Coagulation of dissolved organic matter in surface water by novel titanium (III) chloride: Mechanistic surface chemical and spectroscopic characterisation

Highlights

• Coagulation performances of novel titanium (III) chloride (TiCl₃) was studied.

• TiCl₃ showed a significantly higher capacity for DOC removal at optimum pH and dose.

• Flocs formed by TiCl₃ were larger in size than that from alum at optimum pH and dose.

• Charge neutralization was the dominant removal mechanism for DOC removal by TiCl_3.

• TiCl₃ could be used in the treatment of low alkalinity water and low pH wastewaters.

Abstract

Problems caused by residual organics in treated water include the formation of disinfection by-products (DBP) following reaction with chlorine and being a substrate for microbial regrowth in the drinking water distribution system. Dissolved organic matter (DOM) can only be partially removed by conventional treatment process i.e. coagulation by Al- and Fe-based salts. In the present study, the performance of titanium trichloride (TiCl₃) as a coagulant for surface water treatment was studied and compared with conventionally used aluminum sulfate (alum). Jar test experiments were performed at various coagulant doses and pH levels to determine the optimum conditions based on removal efficiencies of dissolved organic carbon (DOC). The zeta potential values were analysed for assessing the destabilisation mechanism of DOM flocs. The TiCl₃ showed a significantly higher capacity for DOC removal at pH around 3 at which charge neutralization was found to be the dominant mechanism for the floc formation. This was further evident from the relatively larger floc sizes obtained with TiCl₃ treatment. However, destabilization of Ti-flocs occurred at pH 4.5 through an adsorption-enmeshment mechanism due to a highly negative zeta potential. Additionally, fluorescence spectroscopic analyses showed that TiCl₃ was more efficient than alum in removing humic compounds. A two-stage treatment process by alum and TiCl₃, either as the same chemical or both showed better performance than a single dose treatment. The results indicate that TiCl₃ could be an effective alternative coagulant for the treatment of waters, particularly those of low alkalinity and high DOC concentration and low pH wastewaters for removal of hydrophobic compounds and particulate matter.

Introduction

The removal of dissolved and particulate organic matter from surface waters for drinking water supply using physio-chemical processes such as coagulation, adsorption, ion exchange, membrane filtration and advance oxidation have been well documented in the literature. Among them, coagulation is a widely used method for the removal of dissolved organic matter (DOM) and suspended particles [1], [2], [3] despite that DOM can only be partially removed by this process. Enhanced coagulation refers to maximizing the removal of DOM from drinking water sources by increasing the coagulant dose and/or optimizing coagulation pH. This removal occurs primarily through two major mechanisms: (1) charge neutralization, and (2) sweep coagulation or adsorption/entrapment where organic matter adsorbs onto the surface of insoluble metal hydrous oxide precipitates [4]. Conventional coagulants such as Al and Fe based salts (e.g. alum, ferric chloride, polyferric sulphate and polyaluminum chloride) are widely used for drinking water treatment for their reliable performances, commercial availability and relatively inexpensive costs. However, the use of high doses of these coagulants for maximizing DOM removal from source waters with high concentrations can result in production of large amounts of sludge that requires further treatment and solids waste disposal [5], [6]. Practices such as incineration and disposal to landfills are costly with potential environmental impacts [7]. Thus, water treatment plant operators and managers can face significant challenges in treatment processes when Al or Fe based coagulants are used, especially when operational conditions are challenged by poor source water quality.

With high concentrations of residual organic matter present in treated waters, potential problems can occur such as reduced aesthetic quality (colour, taste and odour compounds), reaction with chlorine lowering or removing chlorine residual and bacterial regrowth in the distribution system. Residual DOM in treated drinking water can readily react with chemical (oxidizing) disinfectants such as chlorine resulting in the formation of potentially carcinogenic, cytotoxic or genotoxic disinfection by-products such as trihalomethanes (THM) and nitrogenous DBPs (N-DBPs) such as haloacetonitriles (HAN) and halonitromethanes (HNM) [8], [9]. This is a general concern for drinking water supply companies and authorities globally. In recent years, it has been reported that DOM levels in water resources have increased, which may be due to climate change [4], [10] and in Australia, to extreme climate events [11], [12]. In 2010–2011 and 2011–2012 strong La Niña events occurred in Australia that resulted in the Murray-Darling Basin experiencing high rainfall which led to major and widespread floods [13]. These events resulted in significant declines in water quality where dissolved organic carbon (DOC) levels exceeded 15 mg/L [12].

Drinking water treatment using conventional metal-based coagulants (Al and Fe) is able to remove only a fraction of organic matter present dependent on its characters where aromatic, hydrophobic and high-molecular weight (HMW) compounds as of humic substances are amenable to removal but low-molecular weight (LMW), hydrophilic compounds tend to be recalcitrant to removal by coagulation [14]. Research and development on particular hydrolyzed metal species such as Al₁₃ as reported by Lin et al. [15] exemplifies efforts to improve performances of metal coagulants, including for improved DOC removals.

Research has been conducted to find alternative metal salts that have higher charge neutralization and greater DOC removal capacity with the formation of larger size flocs that have higher settling rates than conventional salts [16], [17], [18], [19]. Highly charged Ti (IV) and Zr (IV) based coagulants such as titanium tetrachloride (TiCl₄) and zirconium tetrachloride have been shown to have capacity for higher DOC removals with the formation of larger sized flocs than Al and Fe based coagulants [6], [17], [18], [20]. Various hydrolysed species of Ti such as Ti(OH)³⁺, Ti(OH)₂²⁺, Ti(OH)₃⁺, Ti(OH)₄⁰, Ti(OH)₅¹⁻, Ti (O₂)₂ (OH)₂² and Ti (O)(O₂) (OH)₂²⁻ are formed at different pH levels and Ti doses, and these species play an important role during the particle stabilization [21], [22]. The first investigation of Ti salt for coagulation was reported by Upton and Buswell [23] who found that Ti(SO₄)₂, as a tetravalent cationic salt, showed a better coagulation efficiency for fluoride removal than trivalent Al or Fe salts. Zhao et al. [24] developed a novel polytitanium tetrachloride (PTC) as pre-hydrolysed coagulant which showed better performance than TiCl₄. Shon et al. [16] reported that TiCl₄ had good performance for the removal of various apparent molecular weight compounds of DOM from wastewater. They also reported that the Ti-based sludge formed following coagulation treatment can be converted to value added materials (e.g., TiO₂) in a simple process [16], [25], [26]. Zhao et al. [18] reported that coagulation behavior of titanium tetrachloride (TiCl₄) was very similar to Al and Fe based salts. However, TiCl₄ is volatile and forms cloudy TiO₂ and HCl in humid air conditions at room temperature and being hazardous [27], there is need for other Ti based salts that are more stable under ambient conditions, are safe, reliable and readily prepared. This study investigated the potential of titanium (III) chloride as a coagulant under a range of conditions including coagulation pH and at various dose rates in comparison to and in combination with alum (aluminium sulphate). This study investigated the coagulation mechanisms of TiCl₃ by exploring the surface chemistry of flocs and spectroscopic characterization of residual DOM post TiCl₃ treatment.