The adsorption performance of aluminum onto powdered activated carbons: Effects of pH and carbon properties

Aluminum salt is widely used in water treatment plants to facilitate the coagulation of particles, colloids and dissolved organic matter, etc. (Wu et al. 2007; Xu et al. 2018), which always arises the issue of residual Al. Till now, no research has reported that Al has any positive health effects on humans, but some studies suggested that Al is neurotoxic and related to amyotrophic lateral sclerosis, osteomalacia and Alzheimer's disease (Srinivasan et al. 1999; Yokel 2000; Rahman et al. 2018). Thus, the Standards for drinking water quality (GB5749-2006) and the U.S. Environment Protection Agency demand the residual Al concentration in drinking water must be lower than 0.20 mg/L (Zhang et al. 2016) and 0.05–0.20 mg/L, respectively (Al-Muhtaseb et al. 2008). However, the coagulation-treated effluent inevitably contains a certain amount of residual Al, ranging from 0.04 to 2.70 mg/L Al after sand filtration (Miller et al. 1984; Shu-xuan et al. 2014), which exceeds the residual Al standard (Gao et al. 2019). He et al. (2021) found that Al close to 0.20 mg/L existed in drinking water pipes networks in both southern and northern regions of China. Therefore, further investigation is still needed regarding residual aluminum control.

Currently, approaches to control residual Al concentrations during coagulation-flocculation in treated water have been extensively investigated (Jiao et al. 2015; He et al. 2022b), including adjusting the coagulant type/dosage and hydraulic conditions (Yang et al. 2010a, 2010b). However, it has been proved that the control effect of residual Al by only optimizing coagulation conditions is limited. Once the construction of the water plant is completed, the optimization of the coagulation process stage only can be realized in a very narrow space for transformation (He et al. 2022b). Thus, other strategies need to be further investigated for the control of residual Al. Activated carbon (AC) is a widely used adsorbent due to its simplicity, cost-effectiveness and efficiency (Ali & Gupta 2006; Zhang et al. 2013; Al-Qahtani 2017), which has been proven to be an effective adsorption removal of a variety of metal ions including Al (Özyonar & Karagozoğlu 2012; Gupta et al. 2015). Previous studies by Basheer et al. (2021) have proved that Al can be effectively removed by the mesoporous palm active carbon fiber with a high specific surface area. Delgadillo-Velasco et al. (2021) have also suggested that Al ions can be removed by the iron-modified coconut shell-AC through electrostatic adsorption interactions. Ghazy & El-Morsy (2007) used the PAC prepared from olive stones modified with HNO₃ to remove Al with an initial concentration of 1.35–2.75 mg/L and found about 100% of Al was eliminated from water. Therefore, residual Al control by AC adsorption is feasible. Al-Muhtaseb et al. (2008) have studied the removal of Al from acid-aqueous solutions about date-pit and BDH AC, suggesting that the adsorption of Al on BDH AC was controlled by an ion exchange, whereas its adsorption on date-pit AC is controlled by physical or chemical adsorption mechanisms. However, to the best of our knowledge, no study focused on the effects of carbon properties on the Al adsorption performance under various pH.

Therefore, this study systematically investigated the Al (200 μg/L) adsorption performance under pH 5.5, 6.5 and 7.5 by nine PACs made from different source materials including coal, wood and coconut. Since the physical and chemical properties of PACs of the same material can differ, three groups of PACs were also selected for each material. The study also evaluated the correlation between the physical/chemical properties of carbon and Al removal performance. Moreover, the adsorption mechanism of Al was analyzed using XRD and FTIR to examine the adsorption behavior of Al on PAC.

Nine commercial PACs were purchased from Ningxia Guanghua Co. Ltd (China). The PACs were classified based on their source material and iodine value. The iodine value provided by the manufacturer and the abbreviation are listed in Tables 1. Therefore, three wood-based PACs, three coal-based PACs and three coconut-based PACs with different iodine values (around 800, 900 and 1,000) were selected for the adsorption experiments to compare the adsorption behavior of different materials of PAC on Al. The functional groups, pore size and pore volume of PAC are shown in Tables S1 and S2. Before the adsorption experiment, all the PAC samples were washed with deionized water until the eluant was neutral (pH 7.00 ± 0.50). Then the PAC samples were dried at 110 °C for 24 h before use. Aluminum chloride (A.R., Macklin Shanghai, China) was prepared weekly as a 1 g/L solution and stored at 4 °C. All solutions were prepared using ultra-purified water (resistivity >18.2 MΩ·cm) obtained from a Millipore Milli-Q water purification system.

Adsorption kinetics experiments were conducted in 250 mL glass bottles. A magnetic stirrer (Model 84-1A, China) was used to ensure complete mixing at a speed of 200 r/min at room temperature. The initial concentration was diluted with 1 g/L Al stock solution to a concentration of 200 μg/L as Al, and NaCl (0.5 mM) was added to provide a suitable ionic strength (Chen et al. 2021). Dilute NaOH and HCl solutions were used to adjust the pH to 5.5, 6.5 and 7.5. PAC (10 mg/L) was added at the start of the adsorption experiment, and 5 mL evenly mixed samples were collected at different intervals (0, 15, 30, 60, 90, 120, 180, 240, 360 min) and filtered through a 0.45-μm polyether sulfone filter. Inductively coupled plasma mass spectrometry (ICP-MS, Agilent-7850, American) was used to quantify the dissolved Al after being acidized with HNO₃. Different pH values Al species were calculated using the Visual MINTEQ model (Version 3.1) (Wang et al. 2019; Zhang et al. 2021).

The pH value at the point of zero charges (pH_pzc) was determined by the previous method (Lee et al. 2011; Qian & Chen 2013). The Boehm titration method was used for the quantification of the basic, acidic sites, phenolic hydroxyl group, carboxyl group and loctons group (Noh & Schwarz 1990; Biniak et al. 1997). The surface area and pore size distribution of the PACs were measured using an AUTSORB (Quantachrome, USA) computer-controlled surface analyzer. X-ray diffraction (XRD) patterns were obtained with a Smart Lab SEX-ray diffractometer (Japan) equipped with a Cu Kα radiation source. The diffractometer was operated at 40 kV and 30 mA and data were collected over the 2θ range 10–80° using Cu Kα radiation with a scan speed of 10°/min. The infrared spectrometer (FTIR) spectra were recorded in the 4,000–400 cm⁻¹ region by a Nicolet FTIR spectrophotometer (Perkin Elmer Frontier, American) with a resolution of 0.2 cm⁻¹.

IMB SPSS 26 was used for statistical analysis. The correlations between adsorption rate and PAC physical/chemical properties were investigated by Spearman's correlation analysis. P-value of <0.05 was considered as a statistically significant correlation.

The adsorption parameters, including the pseudo-first-order rate constant k₁, pseudo-second-order rate constant k₂, experimental equilibrium adsorption amount q_{e, exp} and theoretical equilibrium adsorption amount q_{e, cal} for the Al and regression coefficients (R²) are presented in Tables 2 and 3. It should be noted that the R² values for the pseudo-second-order kinetic model (R² > 0.95) at different pH were all greater than those of the pseudo-first-order kinetic model. In addition, the q_{e, cal} of the pseudo-second-order fit was closer to the experimental q_{e, exp}, which also indicated that the Al adsorption process fitted better with pseudo-second-order kinetic model. The Al adsorption by the PAC was dominated by the physical-chemisorption rather than the transport process.

The pseudo-second-order kinetic constant was applied to analyze the effects of pH on the Al adsorption by nine PACs, as shown in Table 3. At pH 5.5 and 6.5, the adsorption rate of P-B2 was the highest (0.0633 and 0.0149 mg/μg·min, respectively). At pH 7.5, the adsorption rate of P-B1 (0.0215 mg/μg·min) was greater than that of other PACs. Moreover, it could be noted that the adsorption kinetics was not consistent with the iodine value. For example, for coal-based PAC at pH 7.5, P-A3 with the lowest iodine value achieved the fastest adsorption rate. Generally higher iodine values usually exhibit better adsorption efficiency (Kim et al. 2001), the Al adsorption performance could not be evaluated by the iodine value.

It could be noted that at pH 6.5, the Al adsorption performance was positively correlated with the mesopore surface area/volume and the total pore volume. Different from the situation at pH 5.5, Al mainly presented in the form of ⁠, Al(OH)₃, and a small amount of at pH 6.5, as shown in Fig. S1, which are easy to precipitate and form aluminum hydroxide colloid aggregates. Thus, the adsorption of aluminum hydroxide colloid onto PAC should be mainly through surface deposition. Yu et al. (2018) measured the average particle size of colloidal nanoparticles of aluminum hydroxide initial agitation to be 48 nm. It could be inferred that under the condition of pH 6.5, aluminum hydroxide colloids’ adsorption on AC pore was mainly due to surface deposition, especially the mesoporous (2–50 nm).

For pH 7.5, the BET-specific surface area, acid group and phenolic hydroxyl group were significantly correlated to the Al adsorption kinetic. In addition, the BET-specific surface area of P-B1 was 1,630.17 m²/g (in Table S2), and its adsorption rate was much higher than that of the other eight PACs. It is worth noting that the addition of PAC caused a dramatic drop in the solution pH to around 6.5 within 15 min, as shown in Fig. S3c). This drop may be due to the hydrolysis of the Al on the acidic micro surface of the PAC. Although the bulk solution was slightly alkaline, the acidic group on the PAC surface might be able to induce the decline of the surrounding micro-area, and thus lead to the hydrolysis of Al. The pKa of the carboxyl group is around 4.23 (Robinson 1967; Wu et al. 2011; Smith & Rodrigues 2015), which could cause a decrease in pH in the micro-area. When Al hydrolyzed to aluminum hydroxide colloids, its adsorption behavior was similar to that at pH 6.5, which was mainly attributed to the complexion. Thus, at pH 7.5, the Al adsorption performance was not only related to the acidic and phenolic hydroxyl group, but also correlated with the surface area.

When pH was 6.5, XRD spectra indicated that there were Bayerite and Gibbsite on the surface of PAC, which might be due to the precipitation of Al(OH)₃ colloids. In FTIR spectra, the Al–OH bond was observed at 576 cm⁻¹ and was the weakest among the three pH and it could be inferred that the complexion effect of the Al and carbon surface group did not play a significant role.

The XRD pattern of pH 7.5 was similar to that at pH 6.5, and it was an obvious peak of Bayerite and Gibbsite, suggesting the deposition of Al colloids on the carbon surface. In FTIR, the stretching band of the –OH at 1,161 cm⁻¹ disappeared and the band at 610 cm⁻¹ related to the bending vibration of Al–OH. It could also confirm the surface complexation of Al and complexation with the phenolic hydroxyl group on the carbon surface.

The XRD and FTIR spectra studies further confirmed the variability of Al and reactions at different initial pH conditions. The adsorption of Al on PAC in addition to surface complexation and precipitation also involved the combination of Al and the minerals on PAC. This verification highlights the differences in the adsorption of residual aluminum under different pH conditions.

In this study, the Al adsorption behavior on nine kinds of commercially available PACs was investigated, and the adsorption mechanism of Al at different pH was compared. The obtained results can be summarized as follows:

• (1)

  The Al adsorption performance by PAC at pH 5.5 is better than that at pH 6.5 and 7.5.

• (2)

  Under the condition of pH 5.5, Al was adsorbed by complexation, and surface acidic functional group and carboxyl group are the main influencing factors.

• (3)

  Under the condition of pH 6.5, deposition was the main adsorption mechanism, thus mesopore volume, especially in the mesopore volume, is the main influencing factor.

• (4)

  Under the condition of pH 7.5, the surface complexion and deposition were both dominated in the adsorption process, thus the specific surface area and surface functional group effects on aluminum adsorption.

Therefore, this study well understands the adsorption behavior of residual Al onto AC at various pH and could give a sufficient guidance for carbon selection in residual aluminum control.

This study was supported by a grant from the National Natural Science Foundation of China (No.52270013).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.