Excessive supply of phosphorus (P), a vital macronutrient for all organisms, can cause unwanted environmental consequences such as eutrophication. An increase in agricultural and industrial activities has created a considerable imbalance in the P cycle with continuing adverse effects on sustainability and ecosystem health, thereby stipulating/postulating the significance of P removal. However, it is challenging for conventional P removal technologies to remove both high and low concentration of P efficiently and economically from aqueous solution. Therefore, in the present study a unique and sustainable concept for the removal of P through the utilization of waste bivalve seashells was proposed.
Continuous dumping of seashells in open fields has been a global issue, causing serious problems in the water and human health. The conversion of those wastes into value-added products is highly desirable. Here, nano-calcium hydroxide (N-CH) were first synthesized from waste seashells by a chemical precipitation method in an aqueous medium at 90 °C without using any additives. The crystal structure with a hexagonal portlandite [Ca(OH)2], crystal size of around 100?400 nm, and specific surface area with 4.96 m2 g-1 were confirmed. In addition, a schematically organized new qualitative model for a mechanism was proposed to explain the genesis and evolution of N-CH from raw seashells. Furthermore, experimental results revealed successful removal of high concentration of P (> 20 mg L-1) from aqueous solution from N-CH synthesized from various type of seashell with similar P removal efficiencies of ~ 97 %. An optimization study has been conducted using the Box?Behnken design of response surface methodology, which highlights that with a calcium/phosphorus mass ratio, pH, and temperature of 2.16, 10.2, and ~ 25 °C respectively, a P removal efficiency of 99.33% can be achieved in a residence time of 10 min. Also, under the same conditions, diluted human urine was analyzed and P removal efficiency of ∼ 95 % was observed. Through experimental results, semiquantitative phase analysis, and transmission electron microscopy, it has been found that the reaction was diffusion-controlled, which was further confirmed through shrinking core diffusion modeling. The present study manifests the promising potential of waste seashell-derived nano-calcium hydroxide for phosphorus treatment and its precipitation in the form of value-added hydroxyapatite.
Even in low P concentration (≤ 1 mg L-1), P can be detrimental for ecosystem’s health, but this aspect has not been thoroughly investigated. The elimination of low P content is rather expensive or complex. Therefore, a sustainable method has been proposed in which valorized bivalve seashells can be used for the removal of low P content. Initially, acicular shaped aragonite particles (~ 21 μm) with an aspect ratio of around 21 have been synthesized through the wet carbonation process. Also, a schematic crystal growth mechanism was proposed to demonstrate the genesis and progression of aragonite crystal. Green aragonite can bridge the void for numerous applications and holds the potential for the commercial-scale synthesis with bivalve seashells as low-cost precursors. Green aragonite were then subsequently used to treat aqueous solutions containing P in low concentration (P ≤ 1 mg L-1). Response surface methodology-based Box-Behnken design has been employed for optimization study which revealed that with aragonite dosage (140 mg), equilibrium pH (~ 10.15), and temperature (45 ℃), a phosphorus removal efficiency of ~ 97 % can be obtained in 10 h. The kinetics and isotherm studies have also been carried out (within the range P ≤ 1 mg L-1) to investigate a probable removal mechanism. Also, aragonite demonstrates higher selectivity (> 70 %) towards phosphate with coexisting anions such as nitrate, chloride, sulfate, and carbonate. Through experimental data, elemental mapping, and molecular dynamic simulation, it has been observed that the removal mechanism involved a combination of electrostatic adsorption of Ca2+ ions on aragonite surface and chemical interaction between the calcium and phosphate ions. The present work demonstrates a sustainable and propitious potential of seashell derived aragonite for the removal of low P content in aqueous solution along with its unconventional mechanistic approach.