooctanoic acid (PFOA), an emerging drinking water contaminant: A critical review of recent literature. Environmental Research, 2012, 116: 93–117.
 Hawley, E.L., Pancras, T., Burdick, J., Remediation technologies for perfluorinated compounds (PFCs), including perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), 2012.
 Ochoa-Herrera, V., Sierra-Alvarez, R., Somogyi, A., Jacobsen, N.E., Wysocki, V.H., Field, J.A., Reductive defluorination of perfluorooctane sulfonate. Environ. Sci. Technol., 2008, 42, 3260e3264.
 Dickenson, E., & Higgins, C. Treatment mitigation strategies for poly-and Perfluoroalkyl substances. Water Research Foundation Web Report, 2016, 4322.
 Van Der Bruggen, B., Vandecasteele, C., Van Gestel, T., Doyen, W., & Leysen, R. A review of pressure-driven membrane processes in wastewater treatment and drinking water production. Environmental Progress, 2003, 22, 46–56.
 W.F. Bradley, The structural scheme of attapulgite, Am. Mineral., 1940, 25, 405– 410.
 Frost, R. L., Locos, O. B., Ruan, H., & Kloprogge, J. T. Near-infrared and mid infrared spectroscopic study of sepiolites and palygorskites. Vibrational Spectroscopy, 2001, 27(1), 1-13.
 Gionis V., Kacandes G.H., Kastritis I.D. and Chryssikos, G.D. On the structure of palygorskite by mid- and near-infrared spectroscopy, American Mineralogist, 2006, 91, 1125-1133.
 López-Galindo, A., Viseras, C., & Cerezo, P. Compositional, technical and safety specifications of clays to be used as pharmaceutical and cosmetic products. Applied Clay Science, 2007, 36(1-3), 51-63.
 Murray, H. H. Traditional and new applications for kaolin, smectite and palygorskite: A general overview. Applied Clay Science, 2000, 17(5-6), 207-221.  Tong, D. S.; Wu, C. W.; Adebajo, M. O.; Jin, G. C.; Yu, W. H. Ji, S. F.; Zhou, C. H. Adsorption of methylene blue from aqueous solution onto porous cellulose derived carbon/montmorillonite nanocomposites. Appl. Clay Sci., 2018, 161, 256−264.
 Frini-Srasra, N.; Srasra, E. Acid treatment of south Tunisian palygorskite: removal of Cd(II) from aqueous and phosphoric acid solutions. Desalination 2010, 250, 26−34.
 Li, N., Fang, J., Jiang, P., Li, C., Kang, H., & Wang, W. Adsorption Properties and Mechanism of Attapulgite to Graphene Oxide in Aqueous Solution. International journal of environmental research and public health, 2022, 19(5), 2793.
 EPA Method 533: Determination of Per- and Polyfluoroalkyl Substances in Drinking Water by Isotope Dilution Anion Exchange Solid Phase Extraction and Liquid Chromatography/Tandem Mass Spectrometry, December 2019.  EPA Method 3535A (SW-846): Solid-Phase Extraction (SPE), February 2007.