Functionalized MgAl-layered hydroxide intercalated date-palm biochar for Enhanced Uptake of Cationic dye: Kinetics, isotherm and thermodynamic studies

Highlights

• A sustainable biochar decorated LDH composites adsorbent was produced from a MgAl-LDH precursor.

• Biochar-MgAl composite exhibit surface area (441.06 mg/g), micropore volume (0.360 cm³/g) and abundant functional groups.

• The B-MgAl composite reveal rapid MB uptake with sorption capacity improved to 302.75 mg/g related to biochar (206.61 mg/g).

• Internal pore filling, ion-exchange and chemical interactions mainly involved in MB adsorption.

Abstract

MgAl-layered hydroxide intercalated date-palm biochar composites (B-MgAl-LDH) functionalized with improved textural properties and surface functionalities were fabricated by a facile co-precipitation technique and fully characterized by various techniques. The B-MgAl-LDH were employed for enhanced aqueous uptake of methylene blue (MB) (Langmuir q_max = 302.75 mg/g at 100 mg/L in 180 min), compared with the virgin biochar (q_max = 206.61 mg/g in 480 min). Higher MB removal is attributed to the excellent hybridization of the two adsorbents which resulted in substantial improvement in surface area, micropore volume, and abundant surface functional groups, facilitating fast and better adsorptive removal of MB from water. The B-MgAl-LDH showed good adsorptive performances of MB at varied pH values (4–10). The kinetic data were best described by the pseudo-2nd-order while the adsorption isotherms fitted best to the Langmuir model. The thermodynamic parameters indicated the exothermic nature of MB adsorption and reduction in the degree of randomness at the B-MgAl-LDH /MB interface. The adsorption mechanism of MB onto biochar-MgAl composite is mainly governed by the surface adsorption and internal pore diffusion along with chemical and ion-exchange interaction as confirmed by BET, SEM, and FTIR characterization after the MB adsorption. Therefore, the modification of date palm biochar with MgAl via co-precipitation method is an effective approach to produce B-MgAl-LDH composite for the excellent removal of toxic dyes from wastewater streams.

Introduction

The presence of toxic organic dyes at high concentration levels in the wastewater streams arising from their widespread demand and utilization in textile, cosmetic, paper, leather, food, and medical industries poses serious threats to the environmental health (Mu'azu et al., 2018b). Methylene blue (MB) is a popularly used toxic dye that is highly stable to light and oxidation and thus difficult to degrade. A high concentration of MB (>7.0 mg/kg) may result in severe effects on human health, such as the increase in blood pressure, nausea, psychological problem, and intestinal pain (Zhu et al., 2018). Subsequently, the development of an efficient purification method for dye-bearing wastewater is of great environmentally important. Amongst various purification techniques, the adsorption has been considered as one of the most effective purification methods due to its simple and low-cost operation, fast removal rate, and higher removal performance compared to other treatment techniques (Zubair et al., 2018a).

During the last two decades, various materials such as carbon nanotube (Rodrigues et al., 2019), graphene oxide (Yang et al., 2019), bentonite (Babu and Antony, 2019), activated carbon (Tong et al., 2018) and magnetic materials (Palza et al., 2019) have been investigated for MB removal from waste streams. However, most of these adsorbent materials exhibit some disadvantages, including high cost, high dosage, long contact time, and limited reusability despite their good sorption performances. Therefore, the scientific domain has focused on designing and developing eco-friendly and sustainable materials exhibiting effective and fast removal of MB and other dyes from wastewaters. Biochars, circular economy-based carbonaceous materials (Lin et al., 2018), have increasingly been explored as adsorbent materials, owing to their several distinctive characteristics such as high microporosity and surface area, abundant oxygen functionalities, and excellent exchange capacity for ions (Park et al., 2019). These unique properties validate biochars as extremely promising, low cost, eco-friendly, and sustainable materials for effective purification of organic or inorganic contaminated wastewater streams (Ahmed et al., 2016).

Date palms are one of the major cultivated trees globally, with more than 300 different species and tens of million tonnes of annual production. This generates an enormous capacity of date palm wastes consisting of fronds, leaves, and trunks (Al-Kutti et al., 2017). Therefore, eco-friendly utilization of this wastes into beneficial products is one of the key points of sustainable development. Recently, a composite of date palm ash with layered double hydroxide (LDH) resulted in an eco-economical and sustainable adsorbent for the removal of anionic dyes (Blaisi et al., 2018). Usman et al. reported the influence of temperature during pyrolysis on the composition and morphology of biochar (Usman et al., 2015).

Recently, biochar-LDH based composites open a new window of research for higher and fast removal rates of various pollutants. For instance, Zhang et al. reported an improved adsorption capacity (410 mg/g) of phosphate ion from water using MgAl co-precipitated onto the biochar matrix (Zhang et al., 2013). Wang et al. reported an efficient removal of arsenic with excellent reusability of biochar-NiMn-LDH compared to NiMn-LDH (Wang et al., 2016). Tan et al. produced decorated MgAl- ramie stem (Boehmeria nivea L.) via simple pyrolysis and reported as an excellent sorption agent for crystal violet dye (Tan et al., 2016a). Similarly, Li et al. demonstrated the molar ratio of MgAl on the biochar matrix as a vital factor affecting phosphate adsorption performance (Li et al., 2016). Meili et al. evaluated the sorption performance of MB on the different molar ratios of MgAl (2:1, 3:1, and 4:1) on bone-biochar. The author reported that biochar-MgAl with molar ratio 2:1 and 3:1 exhibited a greater affinity for MB compared to MgAl and biochar alone (Meili et al., 2019). Previous studies demonstrated that the coupling of biochar with LDH is a favorable approach to produce high performance, sustainable, and eco-economical adsorbent materials. Thus, hybridization of LDH with biochars produced from date palm waste has a potential to produce versatile adsorbents with inherent and exceptional ion exchange ability, high specific surface area, and less toxicity for organic contaminated wastewater treatment. However, research gap along this area involves understanding the influence of biochar/MgAl mass ratios on adsorptive performance and uptake mechanism of dyes removal from water.

The objective of this study is to functionalize biochar with MgAl LDH composites at different biochar content (1–10 g) via a co-precipitation technique. The effect of biochar amount on the final characteristics of biochar-MgAl composites and the sorption performance and mechanism of MB dye were evaluated. The surface functionalities, structure, surface characteristics, and morphology of the produced B-MgAl were examined using various characterization techniques, including FTIR, XRD, BET, SEM, and TEM, respectively. The effect of adsorption parameters and mechanism of MB dye adsorption onto biochar-MgAl composite were evaluated using isotherm, kinetics, thermodynamic studies, and characterizations of the spent biochar-MgAl composite.