Adsorption of nitrate and phosphate ions using ZnCl 2 -activated biochars from phytoremediation biomasses

: Mishandling and disposal of post-harvested phytoremediation biomass results in secondary pollution. Biochar production is one of the available technologies for processing post-harvested phytoremediation biomasses. The main objective of this study was to assess the potential adsorption of PO 43 - and NO 3 - ions from a binary solution by ZnCl 2 -activated phytoremediation biochars. The biochars were activated using ZnCl 2 and analyzed for specific surface area, pore size, volume, surface morphology, point of zero charges (pH pzc ), surface functional groups, and elemental composition. Subsequently, the adsorption potential for PO 43 - and NO 3 - ions of the activated biochar was investigated. Activation of phytoremediation biochars led to the development of new micropores and increased specific surface area range from 1.62-4.72 m 2 g -1 to 4.75-55.50 m 2 g -1 . ZnCl 2 activation reduced the pH pzc values of Cymbopogon citratus , Cymbopogon nardus , and Chrysopogon zizanioides biochars (BCL2, BCC2, and BCV2) from 9.75, 9.50, 9.62 to 5.72, 5.51, and 6.23, respectively. Activated Chrysopogon zizanioides biochar (ACBCV2), activated Cymbopogon nardus biochar (ACBCC2) and activated Cymbopogon citratus biochar (ACBCL2) showed maximum potential phosphate ion adsorption capacities of 115.70, 101.74, and 270.59 mg g -1 , respectively. ACBCL2, ACBCC2, and ACBCV2 indicated maximum potential nitrate ion adsorption capacities of 155.78, 99.42, and 117.71 mg g -1 . BCC2, BCL2, ACBCV1, ACBCV2, and ACBCC2 best fitted the Langmuir linear form 1 model during NO 3 - adsorption. The results obtained in this study showed that ZnCl2-activated phytoremediation biochars have the potential to remove PO 43 - and NO 3 - ions from PO 43 - and NO 3 - ions binary solution.


Introduction
In recent decades, phytoremediation technology has been employed to rehabilitate contaminated lands.This technology involves selecting and utilizing particular plants or vegetation to extract and translocate pollutants from the soil to plant tissues for post-harvest (He et al. 2020).To date, the disposal of heavy-metal-contaminated biomass from phyto-remediated environments remains a global issue.Improper handling of phytoremediation biomass can easily lead to secondary pollution (Ghosh and Singh 2005).In this study, phytoremediation biomass refers to biomass harvested from heavy metal-rich BCL Cu-Ni mine tailings, while nonphytoremediation biomass refers to biomass grown in nonheavy metal-contaminated soil.One of the major drawbacks of industrialization is environmental pollution.The quest for enhanced food production has led to increased applications of fertilizers in the agricultural sector.The leaching of phosphorous and nitrogen compounds from agricultural fields into aquatic systems results in their enrichment, leading to eutrophication (Wang et al. 2015, Yao et. 2012).
Due to the environmental problems caused by by high concentrations of PO 43-and NO 3 -ions, it is crucial to treat wastewater containing nitrates and phosphates before discharging it into the environment.One effective method for purifying wastewater is adsorption, in which carbon-based materials are the adsorbents of choice.While biomass-derived activated carbon and biochar have received considerable attention as adsorbents, the use of phytoremediation biomass as a precursor material has not been adequately explored.Biochar is a fine-grained, carbon-rich biomaterial produced by the slow pyrolysis of biomass (Yağmur and Kaya 2021).Its special properties, such as a porous structure and high carbon content, broaden its application prospects in both environmental and agricultural sectors (Xie et al. 2015).Biochar activation is the process of increasing the adsorption sites or capacity of biochar through physical or chemical mechanisms (Gámiz et al. 2019).Physical activation involves ball milling and grinding of biochar (Pena et al. 2020), gaseous modification by steam, carbon dioxide, air, or ozone, and thermal modification by conventional heating and microwave irradiation.Chemical activation involves treating biochar with oxidizers, acids, or bases (Williams and Reed 2004).
The main aim of biochar activation is to induce reactive functional groups on biochars, enhance the specific surface area, develop porous structure, and alter the surface charge of biochars to improve the affinity between biochar and adsorbates (Borchard et al. 2012, Deng et al. 2021).According to a study by Paredes et al. (2021), various adsorbents can be modified using different activators such as KOH, NaOH, H 3 PO 4, and ZnCl 2 .ZnCl 2 is particularly preferred for the activation of carbon-based materials such as biochars since it enhances pore development, increases the specific surface area of the carbonaceous structure, and typically results in a high yield of modified carbon (Menya et al. 2018).The low melting point of ZnCl 2 (283-293 °C) enables it to easily contact the carbon surface at activation temperatures above 500 °C (Li et al. 2020).Ideally, ZnCl 2 activation should be performed at temperatures above 500 °C taking into consideration the lignocellulosic composition of the feedstock.However, performing ZnCl 2 activation at higher temperatures (above 600 ℃) can result in lower biochar yield (Bouchemal et al. 2009, Hock andZaini 2018).To maximize biochar production for adsorption studies, the procedures by Abd et al. (2019) and Huang et al. (2024) were adopted.
Adsorption technology results in a high accumulation of contaminants on the surface of adsorbent, decreasing their concentrations in treated wastewater (Wu et al. 2019).Contaminant adsorption in wastewater can be achieved by either chemical or physical means.Physical adsorption involves the adsorbate deposition on the surface of the adsorbent without the forming chemical bonds (Inyang et al. 2016), whereas chemical adsorption involves chemical complexation between the adsorbent and contaminants (Xia et al. 2019).The mechanisms for removing PO 4 3-and NO 3 -ions by adsorption generally include hydrogen bonding, ion exchange, co-precipitation, complexation, and electrostatic adsorption (Gizaw et al. 2021, Barquilha andBraga 2021).
The objectives of this study were to: (1) determine the influence of ZnCl 2 activation on the specific surface area, pore size, and volume, point of zero charges, and surface morphology of phytoremediation biochars, and (2) evaluate the potential of activated biochar as adsorbent(s) for PO 4 3-and NO 3 -ions from a binary solution.Several studies have investigated the removal of PO 4 3-and NO 3 -ions from wastewater using ZnCl 2 -activated biochar from different feedstocks (Thue et al. 2022, Biswas et al. 2023, Lie et al. 2018).However, this is the first study to (1) assess the effect of ZnCl 2 activation on the physicochemical properties of biochar derived from phytoremediation biomass, and (2) determine the potential of ZnCl 2 -activated biochar from phytoremediation biomass as adsorbents for PO 4 3-and NO 3 -ions from a binary solution.

Biomass and Biochar production
Phytoremediation and non-phytoremediation biomasses were produced by growing Cymbopogon citratus, Cymbopogon nardus, and Chrysopogon zizanioides grasses in Bamangwato Concessions Limited (BCL) Cu-Ni mine tailings and uncontaminated soil.Biochars were produced by pyrolyzing these biomasses at 550°C in an inert environment for 30 minutes, and the chemical characteristics of the resulting biochar were analyzed.According to Ultra et al. (2022), the BCL mine tailings are highly acidic, with a pH value of 2, and contain heavy metal concentrations of 154.31, 220.27, 1137.53, 552.81, 2025.22, and 5311.62 mg kg -1 for As, Zn, Mn, Pb, Ni, and Cu, respectively.Cymbopogon citratus, Cymbopogon nardus, and Chrysopogon zizanioides grass species were used in this study for the phytoremediation of the BCL Cu-Ni mine tailings.

Activation of phytoremediation and nonphytoremediation biochars
Chemical activation of phytoremediation and nonphytoremediation biochars was carried out using 0.1 M ZnCl 2 (99% purity grade).Biochar and ZnCl 2 solution were mixed in a ratio of 10g:100 mL.The mixing was performed at 25°C for 24 hours.After mixing, the slurry was dried in an oven at 105°C overnight.The modified biochar was then washed with 0.1 M hydrochloric acid (HCl, 36% purity grade) by stirring for 4 hours at room temperature.The activated biochar was then washed several times with deionized water.The resultant biochar was dried at 110°C for 24 hours (Abd et al. 2019).

Physicochemical characterization of non-activated (ordinary) and activated biochars
The specific surface area was determined following the multipoint Brunaer-Emmett-Teller (BET) method in a relative pressure range of 0.05 to 0.3 using a BELSORP-MINI II BET surface area and pore size analyzer (Lee and Park 2013).The t-method was used to estimate micropore volume and external surface area (Lawal et al. 2021, Buentello et al. 2020).The monolayer thickness was computed using the following equation (eq 1) for carbon-like materials: Bamangwato Concessions Limited (BCL) Cu-Ni mine tailings and uncontam Biochars were produced by pyrolyzing these biomasses at 550°C in an inert envi 30 minutes, and the chemical characteristics of the resulting biochar wer According to Ultra et al. (2022), the BCL mine tailings are highly acidic, with a 2, and contain heavy metal concentrations of 154.31, 220.27, 1137.53, 552.81, 2 5311.62 mg kg -1 for As, Zn, Mn, Pb, Ni, and Cu, respectively.Cymbopog Cymbopogon nardus, and Chrysopogon zizanioides grass species were used in th the phytoremediation of the BCL Cu-Ni mine tailings.

Activation of phytoremediation and non-phytoremediation biochars
Chemical activation of phytoremediation and non-phytoremediation biochars was using 0.1 M ZnCl 2 (99% purity grade).Biochar and ZnCl 2 solution were mixed 10g:100 mL.The mixing was performed at 25°C for 24 hours.After mixing, th dried in an oven at 105°C overnight.The modified biochar was then washed hydrochloric acid (HCl, 36% purity grade) by stirring for 4 hours at room temp activated biochar was then washed several times with deionized water.The resul was dried at 110°C for 24 hours (Abd et al. 2019).

Physicochemical characterization of non-activated (ordinary) and activated bioc
The specific surface area was determined following the multipoint Brunaer-Em (BET) method in a relative pressure range of 0.05 to 0.3 using a BELSORP-M surface area and pore size analyzer (Lee and Park 2013).The t-method was used micropore volume and external surface area (Lawal et al. 2021, Buentello et al. monolayer thickness was computed using the following equation (eq) for materials: (eq 1) where t refers to the monolayer thickness and p/p o is the relative pressure of nitrog The micropore surface area was calculated by subtracting the external from the specific surface area obtained using the BET method.The volume of th was obtained by subtracting the micropore volume from the total pore volume.As all pores are cylindrical, straight, and not interconnected, the average pore size w using the following formula: (eq 2) Where d average is the average pore size, V t is the total pore volume, and specific surface area (Hung et al. 2017, Muzyka et al. 2023).The surface mo (eq 1) where t refers to the monolayer thickness and p/p o is the relative pressure of nitrogen.The micropore surface area was calculated by subtracting the external surface area from the specific surface area obtained using the BET method.The volume of the mesopore was obtained by subtracting the micropore volume from the total pore volume.Assuming that all pores are cylindrical, straight, and not interconnected, the average pore size was estimated using the following formula: Bamangwato Concessions Limited (BCL) Cu-Ni mine tailings and uncontam Biochars were produced by pyrolyzing these biomasses at 550°C in an inert envi 30 minutes, and the chemical characteristics of the resulting biochar wer According to Ultra et al. (2022), the BCL mine tailings are highly acidic, with a 2, and contain heavy metal concentrations of 154.31, 220.27, 1137.53, 552.81, 2 5311.62 mg kg -1 for As, Zn, Mn, Pb, Ni, and Cu, respectively.Cymbopog Cymbopogon nardus, and Chrysopogon zizanioides grass species were used in th the phytoremediation of the BCL Cu-Ni mine tailings.

Activation of phytoremediation and non-phytoremediation biochars
Chemical activation of phytoremediation and non-phytoremediation biochars was using 0.1 M ZnCl 2 (99% purity grade).Biochar and ZnCl 2 solution were mixed 10g:100 mL.The mixing was performed at 25°C for 24 hours.After mixing, the dried in an oven at 105°C overnight.The modified biochar was then washed hydrochloric acid (HCl, 36% purity grade) by stirring for 4 hours at room tempe activated biochar was then washed several times with deionized water.The resul was dried at 110°C for 24 hours (Abd et al. 2019).

Physicochemical characterization of non-activated (ordinary) and activated bioc
The specific surface area was determined following the multipoint Brunaer-Em (BET) method in a relative pressure range of 0.05 to 0.3 using a BELSORP-M surface area and pore size analyzer (Lee and Park 2013).The t-method was used micropore volume and external surface area (Lawal et al. 2021, Buentello et al. monolayer thickness was computed using the following equation (eq) for materials: (eq 1) where t refers to the monolayer thickness and p/p o is the relative pressure of nitrog The micropore surface area was calculated by subtracting the external s from the specific surface area obtained using the BET method.The volume of th was obtained by subtracting the micropore volume from the total pore volume.As all pores are cylindrical, straight, and not interconnected, the average pore size wa using the following formula: (eq 2) Where d average is the average pore size, V t is the total pore volume, and specific surface area (Hung et al. 2017, Muzyka et al. 2023).The surface mo (eq 2) where d average is the average pore size, V t is the total pore volume, and SSA is the specific surface area (Hung et al. 2017, Muzyka et al. 2023).The surface morphology of ordinary and activated biochar samples was characterized using JEOL JSM-7100F Field Emission Scanning Electron Microscope at a working distance, voltage, and magnification of 10 mm, 15.0 kV, and x850 µm, respectively.Before surface morphology analysis, biochar samples were first coated with carbon.
The point of zero charge of biochar samples was determined using a modified procedure from the study conducted by Liu et al. (2015).The pH of a 0.01 M NaCl (99% purity grade) solution was adjusted to 2, 4, 6, 8, 10, and 12 using either 0.1 M HCl (36% purity grade) or 0.1 M NaOH (99% purity grade) solutions.At a selected pH, 20 ml of the 0.01 M NaCl solution was mixed with 0.2 g of biochar and oscillated for 48 h.The change in pH (pH f -pH i ) was plotted against the initial pH, and the points of zero charge were determined from where the graphs intercepted the x-axis.Mineralogical, surface functional group, and elemental analyses of activated biochars were conducted using a Bruker D8 Advance powder diffractometer (using a Cu Kα radiation source), a Fourier Transformer Infrared Spectrometer (USA Nicolet IS5), and an X-ray Fluorescence (XRF) spectrometer emitting 3 beams (beam 1 at a voltage of 50 kV, beam 2 at a voltage of 40 kV, and beam 3 at a voltage of 10 kV) using soil mode, with each beam running for 30 seconds.The effect of solution pH (2, 4, 6, 8, 10, and 12) was investigated using a selected biochar dosage (0.04 g) in 40 ml of solution containing PO 4 3-and NO 3 -ions (50 mg/L) at an orbital speed of 180 rpm for 24 h.The pH was adjusted using either 0.1 M HCl or 0.1 M NaOH.The effect of contact time (0.3, 0.5, 1, 2, 4, and 8 h) was evaluated with the same biochar dosage (0.04 g) in 40 ml of a binary solution of PO 4 3-and NO 3 ions (50 mg/L) at a solution pH of 6 and an orbital speed of 180 rpm for 24 hr.The influence of the initial concentration of PO 4 3-and NO 3  -ions (20, 50, 100, 200, 300 to 500 mg L -1 ) was analyzed by adding the selected biochar dosage (0.04 g) into 40 ml of PO 4 3-and NO 3 -solution and shaking it at an orbital speed of 180 rpm for 1h at a solution pH of 6.

Analysis of PO 4
3-and NO 3 -ion concentrations After the adsorption experiments, all mixtures were filtered using a 0.45 µm nylon syringe filter, and the concentrations of PO 4 3-and NO 3 -ions were analyzed using a Thermo Fisher DS 120 Ion Chromatography system.

Computation of adsorption capacities
The adsorption capacity values were computed using the following equations (Mehdizadeh et al. 2014): After the adsorption experiments, all mixtures were filtered using a 0.45 µm ny filter, and the concentrations of PO 4 3-and NO 3 -ions were analyzed using a Thermo 120 Ion Chromatography system.

Computation of adsorption capacities
The adsorption capacity values were computed using the following equations (Me Where Q e is equilibrium adsorption capacity (mg g -1 ), C i -initial concentratio C e -equilibrium concentration (mg L -1 ), V -volume of the solution (L), M -m adsorbate (g).

Adsorption isotherms used in this study
To determine the applicable equilibrium adsorption isotherm, the equilibrium adso were fitted to 4 isotherm models: the Langmuir model, the Freundlich model, t model, and the Dubinin-Radushkevish model (Foo and Hameed 2010, Itodo et al. models are shown in Table 1.The adsorption isotherm constants were compute slopes and intercepts of the linear equations.The best fitting isotherm for PO 4 3-and adsorption data was chosen based on the regression coefficient closest to 1. Table 1 The separation factors were calculated and plotted for biochars that bes Langmuir adsorption isotherm.The separation factor R L was calculated using equat (eq 11) Where R L is the dimensionless separation factor, K L is the Langmuir constan and C i is the initial concentration of the adsorbate (mg L -1 ).The separation factor in favorability of adsorption: for R L >1, R L =1, 0<R L <1, and R L = 0 the ad unfavorable, linear, favorable, and irreversible, respectively (Foo and Hameed 2010

Experimental quality assurance and statistical analysis
The point of zero charge (PZC) and adsorption experiments were replicated enhance the precision of the experimental results.The PZC data were subjected to variance (ANOVA) to assess significant differences between 2 factors (grass biomass sources).Turkey's homogeneity test was used to determine whether th significant difference when the mean comparison was made at a 0.05 probability le

Physicochemical characterization of non-activated (ordinary) and activated bioch
where Q e is equilibrium adsorption capacity (mg g -1 ), C iinitial concentration (mg L -1 ), C e -equilibrium concentration q e : equilibrium adsorption capacity (mg g -1 ) q m : maximum adsorption capacity of the adsorbent (mg g -1 ) K L : Langmuir adsorption constant (L mg - 1 ) C e : equilibrium concentration (5) R: gas constant (Jmol -1 K -1 ) T: absolute temperature (K) b T : Constant related to the heat of adsorption (J mol -1 ) A T : Temkin isotherm equilibrium binding constant (L mg -1 ) ϵ 2 q m : maximum adsorption capacity of the adsorbent B D : constant of adsorption energy (mol 2 kJ -2 ) ϵ: energy of adsorption (10) Note: Eq.no refers to equation number.

Adsorption isotherms used in this study
To determine the applicable equilibrium adsorption isotherm, the equilibrium adsorption data were fitted to 4 isotherm models: the Langmuir model, the Freundlich model, the Temkin model, and the Dubinin-Radushkevish model (Foo andHameed 2010, Itodo et al. 2010).The models are shown in Table 1.The adsorption isotherm constants were computed from the slopes and intercepts of the linear equations.The best fitting isotherm for PO 4 3-and NO 3 -ions adsorption data was chosen based on the regression coefficient closest to 1.
The separation factors were calculated and plotted for biochars that best fitted the Langmuir adsorption isotherm.The separation factor R L was calculated using equation 11: here Q e is equilibrium adsorption capacity (mg g -1 ), C i -initial concentration (mg L -1 ), equilibrium concentration (mg L -1 ), V -volume of the solution (L), M -mass of the ate (g).

ption isotherms used in this study
ermine the applicable equilibrium adsorption isotherm, the equilibrium adsorption data itted to 4 isotherm models: the Langmuir model, the Freundlich model, the Temkin , and the Dubinin-Radushkevish model (Foo andHameed 2010, Itodo et al. 2010).The s are shown in Table 1.The adsorption isotherm constants were computed from the and intercepts of the linear equations.The best fitting isotherm for PO 4 3-and NO 3 -ions tion data was chosen based on the regression coefficient closest to 1.
The separation factors were calculated and plotted for biochars that best fitted the uir adsorption isotherm.The separation factor R L was calculated using equation 11: (eq 11) Where R L is the dimensionless separation factor, K L is the Langmuir constant (L mg -1 ), is the initial concentration of the adsorbate (mg L -1 ).The separation factor indicates the bility of adsorption: for R L >1, R L =1, 0<R L <1, and R L = 0 the adsorption is rable, linear, favorable, and irreversible, respectively (Foo and Hameed 2010).
imental quality assurance and statistical analysis oint of zero charge (PZC) and adsorption experiments were replicated 3 times to e the precision of the experimental results.The PZC data were subjected to analysis of ce (ANOVA) to assess significant differences between 2 factors (grass types and ss sources).Turkey's homogeneity test was used to determine whether there was a cant difference when the mean comparison was made at a 0.05 probability level.

ochemical characterization of non-activated (ordinary) and activated biochars
(eq 11) where R L is the dimensionless separation factor, K L is the Langmuir constant (L mg -1 ), and C i is the initial concentration of the adsorbate (mg L -1 ).The separation factor indicates the favorability of adsorption: for R L >1, R L =1, 0<R L <1, and R L = 0 the adsorption is unfavorable, linear, favorable, and irreversible, respectively (Foo and Hameed 2010).

Experimental quality assurance and statistical analysis
The point of zero charge (PZC) and adsorption experiments were replicated 3 times to enhance the precision of the experimental results.The PZC data were subjected to analysis of variance (ANOVA) to assess significant differences between 2 factors (grass types and biomass sources).Turkey's homogeneity test was used to determine whether there was a significant difference when the mean comparison was made at a 0.05 probability level.

Physicochemical characterization of non-activated (ordinary) and activated biochars
Table 2 presents the results of specific surface area, total pore volume, and average pore diameter for all biochars.Ordinary biochars derived from phytoremediation biomasses (BCV2, BCC2, and BCL2) exhibited higher specific surface areas compared to non-phytoremediation biochars.ZnCl 2 activation increased the specific surface areas of BCV2, BCC2, and BCL2 from 1.61 to 5.61 m 2 g -1 , 4.51 to 5.23 m 2 g -1 , and 4.75 to 50.50 m 2 g -1 , respectively.Similar results have been reported by Yan et al, who observed that ZnCl 2 activation increased the specific surface area of biochar derived from aerobic granular sludge from 6.34 m 2 g -l to 852.41 m 2 g -1 (Yan et al. 2020).In this study, the total pore volume of non-phytoremediation biochars increased from a range of 0.36-0.46cm 3 g -1 to 0.44-13.00cm 3 g -1 after ZnCl 2 modification.For phytoremediation biochars, the total pore volume increased from a range of 8.23-10.05cm 3 g -1 to 13.35-71.84cm 3 g -1 (62.21-614.83%).Activated Cymbopogon citratus phytoremediation biochar (ACBCL2) showed higher micropore and mesopore volumes, corresponding to 2.34 cm 3 g -1 and 69.49cm 3 g -1 , respectively.Variations in specific surface areas among ordinary biochar derived from different grass types could be attributed to differences in their cellular contents.Lee and Shin (Lee and Shin 2021) associated a small biochar surface area with high lignin content, which does not easily decompose at low temperatures.Du et al. attributed the higher specific surface area of phytoremediation biochars to the potential catalytic effect of heavy metals in the decomposition and dehydration of organic material (Du et al. (2019).Our earlier work (Sefatlhi et al. 2023) showed that phytoremediation biochars exhibited higher heavy metal contents.Table S1 also indicates that activated phytoremediation biochars had higher heavy metal contents compared to non-phytoremediation biochars.ACBCL2 had higher concentrations of Al, Ni, Cu, As, and Zn, while ACBCV2 exhibited a higher concentration of Rb, as shown in Table S1.
Similarly, Liu et al. (2014) found out that pyrolysis of biomass loaded with Cu and Fe leads to the generation of biochar with a higher surface area due to the participation of Cu and Fe in the formation of a micro-porous structure during pyrolysis.The comparatively higher heavy metal content in phytoremediation biomass may have a similar effect on the formation of a porous structure, as reported by Liu et al. (2014) for Fe and Cu.Activation of biochar with ZnCl 2 has been shown to decompose cellulosic material, which is beneficial for the generation of micropores and results in a higher surface area (Zhao et al. 2017, Angin et al. 2013).
The average pore diameters of both ordinary and activated biochars derived from phytoremediation and nonphytoremediation biomasses were below 2 nanometers (nm), which are classified as micropores according to the International Union of Pure and Applied Chemistry (IUPAC) system (Gray et al. 2014).The average pore diameters of activated phytoremediation biochars were slightly lower than those of activated non-phytoremediation biochars, except for ACBCV2.ZnCl 2 activation slightly reduced the average pore diameter of all biochars, except for BCC1.A similar effect was observed in a study by Guo et al. (2019) who found decreased average pore diameter (from 3.533 to 2.241 nm) in ZnCl 2activated biochar produced from rice husk (Guo et al. 2019).
Figure S1 illustrates the surface functional group profile of activated biochars derived from phytoremediation and nonphytoremediation biomasses.Principal component analysis, shown in Figure S2, revealed that the composition of alcohols, allenes, thiocyanates, carboxylic acids, alkenes, alkynes, and alkanes increased from ACBCC1 and ACBCL1 to ACBCC2 and ACBCL2.XRD analysis (Figure S3) indicated that the intensities of quartz and sphalerite peaks were higher in ACBCC2 compared to the corresponding activated nonphytoremediation biochars.The higher intensity of the mineral peaks in phytoremediation biochar suggests enhanced crystalline properties, possibly due to enriched heavy metalloids (He et al. 2019).
Figure 2 shows that activated phytoremediation biochars exhibit a more porous structure compared to activated nonphytoremediation biochars.Among them, ACBCL2 displayed a more pronounced porous structure than ACBCC2 and ACBCV2.The ZnCl 2 activation process results in the widening of the existing pores and the development of new ones, which increases the specific surface area and pore volume of the biochar (Angın et al. 2013).The ZnCl 2 activation enhances intercellular-and intracellular spaces by breaking the bonds in cellulose molecules, leading to increased surface porosity (Machado et al. 2020).The high porosity observed in activated phytoremediation biochars may be attributed to the catalytic effect of the impregnated heavy metals, which facilitate the decomposition and dehydration of organic material (Du et al. 2019).
The solution pH of the PO 4 3-and NO 3 -ions binary mixture was lower than all the pHpzc values of both phytoremediation and non-phytoremediation biochars, indicating that biochar samples carried net positive charges at the solution pH.The pH pzc values of ordinary biochars decreasing in the following order: 11.76>9.75>9.73>9.63>9.50corresponding to BCV1, BCV2, BCL1, BCC1, BCL1, and BCC2, respectively.For non-phytoremediation biochars, BCV1 had the highest pH pzc value (11.76), followed by BCL1 (9.73) and BCC1 (9.72).ZnCl 2 activation reduced the pH pzc values of BCV2, BCC2, and BCL2 from 9.75, 9.50, and 9.62 to 5.72, 5.51, and 6.23, respectively.Table 3 indicates that activated phytoremediation biochars had lower pHpzc values compared to activated nonphytoremediation biochars.According to Zhao et al. (2015), the net surface charge of biochar is affected by organic functional groups and present minerals.The study further attributes low pH pzc values to the presence of inorganic constituents such as heavy metals.Table S1 supports that activated phytoremediation biochars had higher heavy metal contents compared to activated non-phytoremediation biochars.The point of zero charge values in the acidic region after activation of biochar using ZnCl 2 indicates the predominance of acidic functional groups (Thue et al. 2022, Zubir andZaini 2020).

PO 4 3-and NO 3 -ions batch adsorption tests
Increasing biochar dosage led to increased adsorption of PO 4 3and NO 3 -ions by both ordinary and activated biochars with most of biochar samples reaching their optimum adsorption point at a dosage of 0.04 g (Figure 3).Beyond 0.04 g, the increase in adsorption was minimal, so this value was adopted as the lowest dosage for further studies.The adsorption capacities of PO 4 3-and NO 3 -ions were higher in biochars derived from phytoremediation biomasses (Figure 3).The PO 4 3-ion adsorption capacities decreased in the following order: 22.13>20.24>16.57mg g -1 , corresponding to BCL2, BCC2, and BCV2, respectively, at a biochar dosage of 0.08 g.ZnCl 2 activation enhanced the PO 4 3-adsorption capacities of BCL2, BCC2, and BCV2 by 7.01(13.47%),2.12 (27.81%), and 5.06 (31.78%) mg g -1 , respectively.Similarly, Ahmed et al. (2016) reported that nitrate and phosphate ions in wastewater were better adsorbed by ZnCl 2 -activated biochars compared to ordinary biochars.The NO 3 -ions adsorption capacities of activated phytoremediation biochars increased in the  1 Means within the same column, followed by the same letter(s) are not significantly different according to Turkey's homogeneity test at 5% significance.* = significant, ** = highly significant, *** = very highly significant; ns = non-significant.
Wang et al. explicated that increasing biochar dosage enhances the reactive functional groups and the contact area for the adsorbate adsorption (Wang et al. 2021).The complex mechanisms of adsorbate adsorption onto the surface of the adsorbent primarily involve ligand exchange, precipitation, hydrogen bonding, surface complexation, ion exchange, electrostatic attraction, and weak van der Waals forces (Priya et al. 2022).Phytoremediation and non-phytoremediation biochars used in this study were protonated at solution pH values lower than their pH pzc values, enabling electrostatic attraction of PO 4 3and NO 3 -ions onto biochar surfaces.The electrostatic attraction process entails the deposition of oppositely charged adsorbates onto a biochar matrix (Feng et al. 2022).This electrostatic attraction process is affected by the solution pH.biochar (Yang et al. 2021), as indicated in equations (eq)14, 15, 16, and 17: (eq 13) M n+ is positive charge on biochar surface.According to Zhang et al. (2020), the primary adsorption mechanism of PO 4 3-ions from wastewater is mainly due to the formation of calcium and magnesium phosphates.Yang et al. described the precipitation reactions of phosphorus oxyanions with Ca 2+ and Mg 2+ on biochar (Yang et al. 2021), as indicated in equations (eq)14, 15, 16, and 17: In this current study, the primary adsorption mechanism by phytoreme (eq 17) In this current study, the primary adsorption mechanism by phytoremediation biochars was related to their higher surface area and the electrostatic attraction process at solution pH values lower than the pH pzc value of each biochar sample.The high PO 4 3-and NO 3 -ions adsorption capacities displayed by biochars from phytoremediation biomasses are partly attributed to their higher surface area compared to non-phytoremediation biochars, as reported in Table 2. Adsorbents with a high surface area tend to have a higher removal capacity for the adsorbate (Gupta and Khatri 2017).
A decrease in the adsorption of PO 4 3-and NO 3 -ions by biochar as the solution pH increases could be attributed to both changes in the surface charge or charge densities of the adsorbate, and the competition effect among the PO 4 3-, NO 3 -, and OH -ions for adsorption sites on biochar surfaces under alkaline conditions (Yin et al. 2018).According to Wang et al. (2018), phosphate ions are preferentially adsorbed over nitrate ions due to their higher valency and higher atomic number.Similar to our current study, Hafshejani et al. (2016) observed minimal PO 4 3-and NO 3 -adsorption when solution pH was greater than the point zero charge, as a result of electrostatic repulsion between negatively charged biochar and the PO 4 3-and NO 3 -ions.Phosphate ions exist in three forms in water: organic phosphate, condensed phosphate, and orthophosphate (Priya et al. 2022).Different forms of orthophosphate include H 3 PO 4 , H 2 PO 4 -, HPO 4 2-and PO 4 3-(Boyd 2019).Solution pH governs the degree of ionization of phosphate ions and consequently affects their adsorption (Yao et al. 2011).According to Choi et al. (2019), H 3 PO 4 , H 2 PO 4 -, PO 4 2-and PO 4 3-are predominant at solution pH <2. 1, 2.1<pH<7.2,7.2<pH<12.7 and pH>12.7,respectively.NO 3  -ions are stable across a pH scale of 0 to 14 under strongly oxidizing conditions (Zhao et al. 2016).Previous studies (Ip et al. 2009, Shen et al. 2021, Xia et al. 2020, Xu et al. 2021) have proved that it is possible to have affinity between adsorbate and adsorbent bearing the same charges.Van der Waals forces can overcome electrostatic repulsion forces, thus allowing the attachment of the adsorbate onto the adsorbent matrix (Ip et al. 2009, Egbedina et al. 2021).Dai et al. (2020) observed a decrease in electrostatic attraction from pH 3 to 6, with negative charges on biochar increasing Figure 5 shows that adsorption by both ordinary and activated biochars increased sharply during the initial time period.The adsorption process reached equilibrium at a contact time of 1 hour.Rapid adsorption of PO 4 3-and NO 3 -ion between 0.3 and 1h of contact time is attributed to the availability of active adsorption sites on the biochar surface (Hafshejani et al. 2016).The lack of significant changes in adsorption capacities with  further increases in contact time indicates that the adsorbates have occupied all adsorption sites, or equilibrium has been achieved (Mishra et al. 2014).Generally, the adsorption of PO 4 3-and NO 3 -ions by both ordinary phytoremediation and non-phytoremediation biochars increased as the initial concentrations of PO 4 3-and NO 3 -ions increased (see Figure 6).
The obtained findings align with the phenomenon that adsorption increases with an increasing initial concentration of the adsorbate (Elaigwu et al. 2010).A continuous increase in the initial concentration results in more molecules being available for adsorption at the active sites on the adsorbent's surface (Elaigwu et al. 2010).This, in turn, increases the adsorption capacity until saturation of the adsorption sites.Increasing the initial concentration of the adsorbate also increases the concentration gradient between the solution and the adsorbent, resulting in better mass transfer (Milmile et al. 2011).The rapid uptake of PO 4 3-and NO 3 -ions at low concentrations is attributed to the higher surface area available on the biochar surface for the adsorption of fewer PO 4 3-and NO 3 -ion species (Berkessa et al. 2019).In chemisorption, where adsorption occurs at higher initial concentrations of PO 4 3-and NO 3 -ions , monolayer adsorption is attained, preventing further removal of PO 4 3-and NO 3 -ions by the biochar surfaces (Sayadi et al. 2020).The biochars reached optimum adsorption at initial concentrations of 300 for PO 4 3-ions and 200 mg L -1 for NO 3 ions, with activated biochars displaying higher adsorption capacities compared to ordinary biochars.This could be attributed to their improved adsorption characteristics, such as highly porous structures, high surface area, pore size, and volume, as shown in Figure 2 and Table 2.
The K L (Langmuir adsorption constant) represents the strength of adsorption.The values in Table 5 indicate weak adsorption strength, with K L values ranging between 0.007 and 0.046 L mg -1 .Figure 6 shows the separation fac tor (R L ) values ranging from 0 to 1 for both PO 4 3-and NO 3 -ions adsorption.The graphs indicate a decreasing trend in the separation factor with increasing initial concentrations of PO 4 3-and NO 3 ions.The separation factor graphs for PO 4 3-ion adsorption descended in the following order: BCV1>ACBCV2> ACBCV1> BCV1>ACBCC2.For nitrate ion adsorption, the separation factor versus initial concentration decreased in the following order: BCV1> BCL2> ACBCC2.The uptake of adsorbate by the adsorbent is determined by the separation factor constant (R L ) (Temkin and Acikel 2022).The adsorption isotherm is considered appropriate or favorable if the R L values range between 0 and 1.The decrease in the separation factor with increasing initial concentration of the adsorbate implies that the adsorption is more favorable at higher concentrations (Chen et al. 2017).The heat of adsorption (K T ) constant from the Temkin adsorption model ranged from 0.006 to 0.024.Nandiyanto et al. (2020) associated low K T values with minimal binding energy between the adsorbent and the adsorbate.B D refers to the free energy of adsorption as the adsorbate diffuses from the bulk solution onto the adsorbent's surface (Olalekan et al. 2013).B D values from Dubinin-Radushkevich adsorption isotherm ranged between 0.001 and 0.004 mol 2 /kJ 2 .

Conclusion
In conclusion, phytoremediation biochars displayed more porous structures, higher specific surface areas, and greater pore volumes compared to non-phytoremediation biochars.The pore sizes of ordinary phytoremediation and non-phytoremediation biochars ranged from 0.02 to 0.08 nm and from 0.04 to 0.12 nm, respectively.These parameters were further enhanced after the activation of ordinary biochars using ZnCl 2 .All biochar samples used in this study were protonated, and the point of zero charge values of the activated biochars was lower than that of ordinary biochars.Among the biochars, ACBCL2 displayed higher PO 4 3-and NO 3 -ion adsorption capacities compared to ACBCC2 and ACBCV2.Specifically, ACBCV2, ACBCC2, and ACBCL2 could remove 115.70, 101.74, and 270.59 mg g -1 of phosphate ions from wastewater, respectively.Additionally, ACBCL2, ACBCC2, and ACBCV2 could remove 155.78, 99.42, and 117.71 mg g -1 of nitrate ions from wastewater, respectively.This study recommends exploring different activation reagents to enhance net positive charges on phytoremediation biochar surfaces, thereby maximizing their potential for adsorption anionic contaminants in wastewater.Different impregnations could be investigated during the activation process to identify the best impregnation ratio that could help in optimizing the adsorption of oppositely charged adsorbate.The mechanisms of adsorption of adsorbates onto adsorbent matrices bearing the same charges should be further studied.
adsorption tests A binary solution of phosphate and nitrate ions was prepared by dissolving sodium nitrate (NaNO 3 ) and potassium dihydrogen phosphate (KH 2 PO 4 ) salts in deionized water with a resistivity value of 5.10 MΩcm following the procedure described by Alagha et al. (2020).Batch adsorption tests for PO 4 3-and NO 3 ions were conducted under varying conditions: (a) biochar dosages, (b) solution pH values, (c) contact times, and (d) initial ion concentrations.The effect of biochar dosage (0, 20, 100, 200, 1000, and 2000 mg/L) was examined in 40 ml of PO 4 3-and NO 3 -ions binary solution (50 mg L -1 ) for 24 h at an orbital speed of 180 r.p.m.
positively charged phytoremediation and non-phytoremediation biochars: Wang et al. explicated that increasing biochar dosage enhances the reactive functio groups and the contact area for the adsorbate adsorption (Wang et al. 2021).The comp mechanisms of adsorbate adsorption onto the surface of the adsorbent primarily invo ligand exchange, precipitation, hydrogen bonding, surface complexation, ion exchan electrostatic attraction, and weak van der Waals forces (Priya et al. 2022).Phytoremediat and non-phytoremediation biochars used in this study were protonated at solution pH val lower than their pH pzc values, enabling electrostatic attraction of PO 4 3-and NO 3 -ions o biochar surfaces.The electrostatic attraction process entails the deposition of opposit charged adsorbates onto a biochar matrix (Feng et al. 2022).This electrostatic attract process is affected by the solution pH.Equations 12 and 13 illustrate the electrostatic attraction of PO charged phytoremediation and non-phytoremediation biochars: M n+ is positive charge on biochar surface.According to Zhang et al. (2020), the primary adsorption mechanism of PO 4 3-i from wastewater is mainly due to the formation of calcium and magnesium phosphates.Ya et al. described the precipitation reactions of phosphorus oxyanions with Ca 2+ and Mg 2+ biochar (Yang et al. 2021), as indicated in equations (eq)14, 15, 16, and 17: (eq 12) Wang et al. explicated that increasing biochar dosage enhances the reactive function groups and the contact area for the adsorbate adsorption (Wang et al. 2021).The compl mechanisms of adsorbate adsorption onto the surface of the adsorbent primarily invol ligand exchange, precipitation, hydrogen bonding, surface complexation, ion exchang electrostatic attraction, and weak van der Waals forces (Priya et al. 2022).Phytoremediati and non-phytoremediation biochars used in this study were protonated at solution pH valu lower than their pH pzc values, enabling electrostatic attraction of PO 4 3-and NO 3 -ions on biochar surfaces.The electrostatic attraction process entails the deposition of opposite charged adsorbates onto a biochar matrix (Feng et al. 2022).This electrostatic attracti process is affected by the solution pH.Equations 12 and 13 illustrate the electrostatic attraction of PO charged phytoremediation and non-phytoremediation biochars: M n+ is positive charge on biochar surface.According to Zhang et al. (2020), the primary adsorption mechanism of PO 4 3-io from wastewater is mainly due to the formation of calcium and magnesium phosphates.Ya et al. described the precipitation reactions of phosphorus oxyanions with Ca 2+ and Mg 2+ Wang et al. explicated that increasing biochar dosage enhances the react groups and the contact area for the adsorbate adsorption(Wang et al. 2021).mechanisms of adsorbate adsorption onto the surface of the adsorbent prim ligand exchange, precipitation, hydrogen bonding, surface complexation, i electrostatic attraction, and weak van der Waals forces(Priya et al. 2022).Phy and non-phytoremediation biochars used in this study were protonated at solut lower than their pH pzc values, enabling electrostatic attraction of PO 4 3-and N biochar surfaces.The electrostatic attraction process entails the deposition charged adsorbates onto a biochar matrix(Feng et al. 2022).This electrost process is affected by the solution pH.Equations 12 and 13 illustrate the electrostatic attraction of PO charged phytoremediation and non-phytoremediation biochar M n+ is positive charge on biochar surface.According toZhang et al. (2020), the primary adsorption mechanism from wastewater is mainly due to the formation of calcium and magnesium pho et al. described the precipitation reactions of phosphorus oxyanions with Ca 2+ biochar (Yang et al. 2021), as indicated in equations (eq)14, 15, 16, and 17: (eq 14) Wang et al. explicated that increasing biochar dosage enhances the reacti groups and the contact area for the adsorbate adsorption (Wang et al. 2021).mechanisms of adsorbate adsorption onto the surface of the adsorbent prim ligand exchange, precipitation, hydrogen bonding, surface complexation, io electrostatic attraction, and weak van der Waals forces (Priya et al. 2022).Phyt and non-phytoremediation biochars used in this study were protonated at soluti lower than their pH pzc values, enabling electrostatic attraction of PO 4 3-and NO biochar surfaces.The electrostatic attraction process entails the deposition o charged adsorbates onto a biochar matrix (Feng et al. 2022).This electrosta process is affected by the solution pH.Equations 12 and 13 illustrate the electrostatic attraction of PO charged phytoremediation and non-phytoremediation biochars M n+ is positive charge on biochar surface.According to Zhang et al. (2020), the primary adsorption mechanism from wastewater is mainly due to the formation of calcium and magnesium phos et al. described the precipitation reactions of phosphorus oxyanions with Ca 2+ biochar (Yang et al. 2021), as indicated in equations (eq)14, 15, 16, and 17: (eq 15) Wang et al. explicated that increasing biochar dosage enhances the reactiv groups and the contact area for the adsorbate adsorption (Wang et al. 2021).T mechanisms of adsorbate adsorption onto the surface of the adsorbent prima ligand exchange, precipitation, hydrogen bonding, surface complexation, ion electrostatic attraction, and weak van der Waals forces (Priya et al. 2022).Phyto and non-phytoremediation biochars used in this study were protonated at solutio lower than their pH pzc values, enabling electrostatic attraction of PO 4 3-and NO biochar surfaces.The electrostatic attraction process entails the deposition of charged adsorbates onto a biochar matrix (Feng et al. 2022).This electrostat process is affected by the solution pH.Equations 12 and 13 illustrate the electrostatic attraction of PO charged phytoremediation and non-phytoremediation biochars: M n+ is positive charge on biochar surface.According to Zhang et al. (2020), the primary adsorption mechanism o from wastewater is mainly due to the formation of calcium and magnesium phosp et al. described the precipitation reactions of phosphorus oxyanions with Ca 2+ a biochar (Yang et al. 2021), as indicated in equations (eq)14, 15, 16, and 17: In this current study, the primary adsorption mechanism by phytoremediat (eq 16) Wang et al. explicated that increasing biochar dosage enhances the rea groups and the contact area for the adsorbate adsorption (Wang et al. 2021 mechanisms of adsorbate adsorption onto the surface of the adsorbent pr ligand exchange, precipitation, hydrogen bonding, surface complexation, electrostatic attraction, and weak van der Waals forces (Priya et al. 2022).P and non-phytoremediation biochars used in this study were protonated at sol lower than their pH pzc values, enabling electrostatic attraction of PO 4 3-and biochar surfaces.The electrostatic attraction process entails the depositio charged adsorbates onto a biochar matrix (Feng et al. 2022).This electro process is affected by the solution pH.Equations 12 and 13 illustrate the electrostatic attraction of PO charged phytoremediation and non-phytoremediation bioch M n+ is positive charge on biochar surface.According to Zhang et al. (2020), the primary adsorption mechanis from wastewater is mainly due to the formation of calcium and magnesium p et al. described the precipitation reactions of phosphorus oxyanions with Ca biochar (Yang et al. 2021), as indicated in equations (eq)14, 15, 16, and 17:

Figure 3 .
Figure 3.Effect of solution pH on PO 4 3-and NO 3 -adsorption by ordinary and activated biochars derived from phytoremediation biomasses.Q: adsorption capacity,

Figure 4 .
Figure 4. Effect of contact time on PO 4 3-and NO 3 -adsorption by ordinary and activated biochars derived from phytoremediation biomasses.Q: adsorption capacity,

Figure 5 .
Figure 5.Effect of initial concentration on PO 4 3-and NO 3 -adsorption by ordinary and activated biochars derived from phytoremediation biomasses.Q: adsorption capacity,

Table 1 .
Equilibrium adsorption isotherm models used in this study

Table 2 .
Specific surface area, total pore volume, and average pore diameter of ordinary and activated phytoremediation and nonphytoremediation biochars

Table 3 .
Point of zero charges of ordinary and activated non-phytoremediation and phytoremediation biochars : is the constant related to the heat of adsorption, K L , A T , and B D are the Langmuir constant, Temkin isotherm equilibrium binding constant, and Dubinin-Radushkevich constant of adsorption energy.