Biosorption of copper using nopal fibres: moolooite formation and magnesium role in the reactive crystallization mechanism

Ara Carballo-Meilan,Tania E Lara-Ceniceros,Paul Russell,José Bonilla-Cruz,Gustavo Sosa-Loyde,Chun-Yang Yin,Alejandra García-García,Zulfiqur Ali,Elizabeth Hernández-Francisco,Francisco Longoria-Rodríguez,Alejandro Arizpe-Zapata

doi:10.1007/s10570-020-03449-3

Ara Carballo-Meilan, Tania E Lara-Ceniceros + Show 9 more

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https://doi.org/10.1007/s10570-020-03449-3

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Abstract

In this contribution, we present findings on biosorption of Cu (II) ions using novel alkali-treated nopal fibres. The biosorption data at equilibrium were fitted to several isotherm models and the biosorbent was characterized by XRD and SEM–EDX. The biosorption mechanism was investigated using a holistic approach of pH shifts, apparent colour variations and changes in the concentration of Cu(II) and dissolved hard ions (calcium and magnesium) in the Cu(II) solution. The correlation between the colour, XRD analysis, pH shifts and hard cations released from the biosorbent into the solution suggested the existence of two crystal formations, malachite and moolooite, in what appears to be a microprecipitation mechanism via reactive crystallization. The role of magnesium during the transformation of malachite into moolooite during the copper binding mechanism is analysed. Magnesium cations were released into the solution during malachite growth but were taken up from the solution during the moolooite crystal growth phase. The shift of the molar ratio Mg/Ca was located between the two inflexion points of the crystal growth transition. This specific location at the sorption isotherm was correlated with the colour evolution by a linear discriminant model confirming its association with the polymorphs.Graphic abstract

Highlights

Opuntia Ficus-Indica, commonly referred to as nopal, is a type of cactus classified in the Opuntioideae genus in the Cactaceae family (Stevens 2017)
In the X-ray diffractometry (XRD) analysis, the peak at around 2h 1⁄4 22 would have been used to determine the degree of crystallinity of the cellulose fibres from Opuntia cactus skin (Ramezani Kakroodi et al 2015)
The following Ca/Mg crystalline compounds were identified on Opuntia ficus-indica (Contreras-Padilla et al 2015, 2016), Opuntia ellisiana (Monje and Baran 2005) and Opuntia basilaris (Ramezani Kakroodi et al 2015) species using XRD and SEM analyses: calcium carbonate [CaCO3], calcium-magnesium bicarbonate [CaMg(CO3)2], magnesium oxide [MgO], monohydrated calcium oxalate [Ca(C2O4),(H2O)] and dihydrated magnesium oxalate [Mg(C2O4),2(H2O)]

Summary

Introduction

Opuntia Ficus-Indica, commonly referred to as nopal, is a type of cactus classified in the Opuntioideae genus in the Cactaceae family (Stevens 2017). In the context of water treatment, biosorbents derived from nopal generally exhibit good adsorption capacities This has been partially attributed to their multi-functional surface characteristics which incorporate phosphate, phenolic, hydroxyl, carboxylic, carbonyl, amide, amine and alkyl groups. If an ion exchange process were responsible for the metal binding mechanism, the metal cations from the solution would generally be replaced with protons or other cations such as calcium or magnesium (the ones typically reported) in equivalent amounts, indicating that the charge released from and taken by the surface is balanced (Schneider et al 2001; Kotrba et al 2011). This is reflected in the following reaction: SH þ Mþ

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