**Two New 4-Hydroxy-2-pyridone Alkaloids with Antimicrobial and Cytotoxic Activities from** *Arthrinium* **sp. GZWMJZ-606 Endophytic with** *Houttuynia cordata* **Thunb**

**Ying Yin 1,2,3,†, Dongyang Wang 1,2,†, Dan Wu 1,2, Wenwen He 1,2, Mingxing Zuo 1,2, Weiming Zhu 1,4, Yanchao Xu 1,3,\* and Liping Wang 1,2,3,\***

**Abstract:** Two new 4-hydroxy-2-pyridone alkaloids furanpydone A and B (**1** and **2**), along with two known compounds N-hydroxyapiosporamide (**3**) and apiosporamide (**4**) were isolated from the endophytic fungus *Arthrinium* sp. GZWMJZ-606 in *Houttuynia cordata* Thunb. Furanpydone A and B had unusual 5-(7-oxabicyclo[2.2.1]heptane)-4-hydroxy-2-pyridone skeleton. Their structures including absolute configurations were determined on the basis of spectroscopic analysis, as well as the *X*-ray diffraction experiment. Compound **1** showed inhibitory activity against ten cancer cell lines (MKN-45, HCT116, K562, A549, DU145, SF126, A-375, 786O, 5637, and PATU8988T) with IC50 values from 4.35 to 9.72 μM. Compounds **1**, **3** and **4** showed moderate inhibitory effects against four Gram-positive strains (*Staphylococcus aureus*, methicillin-resistant *S. aureus*, *Bacillus Subtilis*, *Clostridium perfringens*) and one Gram-negative strain (*Ralstonia solanacarum*) with MIC values from 1.56 to 25 μM. However, compounds **1**–**4** showed no obvious inhibitory activity against two Gram-negative bacteria (*Escherichia coli* and *Pseudomonas aeruginosa*) and two pathogenic fungi (*Candida albicans* and *Candida glabrata*) at 50 μM. These results show that compounds **1**–**4** are expected to be developed as lead compounds for antibacterial or anti-tumor drugs.

**Keywords:** pyridone alkaloids; *Arthrinium* sp.; endophytic fungus; antibacterial; cytotoxicity

#### **1. Introduction**

Since ricinine [1] and ilicicolin H [2] were found in the early 1970s, a series of 4-hydroxy-2-pyridinone alkaloids with diverse structures were reported [3–6]. These alkaloids were mainly isolated from plants and fungi and had good biological activity [7]. According to the position and type of substituents, C-3 was often replaced by alkanes (e.g., septoriamycin from *Septoria pistaciarum* [8]) or terpenes (e.g., tenelin from *Beauveria tenella* and *Beauveria bassiana* [9]), and C-5 was often replaced by phenyl (e.g., sambutoxin from *Fusarium sambucinum* [10]) or cyclohexyl (e.g., torrubiellone A-B from *Torrubiella* sp. [11]). There were also a small number of derivatives whose C-6 was replaced by alkanes (e.g., pyridomacrolidin from *Beauveria basiana* [12]). These kind of compounds usually have anti-inflammatory, antibacterial, cytotoxicity, antimalarial, antiviral, insecticidal, antioxidant, anti-fibrosis, neuroprotection, inhibition of protein tyrosine kinase, and so on [13–16], which have attracted widespread attention.

In the past few years, there were some new 4-hydroxy-2-pyridones discovered from fungi, such as (+)-didymellamide B, (±)-didymellamide E, (+)-*N*-hydroxyapiosporamide,

**Citation:** Yin, Y.; Wang, D.; Wu, D.; He, W.; Zuo, M.; Zhu, W.; Xu, Y.; Wang, L. Two New 4-Hydroxy-2-pyridone Alkaloids with Antimicrobial and Cytotoxic Activities from *Arthrinium* sp. GZWMJZ-606 Endophytic with *Houttuynia cordata* Thunb. *Molecules* **2023**, *28*, 2192. https://doi.org/ 10.3390/molecules28052192

Academic Editor: Giovanni Ribaudo

Received: 2 February 2023 Revised: 22 February 2023 Accepted: 22 February 2023 Published: 27 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and didymellamides F–H which were isolated from *Coniochaeta cephalothecoides* [17], and arthpyrones A and B with novel oxabicyclo[3.3.1]-nonane ring which were isolated from *Arthrinium arundinis* ZSDS1-F3 [18]. Three new 4-hydroxy-2-pyridone alkaloids citridones E–G with antibacterial activity were isolated from the endophytic fungus *Penicillium sumatrense* GZWMJZ-313 in our previous studies [19]. In order to obtain more compounds of this type from endophytic fungus, *Arthrinium* sp. GZWMJZ-606 was isolated from *Houttuynia cordata* Thunb. Further chemical investigation of this fungal strain led to the isolation of two new 4-hydroxy-2-pyridone derivatives (**1** and **2**) which we named furaprazone A and B (Figure 1), along with the known *N*-hydroxyapiosporamide (**3**) [18,20] and apiosporamide (**4**) [21]. Compounds **1** and **2** were the first reported compounds with 5-(7-oxabicyclo[2.2.1]heptane)-4-hydroxy-2-pyridone skeleton. Compared with the previously reported 1,2-epoxyhexane [17,21], 2-oxobicyclo [3.3.1] nonane [18] or benzene [22], 7-oxadicyclo [2.2.1] heptane can improve some biological activities [23–25]. Herein, the isolation, structure elucidation, the antimicrobial and cytotoxic activity of these compounds are described.

**Figure 1.** Structures of compounds **1**–**4**.

#### **2. Results and Discussion**

#### *Structure Elucidation*

Compound **1** was obtained as a yellow crystal. The molecular formula was deduced as C24H31NO7 based on the HRESIMS ion peak at *m*/*z* 468.19861 [M + Na]+ (calcd. for C24H31NO7Na = 468.19927). Its IR (KBr) spectrum exhibited absorptions at 3434 cm−1 (hydroxy), 1649 cm−1 (carbonyl), and 1605/1552/1446 cm−1 (aromatic heterocycle). Compound **1** had the same molecular formula with *N*-hydroxyapiosporamide (**3**) and showed a high degree of similarity in UV absorption. The NMR spectra displayed two methyls, five sp3-methylenes, eight sp3-methines, three sp2-methines, one sp3-quaternary carbon, five sp2-quaternary carbons (including two carbonyls) (Table 1), which was also similar to those of compound **3**, especially for the important 1H NMR signals, such as two methyl groups at H3-11(*δ*H 0.94) and H3-12 (*δ*H 0.82), a single special hydrogen signal at H-16 (*δ*H 7.93), two olefinic protons at H-6 (*δ*H 5.41) and H-7 (*δ*H 5.60). The above evidence suggested that compound **1** has a similar skeleton with compound **3**. The 1H-1H COSY correlations (Figure 2) from H2-1 (*δ*H 0.88 and 1.95) to H-10 (*δ*H 1.58), H-3 (*δ*H 1.50) to H3-11, and H-8 (*δ*H 2.85) to H-12 proved the existence of a decalin moiety. The relative configurations of this part were confirmed by the NOESY correlations (Figure 2) from H-10 to H3-11/H3-12 and H-5 (*δ*H 1.83) to H-3/H-9 (*δ*H 4.45), and indicated that compound **1** has the same decalin moiety as compound **3**. The 1H-1H COSY correlations from H-20 (*δ*H 3.87) to H2-24 (*δ*H 1.63 and 2.25) and the HMBC correlations (Figure 2) from H-21 (*δ*H 4.00) and H2-23 (*δ*H 1.70 and 2.25) to C-19 (*δ*C 89.4) confirmed the presence of an oxygenated cyclohexane moiety in compound **1**. However, there was a large chemical shift difference between these two compounds at C-19/20/21/22 (*δ*C 89.4, 82.1, 82.9, 78.6 for **1**; 70.4, 60.5, 57.7, 67.2 for **3**). Nevertheless, there is still one degree of unsaturation in the structure **1**, implying an oxygen bridge in this cyclohexane moiety, but the HMBC correlations cannot be used to confirm it. The key HMBC correlations from H-9 to C-13 (*δ*C 211.4), H-16 to C-15 (*δ*C 159.9)/C-18 (*δ*C 173.2)/C-19 indicated that the decalin and hexane moieties substituted at C-13 and C-17 of the 4-hydroxy-2-pyridinone part. The crystal of compound **1** was fortunately acquired in methanol/water (*v/v*, 1:1) solution. The

results of the X-ray (Figure 3) analysis (Flack parameter = −0.15 (11), CCDC: 2218951) confirmed an oxygen bridge between C-19 and C-22 forming the furan ring and led to the final determination of its absolute configuration as 3*R*, 5*S*, 8*R*, 9*R*, 10*R*, 19*S*, 20*S*, 21*S*, 22*S*. This novel 4-hydroxy-2-pyridone was named furanpydone A.

**Table 1.** 1H (600 MHz) and 13C (150 MHz) NMR data of **1**–**4**.

a measured in DMSO-*d*6, b measured in methanol-*d*4 solvent.

**Figure 2.** The key 2D NMR correlations of compounds **1** and **2**.

**Figure 3.** ORTEP drawing of **1**.

Compound **2** was obtained as a yellow powder. The molecular formula was deduced as C24H31O6N based on the HRESIMS peak at *m*/*z* 452.20319 ([M + Na]+, calcd. for 452.20436), which has ten degrees of unsaturation as furanpydone A (**1**), but one less oxygen atom than it. According to IR (KBr) spectrum data, they seem to have similar functional groups at 3445 cm−1 (hydroxy), 1652 cm−1 (carbonyl), and 1604/1557/1456 cm−1 (aromatic heterocycle). According to 1D NMR and HSQC data, compound **2** displayed two methyl (*δ*H/C 0.74/17.9, 0.87/22.5), five sp3-methylene (*δ*H/C 1.50 and 2.06/23.0; 0.81 and 1.83/29.6; 1.47 and 2.04/31.8; 0.96 and 1.67/35.1; 0.75 and 1.70/41.4), eight sp3-methines (*δ*H/C 2.74/30.6, 1.46/32.6, 1.45/35.8, 1.75/41.4, 4.33/51.8, 4.30/76.6, 3.58/80.5, 3.78/81.2), three sp2-methines (*δ*H/C 5.37/130.4, 5.57/131.7, 7.33/139.1), one sp3-quaternary carbon (*δ*C 88.0), and five sp2-quaternary carbons (*δ*C 106.7, 110.8, 161.8, 175.7, 209.6), which were extremely similar to compound **1** (Tables 1 and S1) suggested the similar structure of these two compounds. The 1H-1H COSY correlations (Figure 2) from H2-1 (*δ*H 0.81 and 1.83) to H-10 (*δ*H 1.45), H-3 (*δ*H 1.46) to H3-11 (*δ*H 0.87), H-8 (*δ*H 2.74) to H3-12 (*δ*H 0.74), H-20 (*δ*H 3.58) to H2-24 (*δ*H 1.47 and 2.04), the key HMBC correlations (Figure 2) from H-9 (*δ*H 4.33) to C-13 (*δ*C 209.6), H-16 (*δ*H 7.33) to C-15 (*δ*C 161.8)/18 (*δ*C 175.7)/19 (*δ*C 88.0), H-21 (*δ*H 3.78) to C-19 further confirmed that compound **2** and **1** have the same skeleton structure. Analysis of the NMR spectral data revealed that the chemical shift of C-15 (*δ*C 161.8) moved to a lower field, which was similar to compound **4,** the key 1H-1H COSY correlation between H-16 and H-NH (*δ*H 11.38) confirmed the absence of an N-hydroxy group in **2**. The key NOESY correlations from H-10 to H3-11/H3-12, H-5 (*δ*H 1.75) to H-3 (*δ*H 1.47)/H-9 (*δ*H 4.33), as well as the same chemical shift for C-19/20/21/22/23/24 with furanpydone A (**1**) suggested that these two compounds had the same relative configuration. The similarity of electronic circular dichroism (ECD) curve of compound **2** (213 (−5.83), 228 (−11.74), 265 (+5.40), 310 (+7.09), 341 (−1.63) to **1** (217 (−2.73), 242 (−3.51), 270 (+2.00), 316 (+2.75), 343 (−0.27)) (Figure 4) along with the similar optical rotation values (**1**: −89.7, **2**: −80.0) indicated the same absolute configuration for **2** and **1**. This novel 4-hydroxy-2-pyridone was named furanpydone B.

**Figure 4.** Experimental CD spectra of compounds **1** and **2**.

We propose a possible biosynthetic pathway for compounds **1**–**4**. Didymellamide B was the key intermediate in the biosynthesis of these compounds [18]. The intermediates **a** and **b** were obtained by reduction from didymellamide B. Compound **2** was obtained by oxidation, hydration and cyclization reaction from **a**, and compound **1** was syntheszed by further oxidation. Compound **4** was obtained by two oxidation reactions from **b**, and compound **3** was syntheszed by further oxidation. (Figure 5).

**Figure 5.** Proposed biosynthetic pathway for compounds **1**–**4**.

Compounds **1**–**4** were tested for their antimicrobial activities against nine pathogenic microorganisms. As shown in Table 2, compound **4** exhibited broad inhibitory activities against *Staphylococcus aureus*, methicillin-resistant *S. aureus* (MRSA), *Bacillus subtilis*, *Clostridium perfringens*, and *Ralstonia solanacarum* with the MIC values ranging from 1.56 to 6.25 μM. Compounds **1** and **3** showed moderate selective activities against *S. aureus* and MRSA with the MIC values of 12.5–25.0 μM. Compounds **1**–**4** showed no obvious inhibitory activity against two Gram-negative bacteria (*E. coli* and *P. aeruginosa*) and two pathogenic fungi (*C. albicans* and *C. glabrata*) at 50 μM. According to the results, it seems that the compounds with ternary epoxide showed better antibacterial activity than those with furan ring, but the effect of N-OH needs more research to determine.

The antiproliferative activities against 18 cancer cell lines and one normal cell line were assayed by the CCK-8 method. Compound **1** showed significant cytotoxicity against 10 cancer cell lines, compound **3** showed activities against HCT116 and 786-O cell lines (Table 3). The compounds with furan ring showed better antiproliferative activities than those with ternary epoxide. At the same time, nitrogen hydroxyl is the necessary group for maintaining the inhibitory activity.

**Table 2.** Antimicrobial activity of **1**–**4** (MIC, μM), *n* = 3.

\* the positive drug is ciprofloxacin, # the positive drug is amphotericin B.

**Table 3.** Cytotoxic activity (μM, IC50 ± SD), *n* = 3.

#### **3. Materials and Methods**

#### *3.1. General Experimental Procedures*

The NMR spectra were recorded on Bruker Advance NEO 600 spectrometer (Bruker Corporation, Zurich, Switzerland) using TMS as an internal standard. MS analysis were carried out on Agilent 1100 instrument (Agilent Technologies, Santa Clara, CA, USA) and Thermo ultimate 3000/Q EXACTIVE FOCUS mass spectrometers (Thermo Scientific™, Waltham, MA, USA), respectively. Optical rotations were determined on Rudolph Autopol1 automatic polarimeter (Rudolph Research Analytical, Hackettstown, NJ, USA). UV spectra were detected on a Cary 60-UV-Vis spectrometer (Agilent Technologies, Santa Clara, CA, USA). IR spectra were determined on an iCAN 9 infrared spectrophotometer (Tianjin Nengpu Technology Co., Ltd, Tianjin, China) with KBr disks. X-ray data were generated using a Bruker Smart-1000 CCD (Bruker Corporation, Billerica, MA, USA) area detector diffractometer with graphite monochromatic Cu-Kα radiation. Column chromatography was performed on silica gel (200–300 mesh; Qingdao Puke Parting Materials Co., Ltd., Qingdao, China), Sephadex LH-20 gel (Amersham Biosciences, Uppsala, Sweden). HPLC separation was performed on HITACHI Primaide with an ODS-A column (YMC-pack ODS-A, 10 × 250 mm, 5 μm, 4 mL/min). Melting point instrument (SGW X-4).

#### *3.2. Fungal Material*

The endophytic fungus *Arthrinium* sp. GZWMJZ-606 was isolated from the leaves of *Houttuynia cordata* Thunb., which was collected from Longli, Guizhou, China. The leaves were treated with 75% alcohol for 30 s, and the residual alcohol was washed with sterile water. Then 1 g of fresh leaves was grinded into a pulp and 10 mL sterile water added. The suspension (100 μL) was deposited on a rice agar plate, which was prepared from rice powder (10 g), agar (18 g), and 1 L water containing chloramphenicol (0.3%) as a bacterial inhibitor, and incubated at 28 ◦C for 5 days. Monoclonal was selected and streaked to purity using the same agar medium. This strain was determined as *Arthrinium* sp. by the phylogenetic tree (Figure S1) of the ITS sequence (GenBank No. OP810989). The strain was deposited in our laboratory of Guizhou in 20% glycerol at −80 ◦C.

#### *3.3. Fermentation and Extraction*

The fungal strain GZWMJZ-606 was cultured on PDA at 28 ◦C for 3 days and then was cut into 100 × 1000 mL Erlenmeyer flasks, each containing a solid medium prepared from 100 g rice and 110 mL distilled water. These flasks were incubated at room temperature under static conditions for 40 days. The cultures were extracted three times by EtOAc (each 500 mL) and the combined EtOAc solutions were dried in vacuo to yield the extract (480.0 g).

#### *3.4. Isolation and Purification*

The EtOAc extract (480.0 g) was fractionated into 19 fractions (Fr.1–Fr.19) by chromatography on a silica gel column using step gradient elution of petroleum ether (PE)-EtOAc (*v*/*v*, 100:1–1:1) and CH2Cl2-MeOH (*v*/*v*, 20:1–1:1). Fr.17 (9.4 g) was further separated into 15 subfractions (Fr.17.1–Fr.17.15) by Sephadex LH-20 (CH2Cl2-MeOH, *v*/*v*, 1:1). Fr.17.11 (207.6 mg) was purified by semipreparative HPLC on an ODS-A column eluting with 60% MeCN-H2O containing 0.05% trifluoroacetic acid (TFA) to yield compound **1** (35.6 mg, *t*R 11.1 min). Fr.17.14 (75.8 mg) was purified by semipreparative HPLC on an ODS-A column (60% MeCN-H2O containing 0.05% TFA) to yield compound **2** (6.8 mg, *t*R 9.3 min). Fr.17.2 (830.5 mg) was further separated into 7 subfractions (Fr.17.2.1–Fr.17.2.7). Compound **4** (12.2 mg, *t*R 10.1 min) was obtained from Fr.17.2.1 (57.1 mg) by semipreparative HPLC (55% MeCN-H2O containing 0.05% TFA). Fr.16 (1.6 g) was further separated into 6 subfractions (Fr.16.1–Fr.16.6), and Fr.16.4 (120.5 mg) was performed on a semipreparative ODS-A column (61% MeCN-H2O containing 0.05% TFA) to yield compound **3** (38.6 mg, *t*R 8.4 min).

#### *3.5. Physical Properties and Spectral Data of* **1***–***4**

Compound **1**: yellow crystal; m.p. 167.5–168.5 ◦C; ECD (1.12 *m*M, MeOH) *λ*max (Δ*ε*) 217 (−2.73), 242 (−3.51), 270 (+2.00), 316 (+2.75), 343 (−0.27) nm; [α] 22 D −89.7 (*c* 0.58, MeOH); UV (MeOH) λmax (log *ε*) 281 (0.75), 341 (0.72) nm; IR (KBr) *ν*max 3434, 2913, 2953, 1649, 1605, 1446 cm−1; 1H NMR and 13C NMR data see Tables 1 and S1 and Figures S3–S10; HRESIMS *m*/*z* 468.19861 [M + Na]+ (Figure S2), molecular formula: C24H31NO7.

X-ray crystallographic analyses of **1**: C24H31NO7·CH3OH, orthorhombic, *M* = 477.54, *a* = 7.6539 (3) Å, *b* = 14.5093 (6) Å, *c* = 21.7608 (11) Å, *α* = 90◦, *β* = 90◦, *γ* = 90◦, *V* = 2416.60 (18) Å3, *T* = 150 K, space group *P*21 21 21, *Z* = 4, *μ* (Cu Kα) = 0.807 mm−1, 8074 reflections measured, 4534 independent reflections (*Rint* = 0.019). The final *R1* values were 0.0724 (*I* > 2*σ* (*I*)). The final *wR* (*F*2) values were 0.1921 (*I* > 2*σ* (*I*)). The final *R1* values were 0.0767 (all data). The final *wR*(*F*2) values were 0.1991 (all data). The goodness of fit on *F*2 was 1.022. Flack parameter = −0.15 (11). CCDC: 2218951.

Compound **2**: yellow powder; ECD (1.17 *m*M, MeOH) *λ*max (Δ*ε*) 213 (−5.83), 228 (−11.74), 265 (+5.40), 310 (+7.09), 341 (−1.63) [α] 22 D −80.0 (*c* 0.20, MeOH); UV (MeOH) λmax (log *ε*) 235 (1.05), 270 (0.50), 338 (0.74) nm; IR (KBr) *ν*max 3445, 2909, 1652, 1604, 1456 cm−1; 1H NMR and 13C NMR data see Table 1 and Figures S12–S17; HRESIMS *m*/*z* 452.20319 [M + Na]+ (Figure S11), molecular formula: C24H31NO6.

Compound **3**: yellow solid; the molecular formula is C24H31NO7 (*m*/*z* 444.1 [M − H]−) determined by ESIMS. [α] 22 D −57.4 (*c* 2.3, MeOH); based on 1H NMR and 13C NMR data (Table 1, Figures S18 and S19) proved that compound **3** was N-hydroxyapiosporamide. Compound **4**: faint yellow solid powder; the molecular formula is C24H31NO6 (*m*/*z* 452.5 [M + Na]+) determined by ESIMS. [α] 22 D −32.2 (*c* 0.87, MeOH); based on 1H NMR and 13C NMR data (Table 1, Figures S20 and S21) proved that compound **4** was apiosporamide.

#### *3.6. Antimicrobial Activities Assay*

The isolated compounds were evaluated for antibacterial activity against pathogenic microorganisms including three Gram-negative strains (*Escherichia. coli* ATCC11775, *Pseudomonas aeruginosa* ATCC10145, *Ralstonia solanacarum* [26]), and four Gram-positive strains (*Staphylococcus aureus* ATCC6538, methicillin-resistant *S. aureus* ATCC43300 MRSA, *Clostridium perfringens* ATCC13124, and *Bacillus subtilis* ATCC6051), and two pathogenic fungi (*Candida albicans* ATCC10231 and *Candida glabrata* ATCC2001). The tested bacterial suspensions were incubated in Luria–Bertani (LB) medium and fungi in Mueller–Hinton agar (HMA) medium at 28 ◦C for 12 h and diluted to be 1 × 106 CFU/mL by the same medium. Then, the DMSO solution of each compound was diluted into the corresponding concentration using the LB or MHA medium; 100 μL solution of compound was added into the first well of a 96-well plate and resulted the initial tested concentration of each compound to be 50 μmol/L (DMSO < 0.5‰ in each well) and the concentration of each compound to be 25 μmol/L (DMSO < 0.5‰ in each well) in the second well of a 96-well plate after then following this method in sequence, adding 100 μL microbial suspension into a 96-well plate. The ciprofloxacin and DMSO were used as the positive and negative controls, respectively. All experiments were repeated three times. MIC values were assessed by whether compounds can inhibit the growth of microorganisms [19].

#### *3.7. Cytotoxic Activity Assay*

Cell proliferation was measured with the CCK-8 method. By the dye of WST-8 (2-(2 methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4disulfophenyl)-2H-etrazolium, monosodium salt) was reduced by dehydrogenase in cells to form a water-soluble tetrazolium salt product (formazan dye) with orange color. In the measurement, the amount of the formazan dye is proportional to the number of living cells. Finally, the cell viability can be estimated by recording the optical density (OD) of formazan dye at 450 nm using a microplate reader [27].

A cell suspension of 100 μL was dispensed (adherent cell viewed 5 × 104/mL and suspension cell viewed 9 × 104/mL) in 96-well plates. With doxorubicin hydrochloride as positive drug and DMSO as control, plates were pre-cultured for 24 h, followed by treatments with various concentrations of compound (eight concentration gradients were set for each sample for IC50 determination and three multiple holes were set for each concentration, *n* = 3). Keep the 96-well plates at 37 ◦C in an incubator with 5% CO2 for 72 h. After the aspiration of the old medium, the 10-fold diluted CCK-8 (100 μL) solution was added to each well of the plate, which was then incubated for another 3 h. An absorbance microplate reader was used to measure the absorbance at 450 nm. The optical density values (OD) of each well represented the survival/proliferation of cells. The toxicity is expressed by cell inhibition. The half inhibitory concentration (IC50) was defined as the concentration causing 50% inhibition, each group of data has 8 concentration gradient responses. The IC50 value is calculated by curve fitting using the software GraphPad Prism 8 (version 8.0.2, from GraphPad Software Inc., Boston, MA, USA), the experimental results are expressed in IC50 ± SD [28,29].

Cell inhibition rate = (ODControl − ODDrug)/(ODControl − ODBlank) × 100%.

#### **4. Conclusions**

Two new 4-hydroxy-2-pyridone alkaloids were isolated from an endophytic fungus *Aspergillus* sp. GZWMJZ-606, which was obtained from *Houttuynia cordata* Thunb. Compounds **1** and **2** are the first example of 4-hydroxy-2-pyridone alkaloids possessing novel 7-oxidicyclo[2.2.1]heptane part. Compound **1** exhibited broad-spectrum cytotoxicity against 10 cancer cell lines with the IC50 values of 4.35–9.72 μM, and showed selective activities against *S*. *aureus* and MRSA *S*. *aureus* with MIC values of 12.5 μM. The discovery of novel 4-hydroxy-2-pyridinone alkaloids can provide a material basis for the discovery of potential drug molecules.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28052192/s1, ITS1 gene sequences of *Arthrinium* sp. GZWMJZ-606; Figure S1: Species identification of endophytic fungi strain; Table S1: 1H (600 MHz) and 13C (150 MHz) NMR data of compound **1** DMSO-*d*6; Figure S2: HRESIMS spectrum of **1**; Figure S3: 1H NMR spectrum (600 MHz, Methanol-*d*4) of **1**; Figure S4: 13C NMR spectrum (150 MHz, Methanol-*d*4) s of **1**; Figure S5: HSQC spectrum (Methanol-*d*4) of **1**; Figure S6: HMBC spectrum (Methanol-*d*4) of **1**; Figure S7: 1H-1H COSY spectrum (Methanol-*d*4) of **1**; Figure S8: NOESY spectrum (Methanol-*d*4) of **1**; Figure S9: 1H NMR spectrum (600 MHz, DMSO-*d*6) of **1**; Figure S10: 13C NMR spectrum (150 MHz, DMSO-*d*6) of **1**; Figure S11: HRESIMS spectrum of **2**; Figure S12: 1H NMR spectrum (600 MHz, DMSO-*d*6) of **2**; Figure S13: 13C NMR spectrum (150 MHz, DMSO-*d*6) of **2**; Figure S14: HSQC spectrum (DMSO-*d*6) of **2**; Figure S15: HMBC spectrum (DMSO-*d*6) of **2**; Figure S16: 1H-1H COSY spectrum (DMSO-*d*6) of **2**; Figure S17: NOESY spectrum (DMSO-*d*6) of **2**; Figure S18: 1H NMR spectrum (600 MHz, Methanol-*d*4) of **3**; Figure S19: 13C NMR spectrum (150 MHz, Methanol-*d*4) s of **3**; Figure S20: 1H NMR spectrum (600 MHz, Methanol-*d*4) of **4**; Figure S21: 13C NMR spectrum (150 MHz, Methanol-*d*4) of **4**.

**Author Contributions:** Y.Y. performed the experiments and wrote the original draft; D.W. (Dongyang Wang) analyzed the data and confirmed the structure; D.W. (Dan Wu) and M.Z. performed the biological activity test; W.H. performed the fermentation and extraction; W.Z. helped to modify the manuscript; Y.X. isolated the strain and directed the implementation of the study; L.W. designed the study and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Natural Science Foundation of China, grant numbers 22207020 and U1812403; Guizhou Provincial Science and Technology Projects, grant numbers QKHJC-ZK[2021]ZD017, QKHZC[2022]YB191, QKHJC-ZK [2022]YB392, and QKHZYD[2022]4015; "Light of the West" Talent Cultivation Program of Chinese Academy of Sciences, grant numbers RZ [2022]4 for L. Wang, GMU (J [2020]006, 19NSP078, 20NSP065), and 100 Leading Talents of Guizhou Province for W. Zhu.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not available.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.

#### **References**

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2. Impediments of dual-band near-field wireless energy transfer resonator designs

For a single-band near-field WET resonator design, designers delve into achieving maximum power transfer efficiency (PTE) between a pair of coupled resonators by designing the highest possible quality factor (Q-factor). However, there is an apparent complexity for designing resonators operating in more than one frequency band. PTE for either one of the frequency bands tends to surpass its counterpart. As such, concurrent capitalization on PTE for both frequency bands, f1 and f2, remains as one of the ultimate challenges.

Another concern is the inversely proportionate relation between Q-factor and bandwidth. Increasing PTE of the affected frequency band is feasible by developing resonators with enhanced Q-factor. This comes at the expense of higher bandwidth which is pivotal especially for resonators aiming for concomitant power and data transfer functionalities as portrayed in Figure 1. PTE for f1 is higher with improved Q-factor than f2 but falls behind in terms of bandwidth since it is constrained by Q-factor. On the other hand, redesigning higher Q-factor resonator in contemplation of attaining improved PTE at f2 unfortunately leads to bandwidth degradation. As such, there is always a dilemma between achieving PTE equilibrium for f1 and f2 and maximum PTE and bandwidth for each frequency bands.

Imperfect positioning of loop resonator is yet another impairment of WET system which impacts its performance metrics specifically coupling coefficient, k, and PTE [7–9]. Strict adherence toward perfect alignment between a pair of loop resonator in assuring maximum transfer efficiency is seemingly impossible in practical sense because of misalignment be it planar, lateral, or angular frequently supervened [10]. Referring to Figure 2, planar displacement refers to the angle of rotation ar when both centers are axially aligned. Separated at a fixed axial distance, z, the center of receiving loop resonators is shifted by a distance, ax, known as

horizontal lateral displacement, while ay is referred as vertical lateral displacement. The occurrence of simultaneous planar and the respective lateral displacements are visualized by ar with either ax or ay offsets. Angular displacement occurs when the plane of receiving loop resonator is being tilted by an angle θ [11]. Similarly, the consequences of displacement should be taken into consideration in the design of

Types of displacements between near-field coupled resonators: (i) planar; (ii) concurrent planar and lateral (x-axis); (iii) concurrent planar and lateral (y-axis); (iv) lateral (x-axis), (v) lateral (y-axis);

dual-band coupled resonators.

Figure 1.

Figure 2.

51

(vi) angular.

Transfer efficiency equilibrium for dual-band near-field coupled resonators.

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications

DOI: http://dx.doi.org/10.5772/intechopen.89218

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications DOI: http://dx.doi.org/10.5772/intechopen.89218

#### Figure 1. Transfer efficiency equilibrium for dual-band near-field coupled resonators.

#### Figure 2.

Provision of more than one frequency band is enabled by resonator which is intended for several functions either concurrent power and data transfer or concurrent wireless charging at multiple standards. At present, there are two principal standards for wireless charging of consumer electronics specifically smartphones. The Wireless Power Consortium, otherwise known as Qi, is one of the leading wireless charging standards operating in low-frequency (LF) band, 110–205 kHz [2]. AirFuel Alliance is the merger between Power Matters Alliance (PMA) and the Alliance for Wireless Power (A4WP, also known as Rezence) [3]. A4WP employs magnetic resonance coupling technique operating at 6.78 MHz 15 kHz [4], while both Qi and PMA engage in inductive charging technique. The range of PMA's operating frequency is from 277 to 357 kHz [5]. Nonetheless, unlicensed industrial, scientific and medical (ISM) radio bands of up to 13.56 MHz are commonly selected to be the operating frequency for both inductive and magnetic resonant techniques

This chapter commences with challenges in relation to dual-band near-field WET design and the corresponding key performance metrics followed by design approaches and rectification techniques currently employed to alleviate the adverse

effects on performance metrics with the intention of providing insights for designers in deciding and further enhancing current rectification options available in the design of dual-band near-field resonators. This framework, however, solely pertains to front-end resonator designs excluding end-to-end scope of near-field

2. Impediments of dual-band near-field wireless energy transfer

For a single-band near-field WET resonator design, designers delve into achieving maximum power transfer efficiency (PTE) between a pair of coupled resonators by designing the highest possible quality factor (Q-factor). However, there is an apparent complexity for designing resonators operating in more than one frequency band. PTE for either one of the frequency bands tends to surpass its counterpart. As such, concurrent capitalization on PTE for both frequency bands, f1 and f2, remains

Another concern is the inversely proportionate relation between Q-factor and bandwidth. Increasing PTE of the affected frequency band is feasible by developing resonators with enhanced Q-factor. This comes at the expense of higher bandwidth which is pivotal especially for resonators aiming for concomitant power and data transfer functionalities as portrayed in Figure 1. PTE for f1 is higher with improved Q-factor than f2 but falls behind in terms of bandwidth since it is constrained by Q-factor. On the other hand, redesigning higher Q-factor resonator in contemplation of attaining improved PTE at f2 unfortunately leads to bandwidth degradation. As such, there is always a dilemma between achieving PTE equilibrium for f1 and f2 and

Imperfect positioning of loop resonator is yet another impairment of WET system which impacts its performance metrics specifically coupling coefficient, k, and PTE [7–9]. Strict adherence toward perfect alignment between a pair of loop resonator in assuring maximum transfer efficiency is seemingly impossible in practical sense because of misalignment be it planar, lateral, or angular frequently supervened [10]. Referring to Figure 2, planar displacement refers to the angle of rotation ar when both centers are axially aligned. Separated at a fixed axial distance, z, the center of receiving loop resonators is shifted by a distance, ax, known as

in the current research community [6].

Recent Wireless Power Transfer Technologies

WET systems.

50

resonator designs

as one of the ultimate challenges.

maximum PTE and bandwidth for each frequency bands.

Types of displacements between near-field coupled resonators: (i) planar; (ii) concurrent planar and lateral (x-axis); (iii) concurrent planar and lateral (y-axis); (iv) lateral (x-axis), (v) lateral (y-axis); (vi) angular.

horizontal lateral displacement, while ay is referred as vertical lateral displacement. The occurrence of simultaneous planar and the respective lateral displacements are visualized by ar with either ax or ay offsets. Angular displacement occurs when the plane of receiving loop resonator is being tilted by an angle θ [11]. Similarly, the consequences of displacement should be taken into consideration in the design of dual-band coupled resonators.

### 3. Performance metrics

Figure-of-merit (FOM) consideration yields a comparative insight for a diverse WET system design [12]. Derivation of FOM originates from analytical expression of link transfer efficiency (PTE) based on circuit theory (CT) [13] and reflected load theory (RLT) [14] as shown in Eq. (1) and Eq. (2). FOM is reliant on coupling coefficient, k, and Q-factor parameters which are derived from Eq. (3) and Eq. (4). Proportional relationship is shared between coupling coefficient, k, and mutual inductance, M, with an inverse correlation shared between the latter and product of square root of transmitting and receiving self-inductances, L1,2. Q-factor, Q1,2, is proportional to angular frequency, ω, and self-inductances but inversely proportional to transmitting and receiving resistances, R1,2:

$$PTE = FOM\left[\left\{\mathbf{1} + \left(\mathbf{1} + FOM\right)^{0.5}\right\}\right]^{-1} \tag{1}$$

$$FOM = k^2 Q\_1 Q\_2 \tag{2}$$

between each two turns on a pair of loops with turn radii, ri and rj. μ and n1,2 represent conductor permeability and number of loops' turns, while ρ is the factor which is dependent on loop profile. K and E refer to complete elliptic integrals of

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications

ri þ rj � �2 þ z2 h i�0:5

Xn2 j¼1

Other FOMs used as performance quantification are summarized in Table 1.

There are accompanying implications with the choice of operating frequency on WET links. With higher frequency, the manifestation of eddy currents results in a nonuniformly distributed current density in conductive trace. The probability of current flowing toward the surface of a conductor known as skin effect amplifies alternate current resistance followed by power and heat dissipations [25]. Proximity effect is yet another dilemma of nonuniform current distribution due to current crowding on the inner loop edge caused by nearby conductive traces or layers [26]. On the contrary, current density is uniformly distributed at lower frequency in which eddy current effects are negligible. As such, feasibility of achieving higher power transfer efficiency is established with lower operating frequency at kHz than higher operating frequency at MHz at the expense of transfer range extension and

For dual-band WET links, the ratio between frequencies selected is equally important since it is proportional to the transfer efficiency variance between f1 and f2 [28]. Additionally, selecting a larger ratio between frequencies with both power and data being transmitted at significantly lower and higher operating frequency bands turns out to be one of the isolation options available in curbing interference [23]. Table 2 summarizes frequency selections opted for dual-band resonator designs. For example, variance of PTE between kHz and MHz frequencies is larger

References f1 f2 Ratio, f2=f1 Variance, ΔPTEf1\_f2 (%)

[30] 100 kHz 13.56 MHz 136 <10 [29] 100 kHz 6.78 MHz 68 3 [31, 32] 200 kHz 6.78 MHz 34 7.4, <8 [33] 300 MHz 900 MHz 3 8 [24] 300 MHz 675 MHz 2 7 [34–36] 6.78 MHz 13.56 MHz 2 15, 11.93, 1.68 [37] 90.3 MHz 138.8 MHz 2 1 [23] 300 MHz 700 MHz 2 1 [38] 470 MHz 730 MHz 2 11.4

Summary of frequency selections for dual-band near-field WET resonator.

ρ�1 1 � 0:5ρ2 � �Kð Þ� ρ Eð Þρ � � (5)

Mij ri, rj, z � � (7)

(6)

the first and second kind:

4. Frequency selection

reduced displacement sensitivity [27].

Table 2.

53

Mij ¼ 2μ rirj

DOI: http://dx.doi.org/10.5772/intechopen.89218

� �0:5

M ¼ ρ

Xn1 i¼1

ρ ¼ 2 rirj � �0:5

$$k = M(L\_1 L\_2)^{-0.5} \tag{3}$$

$$Q\_{\mathbf{1},2} = aL\_{\mathbf{1},2} \left(R\_{\mathbf{1},2}\right)^{-1} \tag{4}$$

Performance metrics that are commonly used are power transfer efficiency, Qfactor, and coupling coefficient. As discussed in Section 2, PTE and Q-factor share a positive correlation which is further validated from Eq. (1) and Eq. (2). Nevertheless, optimum coupling coefficient is often desirable in maximizing PTE. Since k is dependent on M and M is negatively correlated to transfer distance, z, a cautionary reminder is to ensure optimum transfer distance between transmitting and receiving resonators. This is validated with subsequent Equations [15] that reveal the correlation between M, z, and k [16]. Mij indicates partial mutual inductance

Table 1. Figure-of-merits (FOM). Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications DOI: http://dx.doi.org/10.5772/intechopen.89218

between each two turns on a pair of loops with turn radii, ri and rj. μ and n1,2 represent conductor permeability and number of loops' turns, while ρ is the factor which is dependent on loop profile. K and E refer to complete elliptic integrals of the first and second kind:

$$M\_{\vec{\eta}} = 2\mu \left( r\_i r\_j \right)^{0.5} \rho^{-1} \left[ (\mathbf{1} - \mathbf{0}.5\rho^2) \mathbf{K}(\rho) - E(\rho) \right] \tag{5}$$

$$\rho = 2\left(r\_i r\_j\right)^{0.5} \left[\left(r\_i + r\_j\right)^2 + z^2\right]^{-0.5} \tag{6}$$

$$\mathcal{M} = \rho \sum\_{i=1}^{n\_1} \sum\_{j=1}^{n\_2} \mathcal{M}\_{ij}(r\_i, r\_j, z) \tag{7}$$

Other FOMs used as performance quantification are summarized in Table 1.

#### 4. Frequency selection

3. Performance metrics

Recent Wireless Power Transfer Technologies

tional to transmitting and receiving resistances, R1,2:

FOM Concerned parameters Dual-

PL(Vs)�2 Transfer efficiency, load power, and voltage source [17]

(ARX)�1.5 Transfer efficiency, transfer distance, and area of receiver [18, 19]

and outermost side length [20]

Transfer efficiency, transfer distance, and outermost side lengths of transmitter and receiver [22]

PTEz(do)�1 Transfer efficiency, transfer distance,

PTEz(A)�0.5 Transfer efficiency, transfer distance, and area [21]

PTEz(A)�0.5 Transfer efficiency, transfer distance, and area [23, 24]

transmitting, receiving resonator [12]

Q1 Q2 Coupling coefficient, Q-factor

k2

PTE2

PTEz3

PTEz/ (do\_tx � do\_rx)0.5

Table 1.

52

Figure-of-merits (FOM).

Figure-of-merit (FOM) consideration yields a comparative insight for a diverse WET system design [12]. Derivation of FOM originates from analytical expression of link transfer efficiency (PTE) based on circuit theory (CT) [13] and reflected load theory (RLT) [14] as shown in Eq. (1) and Eq. (2). FOM is reliant on coupling coefficient, k, and Q-factor parameters which are derived from Eq. (3) and Eq. (4). Proportional relationship is shared between coupling coefficient, k, and mutual inductance, M, with an inverse correlation shared between the latter and product of square root of transmitting and receiving self-inductances, L1,2. Q-factor, Q1,2, is proportional to angular frequency, ω, and self-inductances but inversely propor-

PTE ¼ FOM 1 þ ð Þ 1 þ FOM 0:5 h i n o �1

FOM ¼ k2

Q1, 2 ¼ ωL1, 2 R1, 2

Performance metrics that are commonly used are power transfer efficiency, Qfactor, and coupling coefficient. As discussed in Section 2, PTE and Q-factor share a positive correlation which is further validated from Eq. (1) and Eq. (2). Nevertheless, optimum coupling coefficient is often desirable in maximizing PTE. Since k is dependent on M and M is negatively correlated to transfer distance, z, a cautionary reminder is to ensure optimum transfer distance between transmitting and receiving resonators. This is validated with subsequent Equations [15] that reveal the correlation between M, z, and k [16]. Mij indicates partial mutual inductance

band

(1)

Q1Q2 (2)

� ��1 (4)

k ¼ M Lð Þ 1L2 �0:5 (3)

Significance

Q parameters

load

No Independent optimization of k and

efficiency and power delivered to

distance and outermost side lengths

No Take into account of transfer

No Equitable assessment between various transfer distance and

No Take into account of transfer

No Equitable assessment between various transfer distance and

No Equitable assessment between various transfer distance and asymmetrical pair of resonators

Yes Equitable assessment between various transfer distance and

geometry designs

geometry designs

geometry designs

There are accompanying implications with the choice of operating frequency on WET links. With higher frequency, the manifestation of eddy currents results in a nonuniformly distributed current density in conductive trace. The probability of current flowing toward the surface of a conductor known as skin effect amplifies alternate current resistance followed by power and heat dissipations [25]. Proximity effect is yet another dilemma of nonuniform current distribution due to current crowding on the inner loop edge caused by nearby conductive traces or layers [26]. On the contrary, current density is uniformly distributed at lower frequency in which eddy current effects are negligible. As such, feasibility of achieving higher power transfer efficiency is established with lower operating frequency at kHz than higher operating frequency at MHz at the expense of transfer range extension and reduced displacement sensitivity [27].

For dual-band WET links, the ratio between frequencies selected is equally important since it is proportional to the transfer efficiency variance between f1 and f2 [28]. Additionally, selecting a larger ratio between frequencies with both power and data being transmitted at significantly lower and higher operating frequency bands turns out to be one of the isolation options available in curbing interference [23]. Table 2 summarizes frequency selections opted for dual-band resonator designs. For example, variance of PTE between kHz and MHz frequencies is larger

Table 2.

Summary of frequency selections for dual-band near-field WET resonator.

than MHz frequencies with reduced ratio of f2 to f1. However, research work demonstrated in [29] reports on reduced PTE variance with higher frequency ratio by implementing other rectification techniques which will be discussed in subsequent sections. In general, it is observed that there is higher probability of concurrently capitalizing PTE for both frequencies by designing resonators with lowest possible ratio of f2 to f1.

design approach comes with their respective strengths and weaknesses which deserved appropriate assessment as tabulated in Table 3. Single-coil approach is preferred owing to minimum cross-coupling as compared to multi-coil [34]. Even though designing two separate coils in multi-coil approach necessitate more geometrical area, there is a greater potential in exploiting on highest possible transfer efficiency at two different frequencies in a single transmitter as it allows indepen-

Dual-Band Resonator Designs for Near-Field Wireless Energy Transfer Applications

Apart from deciding on the type of design approach to engage with, dual-band support configuration ought to be determined as well. Applicable configurations [28] involve designing either identical support mode as in dual-band transmitter (TX) and dual-band receiver (RX) or nonidentical support mode as in dual-band transmitter and single-band receivers. Bearing in mind that there is no superior design approach, rectification techniques employed address the respective shortcomings. Referring to Figure 3, rectification techniques for dual-band resonator designs which will be discussed in the following subsections encompass aspects relating to resonator design, resonator configuration, and impedance transforma-

There is a penchant in designing printed spiral coils which encompassed conductive trace etched on dielectric substrates over conventional solenoid coils owing to lightweight and compactness features apart from having privilege of freedom in geometrical optimization [45] and conformity capability on malleable substrates [46] as compared to its predecessor. Equivalent series resistance (ESR) and substrate's dielectric losses are commonly contemplated to boost Q-factor of loop resonator designed [47] since the performance metric indicator, kQ, product can be

where ESR = [(R11R22) � (R12R21)]0.5 for two-port system. Despite the fact that higher inductance could be acquired by increasing number of turns, ESR is not exempted. Direct current (DC) and alternate current (AC) resistances are components of ESR for a loop resonator. Hence, it is obligatory to exercise caution in geometrical layout alteration specifically number of turns, n, conductor width, wc, and spacing between conductor trace, sc. Figure 4 illustrates comparison between a conventional layout of spiral resonator with uniform distribution of wc and sc and

By designing inconstant conductor width distributions and spatial distributions, Q-factor enhancement is attained in double-layer printed spiral resonator [49], printed circular resonator [50], and stacked multilayer printed spiral resonator [51]. This is made possible as losses induced by eddy current are hindered with designs that involve gradual decrement of conductive trace widths from the outermost loop to the innermost loop [52] unlike conventional constant width for all loops. It is worth keeping in mind that designing larger constant conductor width for all turns and smaller constant spacing between adjacent conductor trace will only reinforce the total resistance [53]. However, ensuring ratio between conductor width and spacing to be relatively small aids in the reduction of proximity effects [54].

kQ ¼ j j Z21 ESR�1 (8)

derived from impedance matrix components given by [48]:

nonuniform layout refinement between each loop turns.

dent selection of inductance and quality factor [31].

DOI: http://dx.doi.org/10.5772/intechopen.89218

tion network.

55

6. Rectification techniques

6.1 Resonator design


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**4.1. Arsenic removal using modified clay minerals**

Mishra and Mahato [40] evaluated the effectiveness of Fe and Mn pillared bentonite clay in As(V) and As(III) removal from groundwater. Maximum As(III) and As(V) adsorption capacities of 17.57 and 25.77 μg/g were observed for Fe pillared bentonite while adsorption capacities of 25.77 and 26.17 μg/g were observed for Mn pillared bentonite clay. This was higher by many folds compared to 4.31 and 4.33 μg/g for raw bentonite clay. This was attributed to increased surface area in the pillared bentonite and also increased charge density in the pillared clay.

was better than the capacity achieved with unmodified bentonite. Gitari et al. [20] reported maximum adsorption capacity of 2.91 mg/g for the Fe3+ modified bentonite clay which was also better than the unmodified bentonite. The main factor which was leading to improved sorption fluoride capacity was the increased surface area that corresponds to availability of more active sites for fluoride ion adsorption. Furthermore, modification of clay mineral my high density charges polycations increases the amount of positive charges in the surface of

**Table 2.** Comparison of the adsorption capacity between raw and modified clay toward arsenic and fluoride.

**Table 2** summarizes the comparison between raw and modified clay soils toward fluoride and arsenic adsorption from water. It is observed that the adsorption of fluoride and arsenic

General mechanism involved for arsenic and fluoride removal using clay mineral can be summarized by the following three main steps: (i) Mass transfer of ions to the external of the adsorbent, (ii) adsorption of ions on the external particle surface and (iii) Intra-particle diffusion of ions from the exterior surface and the exterior surface and possible exchange with elements on the pore surface inside particles [30, 43]. These major three steps were observed during the

coated bentonite by Mudzielwana et al., [44]. Besides these

**5. Mechanisms for arsenic and fluoride removal using clay soils**

adsorbent leading to increased sorption capacity of anions.

increases by many fold after clay modification.

adsorption of fluoride onto MnO2

**Adsorbent CEC**

CTAMB-Fe montmorillonite

NB: (−) = not reported.

**(meq/100 g)**

**Surface area (m2 /g)**

Kaolinite — 33 As(V) 5 40 0.86 [32] Montmorillonite — 58 As(V) 5 40 0.68 [32] Illite — 28 As(V) 5 40 0.54 [32] Moroccan clays 35 22.5 As(V) 7 25 1.07 [33] Smectite rich clay 79.9 20.35 F− 2 8 0.21 [23] Montmorillonite — 18.5 F− 6.0 8 0.26 [35] Mixed Mukondeni clay 137.7 35.46 F− 2 150 0.08 [24]

Fe-montmorillonite — — As(V) 4–10 4 15.15 [25] Mg2+ bentonite — — F− 3–10 3 2.26 [41] Fe3+ bentonite 188.9 49.95 F− 2–10 20 2.29 [20] Al3+ bentonite 183.3 33.1 F− 2–12 10 5.7 [22]

**Adsorbate pH Adsorbent**

Mineralogical and Chemical Characteristics of Raw and Modified Clays and Their Application…

— — As(V) 4–10 4 8.85 [25]

**dosage (g/L)**

**Adsorption capacity (mg/g)**

http://dx.doi.org/10.5772/intechopen.74474

**Ref**

53

Ren et al. [25] intercalated Fe polycations and cetyltrimethylammonium bromide (CTMAB) onto montmorillonite clay mineral and test it for arsenic removal. The results showed that CTMAB had diffused into the interlayers of the montmorillonite minerals while Fe polycations were adsorbed on the outer surface and had formed flocculent particles. When tested for arsenic removal, CTMAB-Fe montmorillonite and Fe montmorillonite showed a maximum adsorption capacities of 8.85 and 15.15 mg/g for As(V), respectively, and 13.89 and 16.13 mg/g for As(III). Furthermore, the optimum adsorption capacities were observed at wide range of pH of 4–10.

### **4.2. Fluoride removal using modified clay minerals**

Several studies have been reported concerning the adsorption of fluoride using chemically modified clay minerals and the results showed improved adsorption efficiency as compared to unmodified clay soils. Kamble et al. [41] reported better sorption capacity for La, Mn and Mg oxides incorporated bentonite clay compared to bare bentonite. Efficiency of magnesium incorporated bentonite in adsorption of fluoride has also been evaluated by Thakre et al. [42] and maximum adsorption capacity of 2.26 mg/g was achieved over a wide range of pH which Mineralogical and Chemical Characteristics of Raw and Modified Clays and Their Application… http://dx.doi.org/10.5772/intechopen.74474 53

**Table 2.** Comparison of the adsorption capacity between raw and modified clay toward arsenic and fluoride.

Lenoble et al. [21] pillared montmorillonite clay using titanium, iron and aluminum as pillaring solution. Their results confirmed that pillaring increases the basal spacing and the specific surface area by many folds. These results were confirmed by Mishra and Mahato [40] who also observed increased in both specific surface area for Mn and Fe pillared bentonite clay. Beside the side the change in basal spacing and chemical oxide content in the pillared clays, the whole process of pillaring does not change the mineralogical composition of the clay.

Mishra and Mahato [40] evaluated the effectiveness of Fe and Mn pillared bentonite clay in As(V) and As(III) removal from groundwater. Maximum As(III) and As(V) adsorption capacities of 17.57 and 25.77 μg/g were observed for Fe pillared bentonite while adsorption capacities of 25.77 and 26.17 μg/g were observed for Mn pillared bentonite clay. This was higher by many folds compared to 4.31 and 4.33 μg/g for raw bentonite clay. This was attributed to increased surface area in the pillared bentonite and also increased charge density in the pillared clay.

Ren et al. [25] intercalated Fe polycations and cetyltrimethylammonium bromide (CTMAB) onto montmorillonite clay mineral and test it for arsenic removal. The results showed that CTMAB had diffused into the interlayers of the montmorillonite minerals while Fe polycations were adsorbed on the outer surface and had formed flocculent particles. When tested for arsenic removal, CTMAB-Fe montmorillonite and Fe montmorillonite showed a maximum adsorption capacities of 8.85 and 15.15 mg/g for As(V), respectively, and 13.89 and 16.13 mg/g for As(III). Furthermore, the optimum adsorption capacities were observed at wide range of pH of 4–10.

Several studies have been reported concerning the adsorption of fluoride using chemically modified clay minerals and the results showed improved adsorption efficiency as compared to unmodified clay soils. Kamble et al. [41] reported better sorption capacity for La, Mn and Mg oxides incorporated bentonite clay compared to bare bentonite. Efficiency of magnesium incorporated bentonite in adsorption of fluoride has also been evaluated by Thakre et al. [42] and maximum adsorption capacity of 2.26 mg/g was achieved over a wide range of pH which

**4.1. Arsenic removal using modified clay minerals**

52 Current Topics in the Utilization of Clay in Industrial and Medical Applications

**Figure 4.** Schematic diagram of a pillared clay [40].

**4.2. Fluoride removal using modified clay minerals**

was better than the capacity achieved with unmodified bentonite. Gitari et al. [20] reported maximum adsorption capacity of 2.91 mg/g for the Fe3+ modified bentonite clay which was also better than the unmodified bentonite. The main factor which was leading to improved sorption fluoride capacity was the increased surface area that corresponds to availability of more active sites for fluoride ion adsorption. Furthermore, modification of clay mineral my high density charges polycations increases the amount of positive charges in the surface of adsorbent leading to increased sorption capacity of anions.

**Table 2** summarizes the comparison between raw and modified clay soils toward fluoride and arsenic adsorption from water. It is observed that the adsorption of fluoride and arsenic increases by many fold after clay modification.