A constant composition technique for quantifying the effect of As(V) on struvite crystallization under various operational conditions

Highlights

• Constant composition technique accurately measured the crystal growth rate of struvite.

• The roles of the coprecipitation and adsorption of As(V) in its interaction with struvite were evaluated.

• The inhibitory efficiency of As(V) on struvite crystallization was up to almost 50%.

• As(V) scarcely changed the crystal growth mode of struvite.

• The inhibitory effect of As(V) on struvite crystallization is dependent of supersaturation, pH and strength.

Abstract

Struvite crystallization was one of the most promising techniques for nutrient recovery from nutrient-rich wastewater. However, As(V) derived from the wastewater has been proved to adversely affect struvite crystallization. Constant composition technique was employed to quantify the effect of As(V) on struvite crystallization in the presence of As(V) under various operational conditions such as supersaturation, pH and ionic strength. Subsequently, the interaction mechanisms between As(V) and struvite crystals were further elucidated by means of XPS and FTIR. Interestingly, the roles of adsorption and coprecipitation in the interaction under various operational conditions were primarily evaluated. At a supersaturation of 0.4, pH 8.5 and 0.1 mol/L NaCl, the crystal growth rate of struvite was substantially reduced from (14.2 ± 0.3) × 10⁻⁶ to (7.6 ± 0.1) × 10⁻⁶ mol/min with As(V) concentration increasing from 0 to 10 mg/L. Nevertheless, As(V) scarcely changed the surface diffusion growth mechanism of struvite. Moreover, the adsorption and coprecipitation of As(V) accounted for its interaction with struvite. Thereinto, the adsorption mechanism was triggered by the bidentate inner-sphere surface complexes and the electrostatic interaction between As(V) and struvite. Furthermore, the influence of supersaturation and ionic strength on the inhibitory effect was found to be highly dependent of the adsorption of As(V), whereas the coprecipitation of As(V) is mainly responsible for the impact of pH. The finding herein provided a novel approach to elucidate the inhibitory mechanisms of heavy metals on struvite crystallization, which facilitated to obtain pure struvite crystals from the wastewater contaminated by heavy metals.

Introduction

To reconcile the increasing restrictions of effluent nutrient with global phosphorus scarcity, struvite crystallization is considered as one of the most potential technologies for nutrient recovery from nutrient-rich wastewater [1], [2], [3]. Thus, numerous studies have so far been devoted to nutrient recovery from various streams [4], [5], [6], [7], [8]. However, the impurities in the streams may exert a negative impact on struvite crystallization as well as the quality of the crystals [9], [10], [11], thereby limiting the full-scale application as slow release fertilizers in agriculture for substituting P fertilizers.

Various waste streams for nutrient recovery such as manure [12], [13], sludge [14], [15] and urine [16] commonly contain considerable amounts of arsenic with its maximum concentration up to about 10 mg/L [17], which may cause a great risk to human health. Typically, there are two main arsenic species in the wastewater, namely arsenite [As(III)] and arsenate [As(V)]. Although As(V) is less toxic than As(III) in natural environments, it is a predominant species and more prone to adsorb on struvite crystals under alkaline conditions[18], [19]. Moreover, As(V) can compete with the phosphate ion of struvite to form arsenstruvite (MgNH₄AsO₄·6H₂O), which is often considered as the coprecipitation with struvite [20], [21]. Thereby, to clarify As(V) interfere with struvite crystallization is of vital significance.

Recently, several authors have resorted to investigate the effect of As(V) on struvite crystallization as well as the interaction mechanism thereof [17], [21]. These studies quantitatively assessed As(V) removal, phosphorus recovery before and after struvite crystallization as well as the interaction mechanism thereof. Nevertheless, little attention was paid to explore the effect of As(V) on the crystal growth rate of struvite as well as the growth kinetics. The phenomenon was likely ascribed to the fact that the crystal growth rate of struvite is hard to precisely determined due to its successive drop during the crystallization process. Indisputably, the crystal growth rate of struvite not only directly determines the quality and quantity of the targeted crystals derived from the wastewater, but also dominates the inhibitory mechanism of the impurity on struvite crystallization [22], [23]. As such, an approach to accurately quantify the crystal grow rate of struvite in the presence of As(V) was proposed in the present study. Moreover, recent studies also revealed that the interaction mechanism was related to the adsorption and coprecipitation, which was both proceeded by the forming Mg–O–As bonds in struvite and arsenstruvite, respectively [21], [24]. Thereby, to further elucidate their roles in the interaction is indispensable. As a rule, their roles are likely dependent on the various conditions such as supersaturation, pH and ionic strength. However, to date, their roles in the interaction under various operational conditions have remained elusive.

According to the basic principles of constant composition technique [25], the inhibitory efficiency of As(V) is largely related to its adsorption on struvite surface, but has no discernible concern with its coprecipitation with struvite due to its exchange with phosphate in the crystal lattice. Comparatively, the amount of As(V) removal per gram of struvite was controlled by both the adsorption and the coprecipitation. Consequently, to accurately calculate the inhibitory efficiency of As(V) and its removal amount per gram of struvite under various operational conditions can primarily evaluate their roles in the interaction. In other words, constant composition technique is an effective approach to discriminate their roles in the effect of heavy metals on struvite crystallization. However, few studies have been so far resorted to the approach to elucidate the effect mechanism.

Therefore, in the present study, constant component technique was employed to accurately quantify the effect of As(V) on struvite crystallization under various operational conditions such as supersaturation, pH and ionic strength. Moreover, this study also attempted to elucidate the interaction mechanism between As(V) and struvite as well as the roles of adsorption and coprecipitation under various operational conditions.