Kinetic and equilibrium analyses of lactate adsorption by Cu-Al and Mg-Al layered double hydroxides (Cu-Al LDH and Mg-Al LDH) and Cu-Al and Mg-Al layered double oxides (Cu-Al LDO and Mg-Al LDO)

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Abstract

Lactate is an important metabolite generated during the culture of pluripotent stem cells and is known to inhibit the cell growth. With the increase in the prominence of pluripotent stem cell applications, reliable methods are required to remove metabolites such as lactate for enabling the high-density culture of cells. Herein, we report an investigation of layered double hydroxides (LDHs) and layered double oxides (LDOs) as anion-adsorbents for lactate removal to regenerate the cell culture medium. The lactate absorption ability of Cu-Al and Mg-Al LDH and Cu-Al and Mg-Al LDO adsorbents was evaluated. The influence of various process parameters, including temperature and pH, on the lactate adsorption, were investigated. The kinetics of the adsorption process and equilibrium dynamics were studied, and the maximum adsorption amounts were determined. The Cu-Al LDH, Cu-Al LDO, Mg-Al LDH, and Mg-Al LDO exhibited a lactate adsorption that followed a pseudo-second-order kinetic model, and their apparent activation energies for adsorption were 32.0, 47.4, 95.7, and 61.7 kJ/mol, respectively; these values indicated that chemical adsorption was the underlying mechanism of adsorption. The Langmuir and Freundlich models were analyzed and revealed the nature of the anion-exchange process. This study is expected to promote the high-density culture of cells by ensuring the regeneration of the media by the efficient removal of the undesired lactate.

Introduction

With the recent increase in the applications of pluripotent stem cells such as iPS cells in regenerative medicine and drug discovery, the cultivation of large volumes of high-quality cells is required [1]. The floating culture method is a widely used approach for the high-density culture of cells. However, this approach has several limitations. For example, the depletion of nutrients in the culture medium during cell growth and the occurrence of cell cytotoxicity due to the increase in the concentration of metabolites produced during cell metabolism result in the inhibition of the growth rate [2]. Lactate is a metabolite that poses several problems during the high-density culture of cells. For improving the cost efficiency and development of low-cost technologies for the high-density culture of cells, the cell-growth inhibitors should be removed from the culture medium without losing the nutrients. Electrolysis [3] and culture medium modification [4] are known approaches for removing cell-growth inhibitors from culture media. However, these approaches are operationally complicated and expensive. The purpose of this study is to establish a reliable process for the regeneration of culture media. We investigated the removal of lactate by adsorption and developed an operation that is easy to perform and versatile. Lactate has a pKa value of 3.86 and exists in anionic form (CH3CH(OH)COO) in culture media with a typical pH in the range 7.2–7.4. Therefore, we focused on evaluating the performance of layered double hydroxides (LDHs) and layered double oxide (LDOs) as anion-adsorbents for the removal of lactate.

LDH comprises two types of layer structures that are alternately stacked. The first is a positively charged octahedral host layer, wherein the positive charge results from the replacement of some of the M2+ ions in the divalent metal hydroxide M(OH)2 with M3+ ions. The guest layers consist of anions to compensate the positive charge of the host layer, and the interlayer comprises water [5], [6], [7], [8]. The M2+ divalent metal cation is typically Mg2+, Zn2+, Cu2+, or Ni2+, and the M3+ is a trivalent metal cation such as Al3+ and Fe3+. Because repulsion between the positive charges increases when the proportion of trivalent metal ions in the layer increases, the ratio x is generally in the range of 0.20–0.33 range. LDH is a layered compound with anion-exchange capacity and is reported to be highly selective toward anions with high charge density [9]. Recently, LDH has been widely examined for wastewater treatment [10], [11], [12]. Upon heating, the interlayer water of LDH is first released, and with further heating, the LDH decomposes to form LDO [13]. In this study, Mg-Al LDO, which is the most commonly used LDO, has been investigated for lactate adsorption. CO32-intercalated Mg-Al LDH (CO3Mg-Al LDH) is known to release the interlayer water at temperatures below 300 °C. At 400–450 °C, the hydroxyl groups in the host layer are condensed and dehydrated to form a NaCl-type magnesium-aluminum oxide solid solution (Mg-Al LDO) with cationic defects (see Eq. (1)). The composition of Mg-Al LDO is given by the formula Mg1−xAlxO1+x2. In addition, this LDO decomposes into MgO and MgAl2O4 above 800 °C [14]. In aqueous solutions, Mg-Al LDO can intercalate anions and regenerate the LDH structure (see Eq. (2)) [13]. This reaction has been reported to be highly selective toward anions with high charge density, similar to LDH [15]. Mg1xAlx(OH)2(CO3)x∕2Mg1xAlxO1+x2+x/2CO2+H2O(0.20x0.33)Mg1xAlxO1+x2+x/nAn−+(1+x2)H2OMg1xAlx(OH)2Ax∕n+xOH(0.20x0.33) In our previous study [16], we found that NO3•Mg-Al LDH is an excellent lactate adsorbent in culture medium when the intercalated anion is inorganic. Mg-Al LDO was also suitable as a lactic acid adsorbent in the culture medium. Therefore, in this study, the adsorption mechanism was investigated by analyzing the kinetics and equilibrium of lactate adsorption by NO3•Mg-Al LDH and Mg-Al LDO. However, although NO3•Ni-Al LDH has a high lactic acid adsorption property in the culture medium, it is highly toxic, indicating that it is unsuitable as a lactic acid adsorbent in the culture medium. In the adsorption process, a lactate treatment method with high pH stability is required to ensure that the pH of the solution after the treatment does not significantly deviate from that of the culture medium (pH 7.2–7.4). Therefore, in this study, we investigated the use of the pH-stable Cu-Al LDH. Since the solubility product of Cu(OH)2 is much smaller than that of Mg(OH)2, Cu-Al LDH is synthesized at pH 8–10, while Mg-Al LDH is typically synthesized at pH 10–11 [17], [18], [19]. Therefore, Cu-Al LDH is stable under neutral conditions [19]. In addition, Cu-Al LDO was used because LDOs generally have greater adsorption capacity than LDHs. Therefore, the adsorption mechanism of Cu-Al LDHs and Cu-Al LDOs was also investigated by analyzing the kinetics and equilibrium of lactate adsorption. In addition, the lactate adsorption and cytotoxicity of Cu-Al LDHs and Cu-Al LDOs in culture medium were examined.

Section snippets

Experimental

All the reagents were of chemical reagent grade and were used as received without further purification. NO3•Cu-Al LDH with a Cu/Al molar ratio of 3.4 [19], Cu-Al LDO with a Cu/Al molar ratio of 3.3 [20], [21], NO3•Mg-Al LDH with a Mg/Al molar ratio of 2.8 [22], and Mg-Al LDO with a Mg/Al molar ratio of 3.2 [23] were synthesized according to previous reports and used as adsorbents in this study. For each experiment, 20 mL of 1–50 mmol/L lactate aqueous solution (initial pH 7.2) was poured into a

Lactate adsorption in an aqueous solution

We studied the various factors that influence the lactate adsorption by the adsorbents, such as temperature and pH. The effect of temperature on the lactate adsorption by Cu-Al LDH and Cu-Al LDO over time is presented in Fig. 1. For both Cu-Al LDH and Cu-Al LDO, the lactate adsorption increased rapidly and reached a constant value in a short time at each temperature. Fig. S1 shows the effect of temperature on the solution pH during lactate adsorption by Cu-Al LDH and Cu-Al LDO over time. For

Conclusions

The Cu-Al LDH, Cu-Al LDO, Mg-Al LDH, and Mg-Al LDO exhibited a lactate adsorption that follows a pseudo-second-order kinetic model, and their apparent activation energies for adsorption were found to be 32.0, 47.4, 95.7, and 61.7 kJ/mol, respectively. These results suggest that the lactate adsorption is governed by chemical adsorption. Further, the lactate adsorption by Cu-Al LDH, Cu-Al LDO, Mg-Al LDH, and Mg-Al LDO was shown to follow the Langmuir model. The maximum adsorption amount was

CRediT authorship contribution statement

Tomohito Kameda: Conceptualization, Supervision, Writing - original draft, Writing - review & editing. Kazuya Horikoshi: Investigation. Hanako Kikuchi: Investigation. Fumihiko Kitagawa: Conceptualization, Supervision, Writing - original draft, Writing - review & editing. Shogo Kumagai: Resources, Visualization. Yuko Saito: Investigation. Masayuki Kondo: Investigation. Yoichi Jimbo: Project administration. Toshiaki Yoshioka: Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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