Exceptional arsenic (III,V) removal performance of highly porous, nanostructured ZrO₂ spheres for fixed bed reactors and the full-scale system modeling

Highlights

• Highly porous, nanostructured zirconium oxide spheres for As(III)/As(V) removal.

• Dual-pore structure for easy liquid transport and effective contact with arsenic.

• Enhanced arsenic adsorption performance compared with their nanoparticle counterparts.

• Full-scale system modeling using validated pore surface diffusion model.

• Much larger amounts of water could be treated per gram of these ZrO₂ spheres.

Abstract

Highly porous, nanostructured zirconium oxide spheres were fabricated from ZrO₂ nanoparticles with the assistance of agar powder to form spheres with size at millimeter level followed with a heat treatment at 450 °C to remove agar network, which provided a simple, low-cost, and safe process for the synthesis of ZrO₂ spheres. These ZrO₂ spheres had a dual-pore structure, in which interconnected macropores were beneficial for liquid transport and the mesopores could largely increase their surface area (about 98 m²/g) for effective contact with arsenic species in water. These ZrO₂ spheres demonstrated an even better arsenic removal performance on both As(III) and As(V) than ZrO₂ nanoparticles, and could be readily applied to commonly used fixed-bed adsorption reactors in the industry. A short bed adsorbent test was conducted to validate the calculated external mass transport coefficient and the pore diffusion coefficient. The performance of full-scale fixed bed systems with these ZrO₂ spheres as the adsorber was estimated by the validated pore surface diffusion modeling. With the empty bed contact time (EBCT) at 10 min and the initial arsenic concentration at 30 ppb, the number of bed volumes that could be treated by these dry ZrO₂ spheres reached ∼255,000 BVs and ∼271,000 BVs for As(III) and As(V), respectively, until the maximum contaminant level of 10 ppb was reached. These ZrO₂ spheres are non-toxic, highly stable, and resistant to acid and alkali, have a high arsenic adsorption capacity, and could be easily adapted for various arsenic removal apparatus. Thus, these ZrO₂ spheres may have a promising potential for their application in water treatment practice.

Introduction

Arsenic contamination in water is a worldwide problem, which has caused a global issue of chronic arsenic poisoning with skin lesions, cancers and other symptoms either through contaminated drinking water, agriculture products irrigated by contaminated water, or directly exposure to contaminated water (Tripathi et al., 2007). Therefore, the effective removal of arsenic from water sources is essential to ensure the health and life quality for millions of people under arsenic contamination threats. Various approaches had been examined for arsenic removal, including membrane filtration (Ghurye et al., 2004), coprecipitation (Okamoto et al., 2010), ion exchange (Miller et al., 2000), biological method (Cavalca et al., 2013, Katsoyiannis et al., 2008) and photocatalysis (Fostier et al., 2008, Li et al., 2009). Among them, adsorption is believed to be a simple, cost-effective and eco-friendly process for arsenic removal, especially when the arsenic concentration is very low (<0.05 mg/L) (Mohan and Pittman, 2007). Both natural and synthesized adsorbents had been extensively investigated for the removal of arsenic (Altundogan et al., 2000, Amin et al., 2006, Chen et al., 2008, Haque et al., 2007, Lorenzen et al., 1995, Mamindy-Pajany et al., 2009, Mohapatra et al., 2007, Singh and Pant, 2006, Suzuki et al., 2001, Xu et al., 2002). In recent years, synthesized nanoadsorbents, such as zero valent iron, activated alumina, iron oxides, titanium oxides, zirconium oxides, cerium oxides and cupric oxides, demonstrated the superior arsenic removal performance because of their large surface areas from their nano-size and preferred surface properties (Cao et al., 2007, Cui et al., 2012a, Cui et al., 2012b, Dutta et al., 2004, Li et al., 2012a, Kim et al., 2004, Liu et al., 2010, Mamindy-Pajany et al., 2011, Mohan and Pittman, 2007, Pena et al., 2005, Pena et al., 2006, Singh and Pant, 2004, Tang et al., 2011, Tang et al., 2013, Xu et al., 2010). However, the direct use of nanoadsorbents in the commonly used fixed-bed reactor is very difficult because their small size makes the pressure drop too high to obtain a feasible flow rate and they might be released into the treated water to bring potential damages to natural organisms/environment and increase the operation cost. One possible solution to address this difficulty is to develop highly porous macrostructures loaded with these nanoparticles (Kanel et al., 2007, Sandoval et al., 2011, Sun et al., 2012, Thanh et al., 2012, Wang et al., 2008) or formed by these nanoparticles (Li et al., 2012b, Pan et al., 2013), which may maintain the advantages from their nano-characteristics while be convenient for their application in a fixed-bed reactor (Hristovski et al., 2008a).

Due to their stability, non-toxicity, and insolubility, zirconium based oxides could be an attractive choice for drinking water purification. Arsenic removal by zirconium based oxides had been reported in literature by nanoparticles or Zr-loaded resins (Balaji et al., 2005, Bortun et al., 2010a, Bortun et al., 2010b, Chitrakar et al., 2006, Hristovski et al., 2008b, Okumura et al., 1999, Okumura et al., 1998, Ren et al., 2012, Seko et al., 2004, Suzuki et al., 2000, Suzuki et al., 2001, Zhu and Jyo, 2001). Most of these reports only examined the As(V) removal, while reports on the removal of more mobilized/toxic As(III) were limited. For example, Suzuki et al. prepared a porous resin loaded with monoclinic or cubic hydrous zirconium oxide by incorporation of ZrOCl₂·8H₂O into porous spherical polymer beads followed with the hydrolysis and hydrothermal treatment, which had maximum arsenic adsorption capacities of ∼1.5 and 1.2 mmol/g for As(III) and As(V), respectively (Suzuki et al., 2000). Only a few reports had been made on porous, nanostructured ZrO₂ spheres. For example, Hristovski et al. synthesized porous, nanostructured ZrO₂ spheres with sub-millimeter size by the impregnation of macroporous ion-exchange media with zirconium salt followed with a high temperature treatment to remove the organic polymer resin (Hristovski et al., 2008b). However, they also only examined the As(V) removal performance of their ZrO₂ spheres. It was found that the maximum number of bed volumes that could be treated until reaching the maximum contaminant level of 10 ppb was ∼15200 BVs with the initial As(V) concentration at 30 ppb, which was comparable to that of commercially available iron (hydr)oxides (Badruzzaman, 2005, Hristovski et al., 2008b, Westerhoff et al., 2005). Due to their higher production cost than iron based (hydr)oxides, they suggested that their ZrO₂ spheres may be limited to specific applications where iron based (hydr)oxides could not be used (Hristovski et al., 2008b).

In our recent research, we developed zirconium oxide nanoparticles with high adsorption capacities for both As (III) and As (V) at near neutral pH environment with no pre-/post-treatment, which could largely lower the cost and the pollution risk from adding large amounts of chemicals into natural water bodies (Cui et al., 2012a, Cui et al., 2012b). Thus, by the creation of highly porous, nanostructured ZrO₂ spheres based on these ZrO₂ nanoparticles, they may possess a superior arsenic removal performance on both As(III) and As(V), which could subsequently decrease the cost and may make it feasible for their potential applications. Agar is a gelatinous substance extracted from red algae, which has an abundance existence in the sea. Thus, agar could be easily obtained with a relatively low cost. It is a safe food additive and has been widely used in food industry. One of its specific properties is that it has different melting temperature (∼85–95 °C) and solidification temperature (∼32–40 °C) in its water solution (Hjerten, 1964). In this work, highly porous, nanostructured ZrO₂ spheres were prepared by mixing ZrO₂ nanoparticles with agar powder in a boiling water solution, solidifying in the hexamethylene/carbon tetrachloride mixture in an ice bath to form spheres, and removing agar by the following heat treatment. These ZrO₂ spheres demonstrated a high arsenic adsorption capacity on both As(III) and As(V) at near neutral pH environment, even higher than that of their nanoparticle counterparts. The performance of full-scale fixed bed systems with these ZrO₂ spheres as the adsorber was estimated by the validated pore surface diffusion modeling, which demonstrated a much higher treatment capability than that of various lab-prepared or commercially available media that had been evaluated with column tests under similar conditions.