Elsevier

Water Research

Volume 205, 15 October 2021, 117692
Water Research

The suitability of titanium salts in coagulation removal of micropollutants and in alleviation of membrane fouling

https://doi.org/10.1016/j.watres.2021.117692Get rights and content

Highlights

  • Superior floc properties and low residues were the main merits of TXC.

  • TXC overperformed TiCl4 and PTC for toxic oxysalts and antibiotics removal.

  • TXC pre-coagulation could effectively alleviate ultrafiltration membrane fouling.

  • The diketone moieties make TXC less sensitive to coordination reactions with organics.

Abstract

Coagulation is a conventional method in water treatment. In recent decades, with the rapid development of membrane filtration, the use of coagulation is facing some new challenges. How to minimize the membrane fouling became a leading-edge topic in the study of coagulation. Here, the performances of three types of titanium coagulants were evaluated in terms of both the coagulation removal of toxic micropollutants and the alleviation of membrane fouling. Three oxysalts and two antibiotics were taken as representatives of inorganic and organic micropollutants. As compared with titanium tetrachloride (TiCl4) and polytitanium chloride (PTC), titanium xerogel (TXC) with a higher polymerization degree showed much better performances in direct coagulation removal of oxysalts and antibiotics and in pre-coagulation for mitigating membrane fouling in both coagulation-sedimentation-ultrafiltration (CSUF) and in-line coagulation-ultrafiltration (CUF) processes. In the CSUF system, the membrane permeate flux with TXC pre-coagulation (89.5%) was much higher than those of TiCl4 (56.1%) and PTC (57.4%). After a 5 day continuous operation, the transmembrane pressure in the CUF system with TXC coagulation was increased only to 4.9 kPa, while those of PTC and TiCl4 were 12.2 and 18.5 kPa, respectively. The results here demonstrate that TXC is a promising coagulant for pollutant removal and membrane fouling alleviation, due to the following merits: better floc properties, weaker pH-dependence, and higher resistance to coordination with organic pollutants. The observation shed new lights on the fabrication and application of coagulants in a wide variety of scenarios.

Introduction

Coagulation plays an important role in the removal of colloids, organic matter, and inorganic ions (Peydayesh et al., 2021). However, coagulation is not effective enough for the removal of many micropollutants. Therefore, with the increasing demand for deep treatment, membrane filtration becomes a necessary technology. When used in combination with coagulation process, membrane filtration can not only improve the performance of solid-liquid separation, but also improve the effluent quality by removal of trace contaminants (Huang et al., 2021). Toxic oxysalts and antibiotics can seriously threaten human health and damage the ecological system (Han et al., 2021; Ayala and Fernandez, 2020). In water treatment, coagulation and ultrafiltration technologies are recognized as the best options to eliminate non-biodegradable toxic elements and antibiotics because of the low cost, high efficiency and fast removal kinetics (Hu et al., 2015; Guo et al., 2018; Wang et al., 2021; Wang et al., 2018a; Park et al., 2018).

Coagulation-ultrafiltration hybrid processes with compact flow routes and high water quality assurance have received much attention and have been widely applied in water/wastewater treatment (Liang et al., 2021, Liu et al., 2017; Long et al., 2021). However, the application of coagulation-ultrafiltration hybrid processes has been greatly limited by membrane fouling (Yan et al., 2021). Excellent coagulation performance and suitable floc properties are beneficial to mitigate membrane fouling.

In recent years, titanium (Ti) coagulants have shown better coagulation performance than traditional aluminum (Al) and iron (Fe) coagulants for toxic oxysalts and organic matter removal (Chekli et al., 2017; Wan et al., 2019). The advantages of Ti coagulants over Al/Fe coagulants are mainly manifested in the following aspects: (1) greater floc growth rate and settling rate; (2) lower residual metal concentration after coagulation and lower side effects on subsequent processing units; (3) recyclable coagulation sludge with additional value (Gan et al., 2020, Gan et al., 2021; Zhao et al., 2020; Hossain et al., 2021). Due to the outstanding coagulation performance, Ti coagulation as pretreatment can effectively mitigate membrane fouling with or without sedimentation process (Zhao and Li, 2019; Ly et al., 2018). However, when simple Ti salts are used, the effluent pH after coagulation will drop dramatically and the coagulation performance is easily affected by the water qualities (Zhao et al., 2011). Increasing the polymerization degree of Ti coagulants is a way to overcome this inherent drawback (Zhao et al., 2013; Wang et al., 2016).

Titanium xerogel coagulant (TXC) with high polymerization degree has been obtained by controllable hydrolysis and polymerization of titanium tetrachloride (TiCl4) through a sol-gel process with acetylacetone (AcAc) as a hydrolysis inhibitor (Wang et al., 2016). Compared with polytitanium chloride (PTC) obtained from alkaline polymerization, the use of TXC led to a milder pH change (ΔpH < 0.5) (Wang et al., 2016). In addition, TXC exerted better coagulation performance for treatment of kaolin-humic acid (HA) simulated water and algae-containing water with complex biological metabolites (Wang et al., 2016, 2018b). A further analysis of the improved mechanisms indicates that the TXC hydrolysates have a larger network structure, higher surface charge density, more surface sites, and stronger binding affinity to pollutants (Wang et al., 2018c). Hence, TXC is expected to be a good candidate coagulant in the hybrid processes of coagulation-ultrafiltration, such as in-line coagulation-ultrafiltration (CUF) and coagulation-sedimentation-ultrafiltration (CSUF) processes. Nevertheless, the overall performance of TXC in coagulation-membrane filtration processes remains to be investigated.

In this work, the coagulation performance of TXC was evaluated with both inorganic oxysalts (As(III), Sb(III), and Se(IV)) and organic antibiotics (tetracycline (TC) and oxytetracycline (OTC)) as examples of micropollutants and with the other two Ti salts (TiCl4 and PTC) as references. Special attention was paid to membrane fouling in coagulation-ultrafiltration processes.

Section snippets

Materials

All chemicals were analytical reagent grade and used as received without further purification. Titanium isopropoxide (Ti(OPri)4), TiCl4, AcAc and L-antimony potassium tartrate (C8H4K2O12Sb2) were purchased from Shanghai Macklin Biochemical Co., Ltd., China. HCl, NaOH and ethanol (EtOH) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. TiCl4 was purchased from Aladdin Industrial Co., Ltd., China. Humic acid (HA) was purchased from Thermo Fisher Scientific Inc. (Acros Organics),

Coagulation removal of antibiotics

TC belongs to a typical type of antibiotic drugs. When measured with HPLC, the removal rate of TC increased to more than 98% as the dose of coagulants increased, and TiCl4 performed a little better than PTC and TXC (Fig. 1a). However, by measuring the TOC of the effluent, the actual removal rates were far lower than the HPLC data except for TXC. As the dosage was greater than 20 mg/L, the removal rates of TC by TiCl4 and PTC were reduced from 86.6% and 93.9% to 33.7% and 41.7%, respectively.

Conclusions

TXC with a high polymerization degree showed excellent coagulation performance for the removal of toxic oxysalts and antibiotics and performed better in alleviating ultrafiltration membrane fouling as compared with the simple Ti salt, TiCl4, and the lowly polymerized Ti salt, PTC. The merits of TXC coagulation include lower residual turbidity and Ti concentration, milder pH dropping, larger floc size, faster settling rate of flocs, and stronger resistance to coordination with organics. The

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.

Acknowledgments

This work was financially supported by the Key Technologies Research and Development Program of the Ministry of Science and Technology of the People's Republic of China (No. 2019YFC0408302) and the Jiangsu Provincial Key Research and Development Program, China (No. BE2018702).

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