Surface Functionalized Attapulgite for Mercury and PFAS Adsorption

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 [1]. Mercury in refinery hydrocarbon streams can cause catalyst  poisoning, corrosion, and safety issues. Common commercially available mercury  removal technologies include activated carbon adsorption [2], sulfur-impregnated  activated carbon adsorption [3], separation by microemulsion liquid membranes [4],  ion exchange [5] and colloid precipitation [6]. 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) [15]. 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 [23]. 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 [24]. 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. 


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. 

Sample  d10 






Pore Volume  (mL/g) Porosity  (%) Surface Area  (m2/g)


3.89  13.49  33.93  1.1935  68  125

functionalized  attapulgite

5.15  16.01  37.48  1.3046  72  124

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
Perfluorohexadecanoic acid 85.98 1.78 97.9
Perfluorooctadecanoic acid 87.06 2.06 97.6
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
Perfluorooctanesulfonamide (FOSA) 79.34 3.06 96.1
N-ethylperfluoro-1-octanesulfonamide 73.66 2.44 96.7
NMeFOSA 83.42 2.94 96.5
N-methylperfluorooctanesulfonamidoacetic acid (NMeFOSAA) 76.96 1.37 98.2
N-ethylperfluorooctanesulfonamidoacetic acid (NEtFOSAA) 74.24 1.59 97.9
2-(N-methylperfluoro-1-octanesulfonamido) ethanol 78.7 2.64 96.6
2-(N-ethylperfluoro-1-octanesulfonamido) ethanol 80.64 2.09 97.4
4:2 FTS 69.88 17.3 75.2
6:2 FTS 69.8 2.7 96.1
8:2 FTS 81.52 1.8 97.8
10:2 FTS 76.78 1.56 98
HFPO-DA 86.54 18.3 78.9
9-Chlorohexadecafluoro-3-oxanonane-1-sulfonic acid 76.78 2.31 97
11-Chloroeicosafluoro-3-oxaundecane-1-sulfonic acid 71.4 1.29 98.2
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.  


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