Obayomi K S, Bello J O, Yahya M D, Chukwunedum E, Adeoye J B
Department of Chemical Engineering, Landmark University Omu-Aran Kwara State, Nigeria.
Department of Chemical Engineering, Federal University of Technology Minna Niger State, Nigeria.
Heliyon. 2020 Jun 8;6(6):e04174. doi: 10.1016/j.heliyon.2020.e04174. eCollection 2020 Jun.
In this work, multiwall carbon nanotubes (MWCNTs) developed from cobalt-ferrite catalyst on activated carbon (from castor seed), was used as an adsorbent for the removal of cadmium and hexavalent chromium ions. The effectiveness of the adsorbent for the uptake of Cd(II) and Cr(VI)ions from aqueous solution was investigated in a process batch adsorption study. The developed activated carbon and MWCNTs were characterized by Brunauer-Emmett-Teller (BET) surface area analysis, Fourier Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) for the determination of surface area, functional group, and surface morphology, respectively. The BET surface area of activated carbon and developed adsorbent from Co-Fe/AC was 230.24 and 372.42 m/g, respectively. The operational parameters evaluated on the adsorption efficiency were solution pH, temperature, adsorbent dosage initial metal ions concentration, and contact time. The adsorption of Cd(II) and Cr(VI) were found to have attained equilibrium positions in 60 min for the concentration range tested, respectively. The four linearized adsorption isotherm models; Langmuir, Freundlich, Temkin and Dubinin Radushkevich (D-R) tested, when compared, revealed that Langmuir isotherm fitted well to the experimental data judging from the higher correlation coefficient values (R) and lower values of the error functions (chi-square (χ), the sum of square error (ERRSQ/SSE) and the sum of absolute error (EABS))with monolayer adsorption capacities of 404.858 and 243.902 mg/g for Cd(II) and Cr(VI) ions, respectively. Adsorption kinetic models investigated by pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion showed the conformity of pseudo-second-order model to the process adsorption as informed by the higher values R and Adj, R, maximum log-likelihood and smaller ERRSQ/SSE, χ, Akaike information criterion (AIC), Bayesian information criterion (BIC), and Hannan-Quinn information criterion (HQIC). The intraparticle diffusion model plots indicated that intraparticle diffusion was not the only rate-limiting step. Thermodynamic adsorption parameters (ΔH and ΔG, ΔS) showed that the adsorption of Cd (II) and Cr (VI) ions was spontaneous, endothermic, and increased in randomness between the adsorbate-adsorbent. The mean adsorption energy (E), the heat of adsorption (ΔH), and activation energy (E) values, revealed the adsorption mechanism of Cd(II) and Cr(VI) onto MWCNTs as a combination of chemical and physical adsorption but dominated more by chemical adsorption.
在本研究中，以钴铁氧体催化剂在（蓖麻籽来源的）活性炭上制备的多壁碳纳米管（MWCNTs）用作吸附剂，用于去除镉离子和六价铬离子。在间歇吸附过程研究中，考察了该吸附剂从水溶液中摄取Cd(II)和Cr(VI)离子的有效性。通过布鲁诺尔-埃米特-泰勒（BET）表面积分析、傅里叶变换红外光谱（FT-IR）和扫描电子显微镜（SEM）分别对制备的活性炭和MWCNTs进行表征，以测定表面积、官能团和表面形态。活性炭和Co-Fe/AC制备的吸附剂的BET表面积分别为230.24和372.42 m²/g。考察的对吸附效率有影响的操作参数包括溶液pH值、温度、吸附剂用量、初始金属离子浓度和接触时间。在所测试的浓度范围内，Cd(II)和Cr(VI)的吸附分别在60分钟内达到平衡位置。所测试的四个线性化吸附等温线模型；朗缪尔（Langmuir）、弗伦德里希（Freundlich）、坦金（Temkin）和杜宾宁-拉杜什克维奇（D-R）模型，比较发现，从较高的相关系数值（R）和较低的误差函数值（卡方（χ）、平方误差和（ERRSQ/SSE）以及绝对误差和（EABS））判断，朗缪尔等温线与实验数据拟合良好，Cd(II)和Cr(VI)离子的单层吸附容量分别为404.858和243.902 mg/g。通过拟一级、拟二级、埃洛维奇（Elovich）和颗粒内扩散模型研究的吸附动力学表明，拟二级模型与吸附过程相符，这由较高的R和调整后的R值、最大对数似然值以及较小的ERRSQ/SSE、χ、赤池信息准则（AIC）、贝叶斯信息准则（BIC）和汉南-奎因信息准则（HQIC）表明。颗粒内扩散模型图表明颗粒内扩散不是唯一的限速步骤。热力学吸附参数（ΔH、ΔG、ΔS）表明，Cd(II)和Cr(VI)离子的吸附是自发的、吸热的，且吸附质与吸附剂之间的随机性增加。平均吸附能（E）、吸附热（ΔH）和活化能（E）值表明，Cd(II)和Cr(VI)在MWCNTs上的吸附机制是化学吸附和物理吸附的结合，但以化学吸附为主。
