A Sustainable Approach towards the Restoration of Lead-Contaminated Soils through Nutrient-Doped Olive Waste-Derived Biochar Application

: The current study was conducted to investigate the efﬁciency of olive mill waste-derived biochar and its silica-embedded and nutrient-loaded derivatives in immobilizing lead (Pb) and improving nutrients availability in contaminated sandy loam soils. Biochar was produced at 500 ◦ C and latterly modiﬁed with silica and enriched with nutrients (P and N). An incubation experiment was conducted for a period of 45 days to observe the dynamics in heavy metals and nutrient release. The produced biochar and its modiﬁed versions were characterized for physiochemical and structural properties prior to soil application. The results of incubation trials demonstrated that pseudo second-order kinetic model was ﬁtted best to Pb, P, NO 3 − and NH 4+ release characteristics. Silica embedded and nutrient loaded biochars performed outclass and showed up to 85% decline in Pb release against control treatment. Similarly, 38%, 69% and 59% increase in P, NO 3 − and NH 4+ availability, respectively, was observed with modiﬁed biochars, as compared to 22%, 59% and 32% increase with pristine biochar application, respectively. Overall, silica and nutrient doping of biochar resulted in signiﬁcantly higher immobilization of Pb in contaminated soil and increased the nutrient availability, which could be helpful in restoring heavy metal contaminated soil and improving soil fertility.


Introduction
The genesis of soil is a decades long process and in addition to its formation it has resulted in nutrient enriched topsoil, which is more valuable and provides interaction between biotic and abiotic factors present in it [1].However, parallel to the natural soil formation processes, anthropogenic activities from several years have contributed in the contamination of soil around the globe [2].Many kinds of organic and inorganic pollutants, such as toxic heavy metals, solid waste having undissolved salts and dyes released from several industries and pharmaceutical wastes with partially dissolved antibiotic residues, are released and accumulated in soil and water resources.Among these pollutants, heavy metals are the most common and widely spread toxic substances, which on their excessive accumulation in terrestrial and aquatic environment cause sever toxicity in soil and water bodies.Lateral movement of such heavy metals in soil by outflow from rocks weathering and soil erosion, pollute surface and ground water resources.
Lead (Pb) is common heavy metal, which due to its potential toxicity ranks second for its global distribution after arsenic (As) [3].Generally, Pb is released into the environment by emission from vehicle's smoke, leaching from crude oil refineries, electronic waste, waste discharge from industries and agricultural activities [4].Contamination of Pb in soil results in soil degradation, poor soil quality, soil ecological disturbance and ultimately lower soil productivity.Additionally, previous studies have reported several health-related issues, Sustainability 2023, 15, 2606 2 of 16 such as neurological syndrome in children, growth disorder in early age and behavioral ailment in adults by continuous exposure to Pb [5].
Application of effective and feasible remediation techniques to restore Pb contaminated soil is very important.Various kinds of physical, biological and chemical techniques have been employed to analytically restore such Pb contaminated soils and make it suitable for agricultural practices.Application of some metal chelating agents, using soil amendments for metal remediation, solvent extraction, thermal treatment of soil and adsorption are the commonly used and widely accepted remediation techniques for Pb immobilization in soil.Among these techniques, adsorption is the most feasible and globally accepted remediation technique since it is cost effective and practically easy to perform with minimal side effects [6].Biochar (BC) is a cost effective, easily available and an efficient adsorbent for the adsorption of various kinds of soil and aquatic based pollutants.
The BC, which is a solid product obtained from controlled thermal treatment of organic waste, has many salient features, such as high cation exchange capacity (CEC), surface porosity, high specific surface area, net negative surface charge and plenty of functional groups making it an efficient adsorbent for several kinds of organic and inorganic pollutants [7,8].Ho et al. [9] reported mineral coprecipitation mechanism for Pb removal from the aqueous phase by applying sludge derived magnetic BC.Nzediegwu et al. [10] applied wheat straw derived BC to remove Pb from the solution phase and reported great adsorption capacity (165 mg g −1 ) of applied BC for Pb.Moon et al. [11] found promising results in removing Pb from contaminated soil after the application of soybean derived BC.
However, there are some intrinsic constraints in pristine BC including high pH, large particle size and inconsistent removal efficiency for concentrated pollutants.In that context, a number of chemical and physical methods have been employed to fabricate original BC for better adsorption efficiency.Owing to the nonhazardous and noncorrosive nature of Si, tailoring of original BC with Si particles improved its surface characteristics along with a higher efficiency in removing metal toxicity and relieving other biotic and abiotic stress to plants [8,12].Additionally, the nutrient enrichment of BC reinforces its potential in nutrient retention in soil and plant availability, which ultimately enhances plant growth and biomass yield [13]. Wang et al. [14] reported higher specific surface area along with improved thermal stability of BC modified with nanoparticles of Si, which consequently improved its efficiency in removing inorganic pollutants from aqueous solutions.Thus, the current study was conducted to evaluate the efficiency of olive mill-derived BC and its modified derivatives to immobilize Pb and improve nutrient availability in Pb-contaminated sandy loam soil.Olive mill waste-derived biochar was modified via Si embedding and nutrient doping and characterized for physiochemical and structural properties.Thereafter, the synthesized amendments were applied to Pb-contaminated soil and incubated for 45 days to investigate temporal variations in Pb immobilization as well as P and N release characteristics.

Biochar Preparation
The residues of the olive oil mill press waste were collected from the Al-Jawf region, Saudi Arabia.Collected oil mill waste was segregated for large particles and dried in an oven at 60 • C. The particle size of waste was less than 0.6 mm and the certainty of the size was ensured by screening the material through a screen.After getting the required material size, the feedstock originating from the oil mill waste material (OMW) was passed through the process of pyrolysis.Temperature and retention time both were of significant importance and were maintained at 500 • C and 3 h, respectively.Pyrolysis was done in a tube furnace (Carbolite TZF 12/100/900, Hope Valley, UK) at the pyrolysis rate of 5 • C per min.The end product was obtained, weighted, properly rinsed with deionized water, placed in an oven at 65 • C and tagged as BC.

Synthesis of Silica Embedded Biochar
For the synthesis of Si embedded BC (BCSi), 10 g of sodium silicate (Na 2 SiO 3 ) was mixed in 500 mL of 1% hydrofluoric acid (HF) solution in deionized water and the mixture was stirred for complete homogenization.Then, 100 g of synthesized BC was added, and the suspension was sonicated for 30 min using Sonics Vibra-Cell VCX-500 Ultrasonic Processor.The formed suspension was stirred for 2 h for complete homogenization.Separately, 2 g of polyvinylpyrrolidone (PVP) was dissolved in 100 mL of deionized water.The PVP solution was added dropwise into the Si-BC suspension under vigorous stirring.Thereafter, the suspension was stirred at 180 • C for 2 h.The obtained material was cooled down, the solid particles were separated, washed with deionized water and dried at 60 • C.
For the synthesis of nutrient-loaded BCSi (BCSiNP), 0.29 g of P and 1.28 g of K were dissolved in 500 mL of deionized water using KH 2 PO 4 and KNO 3 salts.After proper dissolution, 5 g Na 2 SiO 3 (dissolved in 1%HF) was added and the mixture was stirred for 2 h.Thereafter, 100 g of BC was mixed and the suspension was sonicated and stirred as mentioned above.After adding the PVP solution, the suspension was stirred at 180 • C for 2 h.The resultant material was separated, washed with deionized water and dried at 60 • C.

Characterization
The BC, BCSi, and BCSiNP were subjected to different physiochemical characterization.Proximate analyses, such as ash content, moisture and volatile matter, were analyzed by using the standard procedures of ASTM D1762-84 [15], while the fixed carbon was estimated by the difference method.Electrical conductivity (EC) and pH were determined in a 1:10 ratio (w/v) in deionized water.Cation exchange capacity (CEC) was measured by following the standard procedure of Richard (1954) [16].Nitrate (NO 3 − ) and ammonium (NH 4 + ) were determined by following the standard procedures [17] using colordeveloping reagents, and absorbance was measured at wavelengths of 410 nm and 420 nm, respectively, using UV/VIS spectrophotometer (Lambda EZ 150, PerkinElmer 940 Winter St. Waltham, MA, USA).Available P was extracted by using the AB-DTPA-Ammonium Bicarbonate-diethylenetriaminepentaacetic Acid AB-DTPA extraction method [18].The color reagent was developed by mixing ascorbic acid (HC 6 H 7 O 6 ), sulfuric acid (H 2 SO 4 ), ammonium molybdate (H 8 MON 2 ) and antimony potassium tartrate (K 2 Sb 2 (C 4 H 2 O 6 ) 2 ).The concentration of P in extracts was analyzed by measuring absorbance at a wavelength of 882 nm by using a spectrophotometer (UV/VIS Spectrophotometer Lambda EZ 150, PerkinElmer, Waltham, MA, USA).Pb 2+ contents were analyzed by following the standard procedure [19].Na + and K + were analyzed by following the standard method using a flame photometer (Model:AE-EP8501, A&E Lab (UK) Co., Ltd., London, UK).The morphology and mineralogical composition of the produced materials were analyzed by using an electron microscope (SEM, EFI S50 Inspect, Amsterdam, The Netherlands) and an X-ray diffractometer (MAXima X XRD-7000, Shimadzu, Kyoto, Japan), respectively.

Collection and Characterization of Soil
The soil was collected from Derab agricultural farm located in, Riyadh (24.42 • N 46.57• E).A composite soil sample was collected from 0-30 cm depth, was ground and sieved through a 2 mm screen.Standard procedures were used for physiochemical characterization of soil [16].pH and EC were determined using water suspension ratios of 1:2 (w/v).Soil organic matter content was determined by following the standard procedures of Walkley and Black (1934) [20].CEC was analyzed by following the standard procedures of Richard [16].Saturation percentage was measured by making a saturated soil paste.Soil textural analysis was done by using the Hydrometer method.P, K, Na and Pb analyses were done as mentioned in Section 2.3.

Incubation Trials
The soil samples were spiked with Pb at 100 mg kg −1 and 200 mg kg −1 of soil individually and named Pb1 and Pb2, respectively.The moisture contents were maintained at 60% of the saturation percentage.After homogenous contamination, both the soils were tightly packed in polythene bags and then placed in the dark for 2 weeks for equilibrium.Thereafter, the soils were taken out and dried in air.After grinding, dried soils were converted individually into small fine particles and subjected to incubation trials.
The incubation experiments were carried out by amending the soils with three different types of synthesized amendments individually.BC, BCSi and BCSiNP were mixed with soils S1 and S2 at the application rate of 0%, 1% and 2% (w/w) individually.The experiments were replicated thrice including control (without any amendment).The amended soils were moistened at field capacity and arranged in an incubator in a completely randomized design (CRD).One set of all treatments including control were taken out of incubator after 1, 3, 7, 15, 30 and 45 days of incubation and soil samples were drawn.Table 1 represents the combinations of all the treatments used in the experiment.After withdrawal, soil samples were dried, ground, sieved and subjected to physiochemical analyses as mentioned in Section 2.4.

Statistical Analysis
The assembled data of the experiment was statistically analyzed by using the Statistics 8.1 program [21].The least significant difference (LSD) test was used to compare the effects of 14 different treatments with 6 different time intervals at a 0.05 level of significance.For calculation of minimum, maximum, mean, skewness, kurtosis and coefficient of variation (CV) of data, descriptive statistics were used.

Characterization of Produced and Fabricate Biochars
The outcomes of the proximate analysis of the BC, BCSi and BCSiNP are presented in the Table 2. From the results it was observed that BC yield percentage was 33% after pyrolysis at 300 • C. The moisture contents in BC (3.04%) were reduced as compared to the moisture contents in BM (6.83%), while the maximum decline in moisture contents were noted in BCSi (2.85%) and BCSiNP (2.41%), which indicated lower volatiles and higher stability of charred materials against raw BM [22].The decrease in the moisture contents of produced BCs could be due to the thermal treatment of BM and release of volatiles [23].Ash contents were increased in produced BCs and highest ash contents were found in BCSiNP (30.19%), which indicated condensation of other inorganic compounds [22].Pristine BC showed the highest fixed carbon contents (70.07%) followed by BCSi (68.61%) and BCSiNP (55.13%).Higher resident carbon contents in BCs against BM showed great recalcitrant potential, aromaticity and higher carbon stability of charred materials over raw BM.Initially, the pH of the BM was found to be lowered; however, after the pyrolysis, the pH of BC was increased to 9.62, while the pH of BCSi was noted to be 3.59 and the lowest value of pH was found in BCSiNP (2.77).Reduction in acidic functional groups, release of alkali salts from the organic compounds and increased basic functional groups resulted in the increased pH [24].The EC of BC (2.23 dSm −1 ) was found to be more than BM (0.87 dSm −1 ); however, the EC of BC-Si-NP was observed as 2.26 dSm −1 , while the EC of BC-Si was found to be the lowest (1.68 dSm −1 ) among the samples.The increase in the EC of BC could be due to thermal combustion, which resulted in a loss of volatile compounds and accumulation of inorganic salts and minerals during pyrolysis [25].
The FTIR spectra of raw BM and its derived BCs are described in Figure 1a.A wide band at 3200-3400 cm −1 in BM indicated the presence of hydroxyl (-OH) and carboxylic (COOH) functional groups along with vibrations of phenols and alcohols [26].The peak at 2916 cm −1 indicated the C-H stretching of methyl groups and methylene in BM [27][28][29].A band at 2351.09 cm −1 described stretching of C=N in BM [30].In BC and BCSi, small bands at 2920.99-2930 cm −1 indicated the presence of methyl and methylene group along with the vibration of C-H [31].The band at 1579.25 cm −1 indicated the aromatic carbon rings containing C=C bonds in the BC, BCSi and BCSiNP [25].The band at 1244 cm −1 represented the C-H stretching, OH deformation of COOH with the C-O stretching of aryl esters and ester sulphate stretching in the BC [25].The band between 1025 and 1051.14 cm −1 represented vibrations of sulfones in the BC, BCSi and BCSiNP [32].Band at 871.775 cm −1 indicated presence of minerals containing silicates (Si-O-Si) in the BC [33].In BCSiNP the stretching of C=O was observed at band 1712.69 cm −1 [34].Vibrations of a symmetric and asymmetric ionic carboxyl group were observed in BCSi at peak 1589.25 cm −1 [35].In BCSi and BCSiNP, vibration of the amine C=N, C-O-C group, PO 4 3− and presence of Si-O-Si (Siloxane) were observed at peak between 615 cm −1 and 1100-1250 cm −1 , which showed coprecipitation of minerals with (Si-O-Si) in BCSiNP [36].
Spectra of XRD analysis of the materials showed the presence of crystal material in the pristine and engineered BCs (Figure 1b).Wide peaks of carbonated Si and SiO 2 were observed in the BCSi and BCSiNP, which indicated successful fabrication of BC with incorporated Si.Moreover, the mineralogical analyses indicated the presence of inorganic CaCO 3 compounds [37].Furthermore, the small peaks associated with kalicinite were also identified, which could be due to the interaction of K and CO 2 in pristine BC [38].The XRD spectra also identified the small peaks of P 2 O 5 in the BC-Si-NP, which indicated the presence of P compounds, which were used for the coating of BC [39].In BM, the peak at 21.16Å, 43.96Å, 43.96Å and 77.42Å indicated the presence of SiO 2 , graphite, CaCO 3 and graphite C, respectively [40][41][42].In BC, the peak at 26.14Å and 28.16Å indicated the presence of sylvite and halite, respectively [40], while these sylvite and halite were also identified in BC-Si and BC-Si-NP.The peaks in the range of 29.3Å to 31.5Å represented the presence of CaCO 3 in the BC, BC-Si and BC-Si-NP [40,43].
XRD spectra also identified the small peaks of P2O5 in the BC-Si-NP, which indicated presence of P compounds, which were used for the coating of BC [39].In BM, the pea 21.16Å, 43.96Å, 43.96Å and 77.42Å indicated the presence of SiO2, graphite, CaCO3 graphite C, respectively [40][41][42].In BC, the peak at 26.14Å and 28.16Å indicated the p ence of sylvite and halite, respectively [40], while these sylvite and halite were also id tified in BC-Si and BC-Si-NP.The peaks in the range of 29.3Å to 31.5Å represented presence of CaCO3 in the BC, BC-Si and BC-Si-NP [40,43].

Soil Characterization
Physiochemical properties of the soil analyses are presented in the Table 3.The of the soil was recorded as 2.41 dSm −1 , while the pH was found in the neutral rang 7.21.The texture of the soil was found sandy loam after confirming the clay, silt and s fractions as 3.39%, 33.41% and 63.19%, respectively.A CEC value of 21.19 cmol kg −1 0.62% OM were found in the soil.The NO3 − was found 2.18 mg kg −1 , while availab content was found 9.16 mg kg −1 and CaCO3 content was recorded as 8.45%.The avail Na and K were found 25.81 mg kg −1 and 78 mg kg −1 , respectively.Available Na, K CaCO3 and NO3 − were observed in permissible limits.

Soil Characterization
Physiochemical properties of the soil analyses are presented in the Table 3.The EC of the soil was recorded as 2.41 dSm −1 , while the pH was found in the neutral range as 7.21.The texture of the soil was found sandy loam after confirming the clay, silt and sand fractions as 3.39%, 33.41% and 63.19%, respectively.A CEC value of 21.19 cmol kg −1 and 0.62% OM were found in the soil.The NO 3 − was found 2.18 mg kg −1 , while available P content was found 9.16 mg kg −1 and CaCO 3 content was recorded as 8.45%.The available Na and K were found 25.81 mg kg −1 and 78 mg kg −1 , respectively.Available Na, K, P, CaCO 3 and NO 3 − were observed in permissible limits.

Treatment Effects on Available Heavy Metals
Application of BCs decreased overall soil available heavy metals (Table S1, Supplementary Material).It was observed that the concentration of Fe and Mn was found to be more as compared to Cu, Cr and Zn.It has been observed from the previous studies that the results of our study were similar to the literature [44].From the results, it was observed that all the materials were efficient for the sorption of As and Cd, while the sorption capacity for the other heavy metals was also efficient for the fabricated materials as compared to the pristine BC.It was observed in the case of Zn that the performance of BC with both application rates was more up to 15 days as compared to other materials that could be due to the availability of sorption sites, which were occupied later on with the release of heavy metals into the soil.In case of Cu, the fabricated materials (BCSi and BCSiNP) performed well up to the 45 days indicating the availability of sorption sites.The concentration of Cu was found to be lower where BC-Si and BC-Si-NP were applied with a 2% application rate as compared to 1% amendments rate.Fe and Mn were detected more in controlled treatments; however, their availability was reduced with the fabricated materials.With the increasing time, the availability of Fe and Mn was found to be decreasing, while the availability of Zn was reduced with the application of BC; however, the availability was not affected significantly with the application of fabricated adsorbents.Overall, the fabricated materials performed well and could be used for the sorption of heavy metals with time by increasing the application rate.Moreover, plant growth could also be healthier with the application of materials because of their nutrient-holding capacity.

Treatment Effects on Soil pH and EC
A slight increase in the pH values of soil samples were observed (Table S2).After 45 days, it was observed that pH of the samples was increased as compared to the initial pH level.Overall, the minimum pH was recorded in the controlled treatment, while the maximum pH was found for the BCSiNP where Pb was applied with a 200 mg kg −1 application rate.In all the treatments, the pH was found to be high where amendments were applied with a 2% application rate as compared to a 1% application rate.In the treatments where Pb was applied with 100 mg kg −1 , pH was found to be less when compared to 200 mg kg −1 .In the case of 100 mg kg −1 , the minimum pH with a 1% application rate of amendments was found for BC followed by BCSi and BCSiNP as 7.4, 7.39 and 7.42, respectively, while after 45 days the pH was found 7.53, 7.49 and 7.6 for BC, BCSi and BCSiNP, respectively, while with the 2% application rate of amendments, initially the minimum pH was found for BC followed BCSi and BCSiNP as 7.41, 7.39 and 7.42, respectively, while after 45 days the pH level for all the treatments was found to be increased to 7.59, 7.56 and 7.62, respectively; however, the pH level for BC-Si was found to be reduced where the 2% application rate was applied.In treatments with 200 mg kg −1 Pb, the initial pH level with a 1% application rate of amendments was found to be the minimum for BCSi followed BC and BCSiNP as 7.35, 7.41 and 7.46, respectively, while this pH level was increased to 7.48, 7.59 and 7.61, respectively.Whereas, the initial pH where amendments were applied with the 2% application rate, the minimum pH was found for BCSi followed by BC and BCSiNP as 7.38, 7.46 and 7.48, respectively, while after 45 days the pH level was increased to 7.52, 7.61 and 7.64, respectively.The pH determines the availability of nutrients for plant uptake in the soil.As a consequence, the presence of basic cations as well as ash content in a considerable amount of BC resulted in increased pH values [45].Due to the alkaline nature of BC, the release of basic cations, such as Ca 2+ and K + , along with the exchange of H + and Al 3+ in the soil could be reason for the increase in soil pH [46].The presence of functional groups (hydroxyl, phenolic and carboxyl groups) containing negative charges on the surface of BC binds the excess quantity of H + ions in soil solution and could contribute to the increment of the soil pH [47].Another possible reason behind the increased soil pH could be the initiation of organic anion decarboxylation because of the incorporation of BC resulting in the consumption of an excess amount of H + in the solution of the soil by the microorganisms in a consequence of the increased attack on organic anions [48].

Kinetic Release of Pb
The variation in Pb concentration in soil was studied at regular intervals of time (Figure 2).Kinetic study of Pb release in incubated soil revealed higher release initially, which decreased latterly leading towards gradual equilibrium.The experimental data was further subjected to kinetics models, including first order and pseudo second-order, to investigate the mechanism of Pb release from soil and sorption on applied BCs (Table 4).Based on the R 2 value and calculated parameters, pseudo second-order was confirmed as the best suited model (R 2 = 0.99), while first-order showed a comparatively lower R 2 value (0.78-0.85).Pseudo second-order stated chemical adsorption as a rate limiting step in metal adsorption, which assumed Pb adsorption on tested adsorbents could be mediated by a chemical reaction between metal ions and surface functional groups, which was further controlled by the adsorbent amount and metal concentration [49,50].All added amendments showed excellent adsorption capacity for Pb adsorption against control treatment with no added amendments, which showed their ample potential in immobilizing Pb in soil and decreasing its bioavailability.Also, by increasing the BC application rate from 1% to 2%, an increase in Pb adsorption by applied BCs was noted.Such increase in Pb adsorption could be due to more available active surface sites, which lead to a higher fixation of Pb by BCs [51].More initial release of Pb indicated a higher concentration of Pb in soil, which latterly fixed on BCs surface, immobilized in soil leading towards a lower increase.Treatments receiving BCSiNPs and BCSis amendments showed higher adsorption capacity for Pb and maximum decline in released concentration of Pb even at the highest application rate of Pb.Calculated kinetics parameter confirmed a higher adsorption rate of BCSiNP2-Pb1 (53.90 mg g −1 ) followed by BCSi 2-Pb1 (50.97 mg g −1 ) and BCSiNP1-Pb1 (50.71 mg g −1 ).ditionally, the presence of the P element in nutrient loaded BC (BCSiNP) investigated th precipitation of Pb ions.In his research work, Ding et al. [52] found precipitation of P ions by P enriched bagasse BCs.Additionally, a rapid increase in Pb adsorption by Si em bedded and nutrient-loaded BCs could also be due to a decrease in pH of 2.77-3.59uni [51].Adsorption of P and N (NO3 − and NH4 + ) on tested adsorbents and its release in so were investigated at specific intervals of time (Figure 3).P and N (NO3 − and NH4 + ) relea were noted in two distinctive phases; initially rapid release was found, which decrease with time until a dynamic equilibrium.Nutrient release was further investigated by em ploying kinetics models.Calculated parameters indicated that pseudo second-order w best fitted kinetic model with highest R 2 (0.98-0.99) value followed by the first-ord model (R 2 = 0.75-0.90)(Tables 5-7).Likewise, Pb release fitness of pseudo second-ord illustrated chemical interactions between P and N (NO3 − and NH4 + ) ions and added adso bents.Among the tested adsorbents, the maximum sorption capacity for P was found BCSiNP2-Pb1 (53.90 mg g −1 ) followed by BCSi2-Pb1(50.97mg g −1 ) and BCSiNP1-Pb1(50.7

Adsorption Mechanism
A number of mechanisms are responsible for metal sorption on BC, including electrostatic interaction, surface complexation, precipitation and interlayer diffusion [52].Generally, physical adsorption of metal on the BC surface is mediated by electrostatic interaction between positively charged metal ions (Pb) and the negatively charged BC surface.Additionally, chemical reactions between surface functional groups and metal ions resulted in surface complexation and ion exchange.Sud et al. [53] reported complexation and ion exchange between Pb ions and oxygen-containing functional groups.The presence of additional functional groups (amine) on BCSi and BCSiNP further enhanced complexation interactions between Pb and adsorbent leading toward higher adsorption.Additionally, the presence of the P element in nutrient loaded BC (BCSiNP) investigated the precipitation of Pb ions.In his research work, Ding et al. [52] found precipitation of Pb ions by P enriched bagasse BCs.Additionally, a rapid increase in Pb adsorption by Si embedded and nutrient-loaded BCs could also be due to a decrease in pH of 2.77-3.59units [51].Adsorption of P and N (NO 3 − and NH 4 + ) on tested adsorbents and its release in soil were investigated at specific intervals of time (Figure 3).P and N (NO 3 − and NH 4 + ) release were noted in two distinctive phases; initially rapid release was found, which decreased with time until a dynamic equilibrium.Nutrient release was further investigated by employing kinetics models.Calculated parameters indicated that pseudo second-order was best fitted kinetic model with highest R 2 (0.98-0.99) value followed by the first-order model (R 2 = 0.75-0.90)(Tables 5-7).Likewise, Pb release fitness of pseudo second-order illustrated chemical interactions between P and N (NO 3 − and NH 4 + ) ions and added adsorbents.Among the tested adsorbents, the maximum sorption capacity for P was found in BCSiNP2-Pb1 (53.90 mg g −1 ) followed by BCSi2-Pb1(50.97mg g −1 ) and BCSiNP1-Pb1(50.71mg g −1 ).BCSiNP2-Pb1 (172.36 mg g −1 ) and BCSiNP1-Pb1(167.05mg g −1 ) showed the maximum adsorption capacity for NO 3 − ions and the highest adsorption of NH 4 + was found in treatments receiving BCSiNP2-Pb1(74.40mg g −1 ) and BCSiNP1-Pb1 (71.27 mg g −1 ) followed by BCSiNP2-Pb2 (67.63 mg g −1 ).Additionally, higher release and availability of P in BCSi and BCSiNP amended soils could be due to nutrient enrichment of these adsorbents.

Adsorption Mechanism
The overall chemical reaction between P ions and adsorbents followed a slow sorption process, which indicated rapid sorption on charged soil surfaces initially and surface diffusion on added BCs latterly leading towards slow and constant sorption [54].Such kind of sorption on external soil system and interlayer diffusion on BC's surface describes lower affinity of P ions for soil and more sorption and higher availability by increasing BC application rates [55].Additionally, physical interaction between BCs and P particles in soil could be mediated by electrostatic interaction, especially in BCSiNP, which possessed acidic pH, and it enhances electrostatic interaction between P ions (PO 4 −3 ) and the protonated surface of BCSiNP [51].Chintala et al. [56] reported excellent sorption of P ions on BC surface by precipitation of P as Mg or Ca phosphates due to chemical interactions between divalent cationic ions on the BC surface.A similar mechanism was reported by Rupa et al. [57] where exchangeable Ca and Mg ions enhanced P sorption on the BC surface by Ca and Mg phosphate precipitation and metal complexation.
Plenty of surface functional groups on tested BC's surface attributed to higher sorption of NO 3 − ions on BCs particularly due to hydrogen bonding and ionic bonding among hydroxyl functional groups on the BC surface [58,59].Additionally, NO 3 − retention was further controlled by interlayer diffusion mediated by sorptive characteristics, such as porous surface, complexation with cationic and nutrient elements, H/C and O/C ratio as well [60].Similar to NO 3 − , sorption of NH 4 + is mainly mediated by acidic functional groups, including phenolic OH and carboxylic C=O [61].Also, due to net negative surface charge, electrostatic interaction between cationic ammonium (NH 4 + ) ions and the BC surface has been reported as a main sorption mechanism [62].Additionally, hydrogen bonding at the surface functional groups on BC was also reported as a sorption mechanism for NH 4 + ions [63].

Table 1 .
Different combinations of applied soil amendments along with Pb application rates used throughout experimental study.

Table 2 .
Physiochemical characteristics of biomass and its derived biochars.

Table 3 .
Selected physiochemical properties of the soil.

Table 4 .
Derived parameters of the kinetics release of Pb as affected by biochars.

Table 5 .
Derived parameters of the kinetics release of P as affected by biochars.

Table 6 .
Derived parameters of the kinetics release of NO 3 − as affected by biochars.

Table 7 .
Derived parameters of the kinetics release of NH 4 + as affected by biochars.