Effective phosphorus removal using transformed water hyacinth: Performance evaluation in fixed-bed columns and practical applications

Abstract

This study introduces calcined water hyacinth (CWH), processed at 650°C, as a novel and environmentally friendly adsorbent for phosphorus (P) removal from wastewater. Building on previous findings that identified CWH as a rich source of metal oxides and hydroxides (e.g., Ca(OH)₂, Al₂O₃, MgO, Fe₃O₄), this research explores its application in fixed-bed column systems for continuous adsorption processes. The study demonstrates that CWH effectively removes phosphorus through apatite formation, showcasing its potential for real-world water treatment. The phosphorus adsorption capacity increased from 23.64 to 26.55 mg/g when the flow rate was reduced from 1.5 to 0.5 mL/min. Breakthrough curves fitted to the Thomas, Adams-Bohart, and Yoon-Nelson models provided critical insights into column performance, while the Bed Depth Service Time (BDST) model confirmed the feasibility of employing CWH in continuous-flow systems. The practical tests on synthetic municipal wastewater, which revealed a maximum adsorption capacity of 5.20 mg/g, further demonstrated CWH’s effectiveness for treating wastewater with low phosphorus concentrations, providing reassurance about its real-world applicability. Furthermore, the study found that increasing the adsorbent height improved column performance by extending breakthrough and exhaustion times, whereas higher flow rates led to faster saturation and reduced capacity. The exhausted CWH material can be repurposed as a soil amendment or fertilizer feedstock, supporting nutrient recycling.

Introduction

Phosphorus (P) is a vital macronutrient essential for life and important across various industries. However, the excessive release of P stemming from industrial, domestic, and agricultural activities, as well as its leaching from soil post-fertilizer application, poses significant environmental challenges, notably eutrophication. This phenomenon triggers a rapid proliferation of algae and other planktonic organisms, leading to reduced dissolved oxygen levels and heightened mortality rates among aquatic life forms [1–6]. Consequently, this deterioration in water quality perpetuates resource degradation, profoundly impacting the ecosystem’s health. Harmful algal blooms stemming from eutrophication can contaminate drinking water sources, posing health hazards to both humans and animals. Certain algal toxins are associated with gastrointestinal illness, liver damage, neurological disorders, and skin irritation [7]. Hence, the imperative lies in removing and recovering P from wastewater, which is crucial to mitigating eutrophication and addressing phosphate rock depletion. While no universal consensus exists on acceptable P concentration, a total P concentration exceeding 0.05 mg/L is widely deemed excessive for aquatic ecosystems [8]. Consequently, numerous studies have explored various techniques for P removal and recovery from water. Among these, adsorption [9–12] emerges as a desirable methodology owing to its cost-effectiveness, seamless integration into existing systems, high efficiency, and environmentally friendly nature. The high versatility of this technology resides in the wide variety of adsorbent materials, which can be derived from biomass residues. These include biocomposites like eggshells combined with banana peels [13], orange peels [14], potato peels [15], or fiber palm [16, 17], along with readily available resources such as water hyacinth [18, 19], among other innovative sources [10–12, 18, 20].

Water hyacinth (WH) is one of the most troublesome aquatic weeds globally, renowned for its rapid growth rate [21]. Capable of forming dense volumes exceeding 60 kg/m², WH plants can severely obstruct watercourses [22], disrupting aquatic ecosystems by reducing sunlight penetration and diminishing optimal conditions for aquatic life, including a decline in dissolved oxygen levels [23]. Furthermore, the presence of WH can inflict negative economic repercussions on industries reliant on water bodies, such as tourism [24]. Consequently, WH removal and effective reuse pose significant challenges for these ecosystems. Thus, the task of removing and efficiently reusing water hyacinth is not without its challenges, posing significant hurdles for these ecosystems. However, the potential for repurposing the harvested water hyacinth is vast, offering a multitude of opportunities. These include biofuel production, composting, animal feed, handicrafts, bioremediation, phytoremediation, and biogas production [25]. These avenues not only contribute to economic growth but also offer significant environmental benefits. Moreover, the ongoing research on phosphorus adsorption using biochar derived from water hyacinth pyrolysis, with modifications involving various metal oxides such as Fe, Ca, Al, and Mg, shows potential for more sustainable solutions [18]. The metal phases present in these biochars are mainly responsible for phosphorus removal, facilitating ligand exchange through the formation of Metal–O–P bonds, while precipitation of phosphorus species has been identified as a predominant adsorption mechanism. Interestingly, among the various metals present in these biomasses, calcium has demonstrated remarkable efficacy in recovering phosphorus from aqueous solutions due to its favorable formation of apatite (Ca₅(PO₄)₃OH) [13, 14, 18]. Apatite, a calcium phosphate biomaterial, represents a valuable pathway for phosphorus recovery and has been employed as a soil amendment. In our previous study [23], we determined that WH ashes contain metal oxides and hydroxides, such as Ca(OH)₂, Al₂O₃, MgO, and Fe₃O₄. These play a crucial role as active phases for phosphorus adsorption and serve as active phases for phosphorus adsorption, as apatite [18]. However, previous applications of WH for P removal and recovery have primarily focused on batch experiments [18, 26, 27]. While these studies offer valuable insights into adsorption characteristics such as mechanism, maximum adsorption capacities, optimal conditions of pH and temperature, and behavior of the system as a function of time (adsorption kinetics), they are limited in their ability to simulate the dynamic performance of adsorbers. Hence, fixed-bed column studies are crucial for scaling from laboratory to industrial applications [3, 28]. Fixed-bed column experiments are crucial for scaling up from laboratory to industrial applications, as they enable: a. Design and operational development of large-scale adsorption processes. b. Insights into breakthrough curves, determining the functional lifespan of the adsorbent bed. c. Handling large volumes of wastewater under continuous flow. d. Potential to achieve high treatment efficiencies [29, 30].

Therefore, this work’s novelty lies in conducting fixed-bed column studies, which provide valuable data for translating laboratory findings to practical industrial-scale adsorption systems, bridging the gap between batch and continuous flow conditions. Additionally, this study utilizes water hyacinth as an adsorbent, creating a valuable product from waste biomass and effectively addressing disposal issues.

While previous studies have explored the adsorption characteristics of Water Hyacinth (WH) through batch experiments, more detailed investigations are needed into its continuous adsorption performance under dynamic flow conditions. This gap underscores the necessity for further research into its potential scale industrial applications. Accordingly, this study aims to evaluate the performance of Calcined Water Hyacinth (CWH) in continuous adsorption experiments specifically for phosphorus (P) removal. The primary objective is to identify the optimal operating conditions that effectively reduce the initial phosphorus concentration. Additionally, the study investigates breakthrough curves to understand how bed height, initial phosphorus concentration, and flow rate influence adsorption in fixed-bed columns. To analyze the column adsorption data and predict breakthrough curves, a range of widely used models—including the Thomas, Adams-Bohart, Yoon-Nelson, and Bed Depth Service Time (BDST) models—were employed. Furthermore, the study explores the practical feasibility of using CWH for phosphorus removal from synthetic municipal wastewater, providing valuable insights into its potential real-world applications and contributing to advancements in wastewater treatment technologies. This study promises a significant impact in the field, offering hope for improved wastewater treatment methods.

Materials and methods

Adsorbent material

The aquatic weed Water Hyacinth was harvested from the Cauca River at the Ituango Hydroelectric Plant (Hidroituango), situated between the municipalities of Ituango and Briceño, in the department of Antioquia, Colombia. A composite of all parts of the harvested Water Hyacinth, including roots, stems, and leaves, was subjected to a calcination process to yield the Calcined Water Hyacinth (CWH) adsorbent material. In our previous study [18] the sample, referred to as WH, was washed with distilled water to remove impurities, dried in an oven at 105°C for 1440 min, ground, and finally passed through a sieve with a size < 0.850 mm. The water hyacinth (WH) sample was calcined in a horizontal tube furnace at 350°C, 450°C, 550°C, 600°C, 650°C, and 700°C, with a heating rate of 10°C/min, and held at each temperature for 120 minutes in an air atmosphere (100 mL/min). The resulting materials were ground and sieved to a particle size of 60–100 mesh. All chemical reagents used were of analytical grade (see supplementary material). In our previous research [18], we explored phosphorus adsorption using materials calcined at these various temperatures. Our findings indicated that the material calcined at 650°C (yield = 16.7%) demonstrated the highest adsorption efficiency, primarily due to the formation of Ca(OH)₂, Al₂O₃, and MgO phases, which are known to enhance phosphorus removal. Thus, the material calcined at 650°C was selected as the focus of the current study. The material before (CWH) and after phosphorus adsorption (CWH + P) was characterized to analyze its structural morphology and surface elemental composition. This was done using Scanning Electron Microscopy coupled with energy-dispersive X-ray Spectroscopy (SEM-EDX) on a JEOL-JSM 6490LV instrument equipped with a LaB6 filament, operating at an accelerating 20 kV.

Fixed-bed adsorption studies

Experiments were conducted on fixed bed columns to evaluate column performance under varied operating parameters, including bed volume, initial P concentration, and flow rate. The experiments were carried out in a glass column of 15 cm in height and 1.1 cm in inner diameter (See Fig 1). In brief, phosphate solutions with concentrations of 25, 50, and 75 mg/L (prepared from KH₂PO₄—Panreac, 99.0%) were pumped through the column using an adjustable peristaltic pump (Shenchen, LabV6-III), resulting in continuous flow rates of 0.5, 1.0, and 1.5 mL/min at different bed heights (2, 3, and 4 cm of CWH) (see S1 Table). Effluent samples were collected periodically and analyzed for residual phosphate concentration at different intervals of time. The concentration of P in the effluent was determined using the colorimetric method (Hach, PhosVer^® 3 Phosphate Reagent) with a DR 6000 spectrophotometer UV-VIS (Hach). The breakthrough curves at different experimental conditions were constructed by plotting the dimensionless concentration at the column outlet against time.

[Figure omitted. See PDF.]

Mathematical modeling of breakthrough curves

Breakthrough times and bed volumes play a crucial role in determining the dynamic operating conditions of an adsorption column [2, 3]. The overall performance of continuous flow adsorption columns is significantly influenced by the shape and length of the ion exchange zone that occurs during solid-liquid contact. This exchange zone progresses through the column section, gradually saturated with adsorbate, juxtaposed with the untouched adsorbent section. During the initial stage of the adsorption process, the top adsorbent layer of the column is "hit" by a high concentration of adsorbate; theoretically, this is where the greatest mass transfer occurs [31, 32]. Nevertheless, since the establishment of the mass transfer zone (MTZ) requires both time and column length, the initial behavior of the column cannot be considered reasonable, exhibiting a transient and unsteady-state regime. As the MTZ fully develops within the bed, its progress along the column becomes observable. At the end of the bed, the breakthrough curve reflects the shape of the MTZ [31]. The MTZ indicates the adsorbent efficiency, defined by the length of the adsorption zone in the column, as follows Eq (1):(1)where, Z (cm) is the bed height, t_b and t_s (min) are the breakthrough time and the exhaustion time, respectively. The t_b is determined as the time when the P concentration exiting the column, C_t, reaches 10% of the initial P concentration, C₀, . The t_s is the time when the P concentration exiting the column reaches 90% of the initial P concentration, .

The total amount of P retained by the column, q_total (mg), at a flow rate, Q (mL/min), and a given P concentration is equivalent to the integral of P concentration in the fixed bed, C_ad (mg/L), respect to time, t (min), and it is calculated using Eq (2):(2)

The maximum capacity of the column under steady-state conditions, q_eq (mg/g), is equal to the total amount of the P adsorbed (q_total) per gram of adsorbent (w) at the end of the time, and it can be calculated according to Eq (3):(3)

This study employed four mathematical models, Thomas, Adams-Bohart, Yoon-Nelson, and Bed Depth Service Time, to characterize phosphorus (P) adsorption on CWH in a continuous system. These models are widely utilized in studying the column behavior of adsorbent-adsorbate systems [33], as they effectively describe breakthrough curves and provide parameters crucial for predicting the operational lifetime of a column during scale-up. Breakthrough curves, elucidated by these models, hold paramount importance in the design and operation of continuous adsorption systems.

Thomas model.

The Thomas model assumes the adsorption process adheres to Langmuir isotherm with a pseudo-second-order rate expression. It postulates reversible reaction kinetics for the velocity driving forces devoid of axial dispersion within the adsorption column [33–35]. By fitting experimental data to the Thomas model, it becomes possible to ascertain the maximum adsorbate concentration in the solid phase, q_Th, and determine the rate constant, k_Th. The Thomas model [36], represented by Eq (4), can be expressed in its linear form as shown in Eq (5):(4)(5)where C₀ and C_t (mg/L) are the P initial and at time, t, concentration; k_Th (L/mg min) is the Thomas constant rate; q_Th (mg/g) is the adsorption capacity for CWH. Using a graph of vs t, values of k_Th and q_Th, are calculated from the slope and the intercept, respectively.

Adams-Bohart model.

This model primarily addresses the first region of the breakthrough curve (), operating under the assumption that equilibrium is not immediately achieved. It assumes a rectangular isotherm with a quasi-chemical rate expression, neglecting dispersive effects such as axial dispersion or mass transfer [35, 37, 38]. According to this model, the adsorption rate is directly proportional to the concentration of the adsorbed contaminant and the remaining capacity of the adsorbent. Consequently, the adsorption rate is favorable and is proportional to the residual adsorption capacity of the adsorbent and the concentration of adsorbed species [1, 34, 38, 39]. The Adams-Bohart model [40] equation and its linearized form are shown in Eqs (6) and (7), respectively:(6)(7)where k_BA (L/mg min) is the Adams-Bohart constant rate; N₀ (mg/L) is the column saturation concentration; U₀ (cm/min) is the linear rate and it is calculated by dividing the flow rate (cm³/min) by the cross-sectional area of the column (cm²); values of k_BA and N₀ are calculated from the slope and the intercept of Eq (7), respectively.

Yoon-Nelson model.

The Yoon-Nelson model operates on a simple theoretical premise, obviating the need for data concerning the properties of the adsorbate or the specific type of adsorbent. The main assumption of this model is that the decrease in the adsorption rate of the adsorbate molecule is directly proportional to both the adsorption of the adsorbate and the breakthrough of the adsorbent [2, 33, 39, 41]. The Yoon-Nelson model [42] is shown in Eq (8), and the linear form can be expressed as follows in Eq (9):(8)(9)where k_YN (L/mg min) is the Yoon-Nelson constant rate; τ (min) is the time required for 50% of the adsorbate breakthrough. Using a graph of vs, t, values of k_YN and τ can be determined using the slope and the intercept, respectively.

Bed Depth Service Time model.

The Bed Depth Service Time model (BDST) is a predictive tool for estimating the packed bed material’s duration within the column before requiring regeneration or replacement [43]. It operates under the assumption that the adsorption rate is maintained due to surface reactions between the adsorbate and the unused capacity of the adsorbent, with negligible intraparticle diffusion [2, 33, 41]. This indicates that equilibrium within the bed is not instantaneous, thereby rendering the rate of the adsorption process proportional to the remaining adsorption capacity within the medium [38].

The constants derived from the BDST model are invaluable for scaling the process across different flow rates and concentrations without requiring additional experimentation. It can be applied to predict column performance for any given bed height if values for certain heights are known. The model establishes a linear relationship between the bed height (Z) and the service time (t) of the column [41]. This linear relationship between bed depth (i.e., height) and service time is expressed by Eq (10):(10)where k_BDST (L/mg min) is the rate constant of the BDST model; N₀ (mg/L) is the adsorption capacity; and t is the service time (min). Values of N₀ and k_BDST can be determined from the slope and the intercept of Eq (10), respectively.

Municipal wastewater application

To evaluate the column’s effectiveness in treating real wastewater, we conducted a column adsorption experiment using simulated municipal wastewater prepared according to OECD guidelines (2001) [44, 45] (see S2 Table). The column was packed with 0.9 g of CWH, and the wastewater sample was pumped using a peristaltic pump at a controlled flow rate of 1.0 mL/min. Effluents from the column were collected and analyzed at different periods. The concentration of P remaining in the solution was determined by the colorimetric method (Hach, PhosVer^® 3 Phosphate Reagent) using a DR 6000 spectrophotometer UV-VIS (Hach).

Results and discussions

Fixed bed column studies—Effect of flow rate, initial P concentration, and bed height variations

Fig 2a–2c shows the effect of varying flow rate, initial P concentration, and bed height on the breakthrough curves of P adsorption on CWH. As the flow rate increases, the breakthrough time (t_b) and exhausted time (t_s) decrease. Specifically, according to Table 3, when the flow rate increased from 0.5 to 1.5 mL/min, t_b decreased from 171 to 7.15 min, and t_s decreased from 1786.80 to 701 min. This observation indicates that there is increased contact time between P and CWH at a lower flow rate, leading to a higher probability of interaction with the adsorption active sites. Hence, the flow rate emerges as a crucial parameter in fixed bed columns, given its direct influence on contact time and mass transfer rate.

[Figure omitted. See PDF.]

Breakthrough curves varying (a) flow rate, (b) initial P concentration, and (c) bed height of P adsorption on CWH. (a) C0 = 50 mg/L, bed height = 2 cm; (b) flow = 1.0 mL/min, bed height = 2 cm; (c) C0 = 50 mg/L, flow = 1.0 mL/min. EBCT: empty bed contact time, Ct: P concentration on the time, C0: P initial concentration.

Similarly, when the initial P concentration increases from 25 to 75 mg/L, a comparable trend is observed with t_b and t_s. The time taken for the column to reach 10% saturation decreases from 134 to 25.2 min, while the time to reach 90% saturation decreases from 1402 to 584 min. This indicates that the column saturates approximately 2.4 times faster at a higher concentration of 75 mg/L than 25 mg/L. This acceleration in saturation can be attributed to the rapid occupation of available active sites at higher concentrations [2]. Conversely, as the bed height increases from 2.0 to 4.0 cm, there is an observed increase in t_b from 41 to 216 min, and t_s also increases from 1034 to 1122.67 min. This phenomenon can be attributed to the higher bed volume, which offers increased active binding sites within the CWH to capture P incoming P within the column [2, 32]. Similar studies have linked this behavior to the relationship between bed height and adsorbent surface area, providing more binding sites for adsorbate adsorption [1, 2, 41].

Table 1 shows that when the flow rate varies from 0.5 to 1.5 mL/min and the initial P concentration from 25 to 75 mg/L, the MTZ changes slightly between 1.81 and 1.98 cm and 1.81 and 1.92 cm, respectively. However, when increasing the bed height from 2 to 4 cm, a notable increase in MTZ is observed, rising from 1.92 to 3.23 cm. This indicates that the partial saturation zone moves through the column in the flow direction determined by the adsorbate concentration, adsorbent capacity, and flow rate [41]. From these results, the column can operate until MTZ has reached the end of the column. Before MTZ reaches the end of the column, the effluent is practically free of adsorbate, but when MTZ reaches the end of the column, the P concentration in the effluent starts to increase gradually. The above agrees with the behavior described by Yoon and Nelson (1984) and Patel (2019) [33, 41].

[Figure omitted. See PDF.]

Table 1 illustrates the impact of varying flow rates on the column’s adsorption capacity for P. With an increase in flow rate from 0.5 to 1.5 mL/min, the maximum P adsorption capacity decreased slightly from 22.84 to 20.80 mg/g (~8%). This reduction can be attributed to the shorter residence times of the P solution within the bed column and the heightened turbulence experienced at higher flow rates. A lower flow rate predominates the mass transport of P over intra-particle diffusion, causing insufficient contact time between adsorbate and adsorbent and resulting in weaker interactions. Consequently, P exits the column before reaching the adsorption equilibrium.

Moreover, the accelerated downward movement of the primary adsorption zone in the fixed bed, induced by increased flow rates, further contributes to the decreased adsorption capacity of the column [46]. Conversely, low flow rates ensure prolonged contact time, facilitating stronger adsorbate-adsorbent interactions [3]. It is also observed that greater bed heights extend the residence time between P and CWH. As the bed height increased from 2 to 4 cm of exhausted CWH material, the time required for C_t/C₀ = 10% (t_b) increased from 41 min to 216 min. This finding aligns with the study conducted by Manjunath and Kumar (2021) [47], highlighting that larger bed heights diminish the mass transfer rate by increasing the contact time between the adsorbate and adsorbent.

To evaluate the morphology and surface elemental composition, the material was characterized before (CWH) and after phosphorus adsorption (CWH + P). Fig 3 shows the SEM micrographs of CWH and CWH + P, where the materials exhibit an irregular morphology with a rough, porous surface. The surface is primarily composed of elements such as C, Ca, O, K, Fe, Mg, Al, Si, and P, with a noticeable increase in phosphorus intensity in the CWH + P sample, indicating higher P concentration on the surface after adsorption.

[Figure omitted. See PDF.]

The presence of Ca in the material is particularly significant, as calcium plays a crucial role in phosphorus adsorption by promoting the formation of calcium-phosphate complexes. Additionally, the SEM images reveal swollen surfaces, with pores formed as volatile matter is released during heating, expanding the microstructure of the biomass. The resulting CWH retains its porous surface area in CWH + P, which may enhance the soil’s water retention capacity and improve its efficiency as a fertilizer [22]. This is particularly inspiring as it suggests that the material could play a key role in fostering the growth of beneficial microorganisms and retaining essential nutrients, thereby significantly improving soil quality.

Modeling of breakthrough curves

Table 2 and S1a–S1g Fig provide parameters for the Thomas, Adams-Bohart, and Yoon-Nelson models. The experimental data present a good fit to these models, with correlation coefficients, R², up to 0.98, indicating that these models can represent the experimental values.

[Figure omitted. See PDF.]

Thomas model.

The Thomas model demonstrates a strong fit to the experimental data, as depicted in S1a–S1g Fig. In Table 2, the values of q_Th and k_Th exhibit contrasting trends with the increase in flow rate from 0.5 to 1.5 mL/min. While q_Th decreases from 23.91 to 18.22 mg/g, k_Th increases from 0.06 to 0.09 L/mg min. At higher flow rates, the weakening of mass driving force leads to rapid adsorption of P onto active sites, shortening the contact time between adsorbent and adsorbate. This results in an incomplete adsorption process and, consequently, lower adsorption capacity. Conversely, when the initial P concentration rises from 25 to 75 mg/L, q_Th increases from 18.47 to 20.45 mg/g, while k_Th decreases from 0.14 to 0.07 L/mg min. This is attributed to the heightened resistance to mass transfer caused by higher P concentration, which decreases the velocity constant [2]. Furthermore, Table 2 shows that with an increase in bed depth from 2 to 4 cm at a constant flow rate of 1 mL/min, the adsorption capacity (q_Th) decreases from 20.66 to 16.07 mg/g, while the constant rate (k_Th) increases from 0.07 to 0.09 L/mg min. This behavior aligns with findings by Lv and Li (2023), suggesting that non-adsorbed phosphate remaining in the adsorption layer induces axial diffusion, while the Thomas model is ideally obtained without axial diffusion [4]. These observations are consistent with those reported by other studies [3, 4, 29]. Notably, the calculated values of q_Th using this model align with the experimental values of q_eq presented in Table 1.

Adams-Bohart model.

The Adams-Bohart model effectively fits the initial segment of the breakthrough curve (C_t/C₀ < 0.5). This is a characteristic consistent with its typical application in describing the first 50% of the breakthrough curve, as observed in S1 Fig and corroborated by previous studies [1, 3, 48]. Examining Table 2 reveals that with an increase in flow rate from 0.5 to 1.5 mL/min, the mass transfer coefficient (k_BA) increases from 0.09 to 1.30 L/mg min. At the same time, the column’s saturation concentration (N₀) decreases from 9861.64 to 1873.41 mg/L. This suggests that during the initial part of adsorption on the column, external mass transfer predominantly governs the global kinetics of the system. On the other hand, by increasing the bed height from 2 to 4 cm, k_BA decreases from 0.24 to 0.12 L/mg min, and N₀ increases from 5697.41 to 8566.88 mg/L. These trends are consistent with findings from other research studies [1, 48], underscoring these behavior’s reliability and reproducibility across different experimental setups.

Yoon-Nelson model.

According to the Yoon-Nelson model, the rate constant (k_YN) and the time required to reach 50% breakthrough time (τ) were determined (Table 2). As the flow rate increased from 0.5 to 1.5 mL/min, τ decreased from 806.47 to 209.56 min, while k_YN increased from 3.00 to 4.80 1/min. Similarly, as the initial P concentration rose from 25 to 75 mg/L, τ decreased from 625.03 to 230.75 min, while k_YN increased from 3.70 to 5.60 1/min. These changes are attributed to the heightened mass driving force from the liquid phase to the solid-liquid interface. This results in increased P mobility and occupation of more CWH active sites. Consequently, the mass transfer zone moves faster, reducing the time required to achieve a 50% breakthrough (τ) and increasing k_YN [4, 47]. This phenomenon indicates that CWH saturation on the column occurs more rapidly at higher flow rates [4, 9, 49]. Notably, a higher τ value reflects better column performance, as reported by Omitola et al. (2002) [50]. On the other hand, increasing the bed height from 2 to 4 cm results in a slight increase in k_YN from 3.50 to 4.50 1/min and an increase in τ from 348.31 to 668.49 min. This is attributed to the greater depth of the bed, which increases resistance to mass transfer and extends the contact time between P and CWH [4, 47, 49].

Bed Depth Service Time model.

The BDST model is widely used in designing and operating fixed bed columns for wastewater treatment systems to gauge the longevity of the adsorbent bed before regeneration or replacement becomes necessary [2, 43, 51].

In this work, we employed the BDST model to investigate the effect of bed height on removing P from aqueous solutions using CWH in fixed bed columns. The experiments were conducted using various breakthrough points, ranging from C_t/C₀ = 0.1, 0.2, 0.3, 0.4, and 0.5 and bed depths of 2, 3, and 4 cm (see Fig 4). The parameters obtained by fitting the experimental data of the BDST model are summarized in Table 3. Notably, the analysis reveals a robust linear correlation (R² ≥ 0.93), indicating that the BDST model effectively characterizes the experimental values of P adsorption on CWH.

[Figure omitted. See PDF.]

Ct: P concentration on the time, C0: P initial concentration.

[Figure omitted. See PDF.]

From Table 3, the adsorption capacity (N₀) increases from 4603.66 to 9036.32 mg/L with the increase of C_t/C₀ from 0.1 to 0.5. This may be attributed to the fact that P occupies more active sites on CWH at higher values of C_t/C₀, which improves adsorption capacity [43].

By setting t = 0 and solving Eq (10) for Z₀, produces the following Eq (11) [1]:(11)where Z₀ (cm) is the critical bed height, which is the minimum bed height required to obtain the desired effluent concentration (C_t).

Table 3 reveals a decreasing trend in Z₀, ranging from 1.37 to 0.31 cm as C_t/C₀ increases from 0.1 to 0.5. This indicates that the minimum bed depth required to achieve the desired effluent concentration (C_t) diminishes as the CWH column allows more P to pass through.

These findings underscore the significance of flow rate and bed height in effectively utilizing CWH as a fixed bed column adsorbent for P removal from water. Overall, these results highlight the utility of the BDST model in offering insights into the performance of large-scale wastewater treatment systems aimed at P removal using CWH filled fixed bed columns [43].

Comparison of breakthrough models.

The Thomas, Adams-Bohart, and Yoon-Nelson models proved to be effective in accurately describing the breakthrough curves observed during P adsorption on CWH, enabling the determination of key column properties essential for process design. Notably, the Thomas model demonstrated its efficacy by observing low values of Δq (q_Th − q_eq), ranging between 0.1 and 5.42 (See Table 4). These results suggest that maintaining high bed adsorbent levels alongside moderate flow rates enhances column efficiency in P removal over prolonged operational periods. Additionally, the BDST model exhibited a good linear relationship between the amount of CWH employed and the service time, thereby facilitating the determination of the requisite amount of adsorbent needed to attain a specified breakthrough criterion. This behavior underscores the feasibility of implementing the fixed bed column within a continuous-flow wastewater system.

[Figure omitted. See PDF.]

Municipal wastewater application

A packed column was employed to assess CWH’s efficacy in removing P from municipal wastewater. Fig 5 illustrates the breakthrough curve depicting P adsorption on CWH using synthetic municipal wastewater, in while Table 5 shows the experimental breakthrough parameters. Notably, t_b and t_s are 108 min and 1166 min, respectively, with a maximum adsorption capacity of P on the column of 5.20 mg/g.

[Figure omitted. See PDF.]

C0 = 9 mg/L, flow = 1.0 mL/min, and bed height = 2.0 cm. Ct: P concentration on the time, C0: P initial concentration.

[Figure omitted. See PDF.]

The findings presented above underscore the suitability of a CWH-packed column for continuous P adsorption experiments without compromising the material’s active phase. Table 6 compares phosphorus adsorption capacities determined by the Thomas model across various adsorbent materials. Water hyacinth (CWH) demonstrates a notably higher Thomas adsorption capacity for phosphorus than other materials derived from physical and chemical treatments in continuous systems, highlighting a distinct advantage of CWH over alternative adsorbents. Additionally, this table now emphasizes not only the effectiveness of water hyacinth for phosphorus removal but also its potential for removing heavy metals, dyes, and pesticides. These additions further underscore the versatility and significance of water hyacinth as an adsorbent material for addressing a wide range of environmental pollutants.

[Figure omitted. See PDF.]

Conclusions

In conclusion, this study marks a significant advancement in applying calcined water hyacinth (CWH) as an adsorbent material for phosphorus (P) removal in continuous-flow systems. Our investigation provides a comprehensive evaluation of how key operational parameters—specifically influent flow rate and adsorbent height—impact the system’s adsorption performance and breakthrough characteristics. These findings have significant implications for the fields of environmental engineering, water treatment, and sustainable agriculture, as they offer a potential solution for phosphorus removal and soil enrichment.

The results demonstrate that increased flow rates accelerate the saturation of the CWH surface with phosphorus, reducing both breakthrough and exhaustion times. Specifically, we observed an 8.93% decrease in the maximum adsorption capacity of the material as flow rates increased. This finding indicates that higher flow rates enhance throughput and result in quicker exhaustion of the adsorbent, necessitating careful optimization to balance efficiency and capacity.

Conversely, increasing the adsorbent height within the fixed-bed column significantly improved performance by extending breakthrough and exhaustion times. This enhancement suggests that a larger adsorbent volume provides increased surface area and longer contact time between phosphorus and the adsorbent, ultimately boosting overall adsorption efficiency and prolonging the effective life of the column.

When applied to synthetic municipal wastewater containing 9 mg/L phosphorus, the continuous-flow system achieved a notable maximum column capacity (q_eq) of 5.20 mg/g. This result underscores the substantial potential of CWH as an effective and sustainable solution for phosphorus removal, highlighting its applicability in real-world water treatment scenarios where efficient and cost-effective solutions are paramount.

Moreover, the study reveals an additional benefit of the exhausted CWH material, which can be repurposed as a soil amendment or fertilizer feedstock. This exciting finding opens up new possibilities for the use of CWH, as previous research has demonstrated the bioavailability of the adsorbed phosphorus in these materials, suggesting that the spent adsorbent could contribute to nutrient recycling and soil enrichment without the need for further reusability studies. This potential for soil enrichment should excite the audience about the additional benefits of the research.

Looking ahead, future research should focus on refining and optimizing the operational conditions of the continuous-flow system to enhance its performance and cost-efficiency. This includes exploring various scaling processes, improving the economic feasibility of large-scale applications, and investigating the long-term stability and effectiveness of the adsorbent under diverse operational conditions. By addressing these areas, the practical implementation of CWH in phosphorus removal technologies can be further advanced, underscoring the urgency and importance of your work in contributing to more sustainable water treatment solutions.

Supporting information

S1 Table. Continuous flow fixed bed column experimental design.

https://doi.org/10.1371/journal.pone.0312432.s001

(DOCX)

S2 Table. Chemical composition of municipal wastewater.

https://doi.org/10.1371/journal.pone.0312432.s002

(DOCX)

S1 Fig. Breakthrough curves modeling by Thomas, Adams-Bohart, and Yoon-Nelson models for the adsorption of P on CWH.

https://doi.org/10.1371/journal.pone.0312432.s003

(DOCX)

S1 Data.

https://doi.org/10.1371/journal.pone.0312432.s004

(XLSX)

Acknowledgments

The authors acknowledge the support provided by Universidad de Medellín.

References

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18. 18. Ramirez-Muñoz A, Pérez S, Flórez E, Acelas N. Recovering phosphorus from aqueous solutions using water hyacinth (Eichhornia crassipes) toward sustainability through its transformation to apatite. J Environ Chem Eng. 2021;9(5):106225.

* View Article

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19. 19. Ramirez-Muñoz A, Pérez S, Muñoz-Saldaña J, Flórez E, Acelas N. Eco-friendly materials obtained through a simple thermal transformation of water hyacinth (Eichhornia Crassipes) for the removal and immobilization of Cd2+ and Cu2+ from aqueous solutions. Environ Nanotechnol Monit Manag. 2021;16:100574.

* View Article

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20. 20. Abdulqader MA, Suliman MA, Ahmed TA, Wu R, Bobaker AimanM, Tiyasha T, et al. Conversion of Chicken Rice Waste into Char via Hydrothermal, Pyrolysis, and Microwave Carbonization Processes: A Comparative Study. AUIQ Complementary Biological System. 2024 Jul 1;1(1):1–9.

* View Article

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* View Article

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* View Article

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Citation: Ramirez-Muñoz A, Flórez E, Ocampo-Perez R, Acelas N (2024) Effective phosphorus removal using transformed water hyacinth: Performance evaluation in fixed-bed columns and practical applications. PLoS ONE 19(11): e0312432. https://doi.org/10.1371/journal.pone.0312432

About the Authors:

Anyi Ramirez-Muñoz

Roles: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing – original draft

Affiliations: Grupo de Investigación Materiales con Impacto (Mat&mpac), Facultad de Ciencias Básicas, Universidad de Medellín, Medellín, Colombia, Laboratorio Nacional de Proyección Térmica (CENAPROT), Centro de Investigación y de Estudios Avanzados Del IPN, Querétaro, México

Elizabeth Flórez

Roles: Conceptualization, Project administration, Supervision, Writing – review & editing

E-mail: [email protected] (NA); [email protected] (EF)

Affiliation: Grupo de Investigación Materiales con Impacto (Mat&mpac), Facultad de Ciencias Básicas, Universidad de Medellín, Medellín, Colombia

Raúl Ocampo-Perez

Roles: Conceptualization, Validation, Writing – review & editing

Affiliation: Centro de Investigación y de Estudios de Posgrado, Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, San Luis Potosí, México

Nancy Acelas

Roles: Conceptualization, Methodology, Project administration, Supervision, Writing – review & editing

E-mail: [email protected] (NA); [email protected] (EF)

Affiliation: Grupo de Investigación Materiales con Impacto (Mat&mpac), Facultad de Ciencias Básicas, Universidad de Medellín, Medellín, Colombia

ORICD: https://orcid.org/0000-0003-4771-0934

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[/RAW_REF_TEXT]

[/RAW_REF_TEXT]

[/RAW_REF_TEXT]

[/RAW_REF_TEXT]

References

1. Nguyen TAH, Ngo HH, Guo WS, Pham TQ, Li FM, Nguyen T V., et al. Adsorption of phosphate from aqueous solutions and sewage using zirconium loaded okara (ZLO): Fixed-bed column study. Science of the Total Environment. 2015;523:40–9. pmid:25847314

2. Hussein FB, Mayer BK. Fixed-bed column study of phosphate adsorption using immobilized phosphate-binding protein. Chemosphere [Internet]. 2022;295:133908. pmid:35143858

3. Ramirez A, Giraldo S, García-nunez J, Flórez E, Acelas N. Phosphate removal from water using a hybrid material in a fixed-bed column. Journal of Water Process Engineering [Internet]. 2018;26:131–7.

4. Lv N, Li X. Phosphorus removal from wastewater using Ca-modified attapulgite: Fixed-bed column performance and breakthrough curves analysis. J Environ Manage. 2023;328(September 2022):116905. pmid:36521218

5. Meina L, Qiao M, Zhang Q, Xu S, Wang D. Study on the dynamic adsorption and recycling of phosphorus by Fe–Mn oxide/mulberry branch biochar composite adsorbent. Sci Rep. 2024 Dec 1;14(1).

6. Liu Y, Wang S, Huo J, Zhang X, Wen HT, Zhang D, et al. Adsorption recovery of phosphorus in contaminated water by calcium modified biochar derived from spent coffee grounds. Science of the Total Environment. 2024 Jan 20;909. pmid:37944608

7. Epa U, of Ground Water O, Water D. Harmful Algal Blooms and Drinking Water Factsheet [Internet]. www.epa.gov/cyanohabs.

8. Cai R, Wang X, Ji X, Peng B, Tan C, Huang X. Phosphate reclaim from simulated and real eutrophic water by magnetic biochar derived from water hyacinth. J Environ Manage [Internet]. 2017;187:212–9. pmid:27912132

9. Hamid A, Wilson AE, Torbert HA, Wang D. Sorptive removal of phosphorus by flue gas desulfurization gypsum in batch and column systems. Chemosphere [Internet]. 2023;320(January):138062. pmid:36746251

10. Pérez S, Muñoz-Saldaña J, Garcia-Nunez JA, Acelas N, Flórez E. Unraveling the Ca–P species produced over the time during phosphorus removal from aqueous solution using biocomposite of eggshell-palm mesocarp fiber. Chemosphere. 2022;287(September 2021). pmid:34563780

11. Lee JI, Kim JM, Yoo SC, Jho EH, Lee CG, Park SJ. Restoring phosphorus from water to soil: Using calcined eggshells for P adsorption and subsequent application of the adsorbent as a P fertilizer. Chemosphere [Internet]. 2022;287(P3):132267. pmid:34537455

12. Liao Y, Chen S, Zheng Q, Huang B, Zhang J, Fu H, et al. Removal and recovery of phosphorus from solution by bifunctional biochar. Inorg Chem Commun [Internet]. 2022;139(March):109341.

13. Ospina-Montoya V, Pérez S, Muñoz-Saldaña J, Forgionny A, Flórez E, Acelas N. Performance of novel Ca-biocomposites produced from banana peel and eggshell for highly efficient removal and recovery of phosphate from domestic wastewater. J Environ Manage. 2024 Feb 1;352. pmid:38184877

14. Pérez S, Giraldo S, Forgionny A, Flórez E, Acelas N. Eco-friendly reuse of agricultural wastes to produce biocomposites with high potential in water treatment and fertilizers. Biomass Convers Biorefin [Internet]. 2022;(0123456789).

15. Quisperima A, Pérez S, Flórez E, Acelas N. Valorization of potato peels and eggshells wastes: Ca-biocomposite to remove and recover phosphorus from domestic wastewater. Bioresour Technol. 2022;343(October 2021). pmid:34637908

16. Ramirez A, Giraldo S, Flórez Yepes E, Acelas Soto NY. Preparación de carbón activado a partir de residuos de palma de aceite y su aplicación para la remoción de colorantes. Revista Colombiana de Química [Internet]. 2017 Jan 1;46(1):33–41. Available from: http://dx.doi.org/10.15446/rev.colomb.quim.v46n1.62851

17. Pérez S, Muñoz-Sadaña J, Acelas N, Flórez E. Phosphate removal from aqueous solutions by heat treatment of eggshell and palm fiber. J Environ Chem Eng. 2021;9(1).

18. Ramirez-Muñoz A, Pérez S, Flórez E, Acelas N. Recovering phosphorus from aqueous solutions using water hyacinth (Eichhornia crassipes) toward sustainability through its transformation to apatite. J Environ Chem Eng. 2021;9(5):106225.

19. Ramirez-Muñoz A, Pérez S, Muñoz-Saldaña J, Flórez E, Acelas N. Eco-friendly materials obtained through a simple thermal transformation of water hyacinth (Eichhornia Crassipes) for the removal and immobilization of Cd2+ and Cu2+ from aqueous solutions. Environ Nanotechnol Monit Manag. 2021;16:100574.

20. Abdulqader MA, Suliman MA, Ahmed TA, Wu R, Bobaker AimanM, Tiyasha T, et al. Conversion of Chicken Rice Waste into Char via Hydrothermal, Pyrolysis, and Microwave Carbonization Processes: A Comparative Study. AUIQ Complementary Biological System. 2024 Jul 1;1(1):1–9.

21. Vinayakumar K, Palliyarayil A, Prakash PS, Kumar NS, Sil S. A facile one pot synthesis of biocarbon derived from water hyacinth and development of pellets for CO2 capture applications. Biomass Bioenergy [Internet]. 2022;167(June):106648.

22. Gaurav GK, Mehmood T, Cheng L, Klemeš JJ, Shrivastava DK. Water hyacinth as a biomass: A review. J Clean Prod. 2020;277.

23. Ramirez A, Pérez S, Acelas N, Flórez E. Utilization of water hyacinth (Eichhornia crassipes) rejects as phosphate-rich fertilizer. J Environ Chem Eng. 2021;9:104776.

24. Amalina F, Razak ASA, Krishnan S, Zularisam AW, Nasrullah M. Water hyacinth (Eichhornia crassipes) for organic contaminants removal in water–A review. Vol. 7, Journal of Hazardous Materials Advances. Elsevier B.V.; 2022.

25. Nandiyanto ABD, Ragadhita R, Hofifah SN, Al Husaeni DF, Al Husaeni DN, Fiandini M, et al. Progress in the utilization of water hyacinth as effective biomass material. Environ Dev Sustain. 2023 Jul 28;

26. Mosa A, El-ghamry A, Tolba M. Functionalized biochar derived from heavy metal rich feedstock: Phosphate recovery and reusing the exhausted biochar as an enriched soil amendment. Chemosphere [Internet]. 2018;198:351–63. pmid:29421750

27. Chen X, Chen X, Wan X, Weng B, Huang Q. Water hyacinth (Eichhornia crassipes) waste as an adsorbent for phosphorus removal from swine wastewater. Bioresour Technol [Internet]. 2010;101(23):9025–30. Available from: http://dx.doi.org/10.1016/j.biortech.2010.07.013 pmid:20674342

28. Hu Q, Yang X, Huang L, Li Y, Hao L, Pei Q, et al. A critical review of breakthrough models with analytical solutions in a fixed-bed column. Vol. 59, Journal of Water Process Engineering. Elsevier Ltd; 2024.

29. Mekonnen DT, Alemayehu E, Lennartz B. Fixed-bed column technique for the removal of phosphate from water using leftover coal. Materials. 2021;14(19):5466. pmid:34639864

30. Rusu L, Grigoraș CG, Simion AI, Suceveanu EM, Dediu Botezatu A V., Harja M. Biosorptive Removal of Ethacridine Lactate from Aqueous Solutions by Saccharomyces pastorianus Residual Biomass/Calcium Alginate Composite Beads: Fixed-Bed Column Study. Materials. 2022;15(13):4657. pmid:35806780

31. Naja G, Volesky B. Behavior of the mass transfer zone in a biosorption column. Environ Sci Technol. 2006;40:3996–4003. pmid:16830573

32. Mahmoud AED, Franke M, Braeutigam P. Experimental and modeling of fixed-bed column study for phenolic compounds removal by graphite oxide. Journal of Water Process Engineering [Internet]. 2022;49:103085.

33. Patel H. Fixed-bed column adsorption study: a comprehensive review. Appl Water Sci [Internet]. 2019;9(3):45.

34. Saadi Z, Saadi R, Fazaeli R. Fixed-bed adsorption dynamics of Pb (II) adsorption from aqueous solution using nanostructured γ-alumina. J Nanostructure Chem. 2013;3(1):48.

35. Ahmed MJ, Hameed BH. Removal of emerging pharmaceutical contaminants by adsorption in a fixed-bed column: A review. Ecotoxicol Environ Saf [Internet]. 2018;149:257–66. pmid:29248838

36. Thomas HC. Chromatography: a Problem in Kinetics. Ann N Y Acad Sci. 1948;49(2):161–82. pmid:18911539

37. Chu KH. Fixed bed sorption: Setting the record straight on the Bohart-Adams and Thomas models. J Hazard Mater [Internet]. 2010;177(1–3):1006–12. pmid:20096997

38. Smaranda C, Popescu MC, Bulgariu D, Măluţan T, Gavrilescu M. Adsorption of organic pollutants onto a Romanian soil: Column dynamics and transport. Process Safety and Environmental Protection. 2017;108:108–20.

39. Chen S, Yue Q, Gao B, Li Q, Xu X, Fu K. Adsorption of hexavalent chromium from aqueous solution by modified corn stalk: A fixed-bed column study. Bioresour Technol [Internet]. 2012;113:114–20. pmid:22189077

40. Bohart GS, Adams EQ. Some aspects of the behavior of charcoal with respect to chlorine. J Am Chem Soc. 1920;42(3):523–44.

41. Sun XF, Imai T, Sekine M, Higuchi T, Yamamoto K, Kanno A, et al. Adsorption of phosphate using calcined Mg3-Fe layered double hydroxides in a fixed-bed column study. Journal of Industrial and Engineering Chemistry [Internet]. 2014;20(5):3623–30. Available from: http://dx.doi.org/10.1016/j.jiec.2013.12.057

42. Yoon YH, Nelson JH. Application of gas adsorption kinetics I. A theoretical model for respirator cartridge service life. Am Ind Hyg Assoc J. 1984;45(8):509–16.

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