Evaluation of parameters controlling calcium recovery and CO₂ uptake from desalination reject brine: An optimization approach

Highlights

• A multistage process for extracting minerals from desalination reject brine while concurrently capturing CO₂ was evaluated.

• CO₂ uptake and calcium recovery were optimized using Response Surface Methodology.

• The optimum conditions for NaOH, CO₂ flowrate and salinity for the process were found to be 5 g/L, 2 L/min and 75 g/L, respectively.

• 100% removal of Ca²⁺ ions from the brine and a CO₂ uptake of 5.4 g/L could be achieved under optimum conditions.

Abstract

A multistage process scheme for extracting valuable minerals from desalination reject brine while simultaneously sequestering CO₂ has been proposed. As a part of the proposed scheme, simultaneous removal of calcium ions and CO₂ uptake of a magnesium free reject brine was investigated in an inert particle spouted bed reactor (IPSBR) by reacting CO₂ with reject brine in presence of NaOH. Response surface methodology was applied to optimize the effect of NaOH dosage, gas flowrate and salinity on the reaction with the target of maximizing calcium removal and CO₂ uptake. A second order regression model was generated for both target responses. The predicted responses for both targets were in good agreement with the obtained experimental response. Based on a multivariate optimization, the optimum conditions for NaOH, flowrate and salinity for simultaneous calcium removal and CO₂ uptake were found to be 5 g/L, 2 L/min and 75 g/L, respectively. Under these conditions, 100% removal of Ca²⁺ ions from the brine could be achieved with a CO₂ uptake of 5.4 g/L. Moreover, the solid products obtained under the optimum conditions were characterized confirming the formation of high purity calcium carbonates. Based on optimized conditions, the proposed multistage process scheme promises to produce highly pure Mg(OH)₂ and CaCO₃, while sequestering 5400 tons of CO₂ for every million tons of brine treated. The scheme can be further improved by incorporating an electrolysis unit to produce HCl along with NaOH that can be recirculated to eliminate the need for any external addition of NaOH, thus, creating a sustainable process for reject brine management and CO₂ sequestration.

Introduction

Rapid growth of population coupled with industrial developments is gradually leading to the depletion and contamination of the already scarce fresh water sources. It was reported that around 20% of the world's pollution is suffering from water scarcity and this number is expected to be doubled by the year 2030 (Abdelkareem et al., 2018). Moreover, by the year 2050, the global demands for water is projected to increase by 55% compared to the demands of the year 2000 (Rao et al., 2018). Consequently, finding alternate sources of fresh water to ensure the social and economic growth has become the top priority for the governments worldwide (Elwakeel, 2020; Mekonnen and Hoekstra, 2016). Since 97% of the world's water body is saline, desalination has naturally emerged as the most sustainable means of water supply (Manju and Sagar, 2017). In a typical desalination process, water from a saline source (seawater) is utilized to produce fresh water for municipal and industrial uses (Aziz and Hanafiah, 2021). Currently, there are over 15 thousand desalination plants in operation in over 177 countries around the world (Ghaffour et al., 2013; Jones et al., 2019). These plants account for a water production capacity of almost 100 million cubic meters per day and almost half of it is being produced from the countries located in the Middle East and Africa (Elsaid et al., 2020).

The potential of desalination technology as a long-term solution to meet the global water demand is undeniable, however, there are several economic and environmental implications associated with their use (Darre and Toor, 2018; Jones et al., 2019). A high salinity effluent commonly known as brine is produced as a by-product of the desalination processes and around 90% of the produced brine is discharged back into the ocean (Crowe et al., 2016). In addition to its high salinity (Total Dissolved Solids >70 g/L), it also comprises of various pretreatment chemicals that are used in different stages of the treatment process which lead to the contamination of the adjacent water bodies (Ihsanullah et al., 2021; Mavukkandy et al., 2019). Studies have also shown that the reject brine tends to have higher density compared to its seawater source, thus, contaminants present with the brine are easily transported and deposited to the ocean floor which can eventually harm the benthic communities (Missimer and Maliva, 2018; Rodman et al., 2018). Although, the estimates for the volume of reject brine being disposed globally appears to vary between studies, a recent study have reported the volume to be around 44 million cubic meters per day (Aziz and Hanafiah, 2021). Consequently, implementation of improved brine management strategies to negate the harmful impacts of the large volume of disposed reject brine is necessary to protect the environment. Moreover, the desalination processes are known to be energy-intensive and mostly powered by the fossil fuel sources, which are notorious for their CO₂ emissions that eventually contributes to global warming and exacerbate water scarcity. Hence, the CO₂ emissions generated from fossil fuel powered desalination plants has become another major concern. Currently, it is estimated that around 76 million tons per year of CO₂ is being emitted from all operational desalination plants. As the demand for clean water continous to grow, this number is expected reach around 218 million tons per year by the end of 2040 (Shahzad et al., 2017). Therefore, modification of current desalination facilities is paramount to address the concerns related to brine disposal and CO₂ emissions to protect the environment and also to ensure sustainable water supply for current and future generations.

Recently, CO₂ mineralization using reject brine has garnered a lot of interest from the scientific community as a solution for brine management (Mustafa et al., 2020). Reject brine is rich in calcium and magnesium ions which can be converted into their carbonate form via reaction with CO₂ by adjusting the pH. Moreover, this approach is considered to be economically feasible as it promises the production of various marketable CO₂-based products while simultaneously ensuring the treatment of reject brine and reduction of carbon footprint in the long term. The Solvay process is one such process that has been applied to sequester CO₂ and reduce the salinity of the brine to produce different products. The process involves the reaction of carbon dioxide with sodium chloride present in the aqueous solutions in presence of a catalyst that raises the pH of the process. Ammonia is traditionally used as a catalyst for this process and it results in the formation ammonium chloride and sodium bicarbonate prepitates. However, the presence of ammonia is a major concern of the process because of its environmental and health hazards. Furthermore, the Solvay process suffers from a low CO₂ capture and salt reduction efficiency that needs to be improved to increase the feasability of the process. This motivated the researchers to replace the ammonia with other alkaline components. El-Naas et al. applied calcium oxide as a promoter in a modified Solvay process and achieved a significant improvement in CO₂ capture efficiency (El-Naas et al., 2017). Mourad et al. adopted a similar approach in their work using potassium hydroxide as promotor. The authors not only observed a significant improvement in CO₂ capture efficiency, but also observed a significant reduction in sodium, calcium and magnesium ions (El-Naas et al., 2017; Mourad et al., 2021, 2022). Since amines can significantly improve the pH of a solution, a few studies have suggested their use for combined sequestration of CO₂ and removal ions from reject brine (Dindi et al., 2015; Kang et al., 2020). Although, the amine based processes can improve the CO₂ capture efficiency and produce of high quality carbonates, their recovery from aqueous brine can be very challenging and may require additional steps.

This work is a continuation of a previous study focusing on the production of magnesium and calcium free feed to a membraneless electrolyzer unit (Mahmud et al., 2022). The electrolyzer will be used to produce acid and base products. Since reject brine contains considerable amounts of magnesium and calcium ions, removing these ions is essential for the long-time durability of the electrodes. In the previous study, a multistage process for step wise removal of magnesium and calcium ions was proposed in accordance to the scheme shown in Fig. 1. The first step of this scheme, involved the precipitation of magnesium ions into magnesium hydroxide via addition of NaOH. The optimum conditions for maximizing magnesium removal in this step was investigated and an overall Mg²⁺ removal of 98.8% was achieved with the production of high purity magnesium hydroxide (Mahmud et al., 2022). The brine effluent obtained after this step was almost free of magnesium but it did not have any significant reduction in Ca²⁺ ions. As illustrated in the proposed scheme, the calcium ions present within the effluent can be recovered in the subsequent steps to produce high purity calcium carbonates. Although, the direct reaction of the obtained effluent with CO₂ resulted in some removal of calcium ions, it was observed that additional NaOH dose was required for the complete removal of calcium ions. In this work, the reaction of Mg-free brine effluent with CO₂ and NaOH is investigated using an inert particle spouted bed reactor (IPSBR). Response surface methodology was adopted to determine the optimum conditions for complete removal of calcium ions and maximizing CO₂ uptake by varying the NaOH dose, CO₂ flowrate and brine salinity. Furthermore, the effect of the three process variables on the two target responses are also investigated and solid products obtained under optimum conditions are characterized using various techniques. To the best of the authors’ knowledge, optimization of such an integrated process is not available in the open literature, therefore, the results presented in this work can provide significant insights in the field of simultaneous treatment of reject brine and CO₂ sequestration.