Biochar from Agricultural by-Products for the Removal of Lead and Cadmium from Drinking Water

: This study reports the adsorption capacity of lead Pb 2 + and cadmium Cd 2 + of biochar obtained from: peanut shell (BCM), “chonta” pulp (BCH) and corn cob (BZM) calcined at 500, 600 and 700 ◦ C, respectively. The optimal adsorbent dose, pH, maximum adsorption capacity and adsorption kinetics were evaluated. The biochar with the highest Pb 2 + and Cd 2 + removal capacity is obtained from the peanut shell (BCM) calcined at 565 ◦ C in 45 min. The optimal experimental conditions were: 14 g L − 1 (dose of sorbent) and pH between 5 and 7. The sorption experimental data were best ﬁtted to the Freundlich isotherm model. High removal rates were obtained: 95.96% for Pb 2 + and 99.05. for Cd 2 + . The BCH and BZM revealed lower e ﬃ ciency of Pb 2 + and Cd 2 + removal than BCM biochar. The results suggest that biochar may be useful for the removal of heavy metals (Pb 2 + and Cd 2 + ) from drinking water.


Introduction
Water pollution by wastewater discharges into rivers or bodies of water by anthropogenic activities has increased due to population growth [1,2]. Heavy metals in water promote toxicity, and they are not biodegradable [3,4]. Low concentration of heavy metals has a great impact on human health and aquatic life. They can cause respiratory problems, weakening of the immune system, damage to the kidneys or liver, genetic and neurological alterations and death [5]. Lead (Pb 2+ ) and cadmium (Cd 2+ ) are abundant in nature; however, they are very toxic. Pb 2+ and Cd 2+ are incorporated into the food chain in low concentrations by water systems, affecting wildlife and people [6].
In South America, some rivers that supply drinking water to cities contain Pb 2+ in high concentrations. The Rímac River in Lima, Peru, in 2009, registered a concentration of 2.15 mg L −1 Pb 2+ [7]. In 2017, the Rímac river maintained a high Pb 2+ concentration (2.064 mg L −1 ) and also reported a Cd 2+ concentration of 0.038 mg L −1 [8]. In Ecuador, some rivers contain heavy metals from mining [9]. The Puyango river, located between Loja and El Oro provinces at southern Ecuador, reported an average content of 0.77 mg L −1 of Pb 2+ . However, the water from Puyango river is used for agricultural application and human consumption by northern Peru. standard [34,39]. The yield was determined by weight mass difference between the biomass and the biochar obtained after calcination [40]. The moisture content was determined by drying 1 g of biochar at 105 • C for 180 min, then the sample was placed in a desiccator until the final weight was registered [41].
The morphology and composition of the biochar were studied in a Scanning Electron Microscopy coupled to the Energy Dispersive Spectroscopy system.
To biochar specific surface was determined by the nitrogen gas adsorption method. An automatic adsorption analyzer (Micrometrics) was used. Trials were performed with three replicates and average values are reported.
The ash content was determined using 0.10 g of dry biochar. The sample was introduced into a muffle preheated at 650 • C between 3 h and 16 h. The calcination was completed when constant weight was obtained [42]. The volatile material was determined by weighing 1 g of biochar, which is preheated in a muffle at 950 • C for 7 min. Finally, the biochar weight correspond to the non-volatile compounds [43]. The moisture, volatile material, ash and fixed carbon of biochar is equal to 100% of the carbon composition. The fixed carbon was determined by mass balance from 100% of the carbon composition, the percentage of moisture, ash and volatile material [39].
The weight: volume ratio as the apparent density was determined according to ASTM D2854 -09 (Standard Test Method for Apparent Density of Activated Carbon). An electro vibrator was used with a uniform flow range of 0.75 cm 3 s −1 to 1 cm 3 s −1 , to improve the density results [44].
Finally, the use of a Hirox KH 8700 digital microscope and Labscope software, the pore size on the grain surface was measured.
where q e (mg g −1 ) is the adsorption capacity; C o (mg L −1 ) is the initial concentration; C e . (mg L −1 ) is the equilibrium concentration; W (L) is the volume of Pb 2+ or Cd 2+ aqueous solution and m (g) is the sorbent mass. The removal percentage (%Removal) were determined by Equation (2) [32].

Effect of pH
The adsorption capacity and removal percentage were evaluated at pH 3, 5, 7 and 9 at room temperature of 18 • C. Synthetic solutions containing 2 mg L −1 Pb 2+ and 2 mg L −1 Cd 2+ at were prepared from Pb (NO 3 ) 2 (purity 99.5%, MERCK) and using a standard of 1000 mg L −1 Cd 2+ , SIGMA -ALDRICH, respectively. The pH of the solution was adjusted using NH4OH at 0.1 mol L-1 or HCl 0.1 M [45]. The amount of biochar and adsorbate was the same for each pH value under evaluation. The flask was stirred for 45 min at 140 rpm. The equilibrated solution were filtered on a 45 µm filter paper and HNO 3 was added (0.1 mol L −1 ) to avoid the precipitation of metal ions [28]. An inductively coupled plasma optical emission spectrometer (Perkin Elmer OPTIMA 8000) was used for determining the metals' concentration. The adsorption capacity and removal percentage were determined.

Effect of Adsorbent Dose
The amount of biochar was evaluated using concentrations of 8, 10, 12, 14 and 16 g L −1 of adsorbent at room temperature at 18 • C. 100 mL of solution containing 2 mg L −1 Pb 2+ and 2 mg L −1 Cd 2+ at pH 5.
The flask was stirred for 45 min at 140 rpm. The equilibrated solution was filtered on a 45 µm filter paper. The equilibrium concentration C e was determined.

Adsorption Isotherms
The initial concentrations of the synthetic Pb 2+ and Cd 2+ solutions were: 1, 2, 3 and 4 mg L −1 . 50 mL of synthetic solutions using 2 mg L −1 of sorbent were equilibrated during 45 min at 18 • C. The equilibrated solution was filtered on a 45 µm filter paper. The equilibrium concentration C e was determined. The experimental data were fitted to de Langmuir and Freundlich isotherm models [28,44]. Langmuir model describe the chemical or monolayer adsorption and Freundlich will indicate whether it is a physical or multilayer adsorption. The Langmuir isotherm model is represented by Equation (3) [46].
where is maximum adsorption capacity required for the formation of monolayer; K L (L mg −1 ) is a Langmuir constant related to the affinity constant between the adsorbent and an adsorbate. The Freundlich equation model is represented by Equation (4) [47].
where K F (L mg −1 ) is the Freundlich adsorption constant, which characterizes the strength of adsorption; n (dimensionless) is a Freundlich intensity parameter.

Adsorption Kinetic
The adsorption kinetics of a 2 mg L −1 Pb 2+ and 2 mg L −1 Cd 2+ solution was performed at 5 • C and 18 • C. In total, 100 mL of a 2 mg L −1 Pb 2+ and Cd 2+ at 5 were equilibrated with biochar at 140 rpm. A 5 mL aliquot was taken from the solution during equilibration at 5, 15, 30, 45 and 60 min [37]. The equilibrated solution was filtered on a 45 µm filter paper. The equilibrium concentration C e was determined. The experimental data were fitted to the pseudo-first order [48] and the pseudo-second order [49] kinetic model. The adsorption of metal ions as a function of time was determined by Equation (5) [37].
where q t (mg g −1 ) is the sorption capacity at t time; C t (mg L −1 ) is the concentration of an adsorbate after a contact time t (min). The Lagergren pseudo-first order model is represented by Equation (6) [48].
where K 1 (min −1 ) is the first order rate constant; t (min) is the contact time. The Ho Model or pseudo-second order is denoted by Equation (7) [49].
Integrating the previous equation for the conditions of q t = 0 for t = 0, Equation (8) [50].
where K 2 (g mg −1 min −1 ) is the second order constant. The adsorption rate is denoted by Equation (9). v = q e t e (9) where v (mg g −1 min −1 ) is the adsorption rate; t e (min) is the equilibrium time determined by the kinetic.

Calcination Conditions
The curves for the fixed carbon content at 30, 45 and 60 min are presented by Figure 1. They are identified as FC (30 min  The intersection of Y and FC curves is the optimal carbonization point. For the BCM, the optimal calcination point was determined at 565 • C for 45 min. The BCH is optimally calcinated at 630 • C for 45 min, and the BZM is optimally calcined at the 600 • C for 45 min. At 30 and 60 min, biochar present high content of FC and a low Y.
These results have been compared with other studies developed with biochar obtained from organic material. N'goran et al. [37] calcined cashew shells at 500 • C for 240 min and walnut shells at 450 • C for 120 min in an electric oven. Colpas et Al. [39] calcined the corn cob at 400 • C for 60 min in a multipurpose furnace. Castellar [51] reports calcination temperatures of 530 • C for 30 min for the cassava shell, carried out in a muffle furnace. Therefore, the carbonization temperature obtained for the organic materials used is in the order of the values reported in similar studies, considering it ideal for the transformation of these biomass.

Biochar Characterization
The result of biochar characterization is presented in Table 1. Peanut shell biochar obtained at 565 • C, "chonta" pulp biochar obtained at 630 • C and corn cob biochar obtained at 600 • C. The moisture content of biochar is according to the recommended values suggested according to the ASTM D2867-04, between 2%-15%. The apparent density of the biochar is in the order of the range established by the ASTM D2854-09 standard, from 0.26-0.65 g cm −3 . The apparent density of the biochar obtained is low, so the mass of these adsorbents should be less in a batch device. This fact can be favorable because fewer particles increase the porosity and a better contact between the adsorbent's surface and the adsorbate can occur. This avoids overlapping of adsorption sites due to large adsorbent masses [44]. The biochars obtained are in accordance with the AWWA B604-90 (Standard for granular activated carbon) [52]. This Standard recommends that the particle of the granular carbon for water treatment should have an effective size between 0.4 to 3.3 mm.
The ASTM D3175-20 (Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke) considers Mv is optimal between 21.25% and 28.84%. ASTM D3175-20 (Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke) establishes a range of 21.25%-28.84% for Mv. The Mv of BCH is higher than BCM and BZM. A biochar with a low content of volatile material is not very combustible; the lower the amount of Mv, the higher the fixed carbon content, an aspect that will favor at the time of adsorption [39,53,54].
The Cc of the biochar are in accordance with the ASTM D-2866-11 (Standard Test Method for Total Ash Content of Activated Carbon) standard establishes a range from 3% to 15%. A high ash content indicates that the biochar is fragile to carbonize but the low Cf content obtained affect its specific surface the adsorption capacity [55,56]. The higher specific surface the better the adsorption capacity. The specific surface of the BCM was higher and had a higher adsorption capacity than the other materials tested. The specific surface of the biochar was compared with other similar materials ( Table 2).

Sorbent Chemical Characteristics
The morphology and chemical compositions of the adsorbents were analyzed using a scanning electron microscope (SEM) coupled Energy Dispersive X-ray spectroscopy (EDX) (EDX) (Figure 2a-c). BZM contains mainly silicon, the whitish coloration can be leathery materials from the plant that generated the carbon. BCH contains a lot of calcium sulphate, gypsum, some phosphorus (typical of organic materials) and a little silicon.

Adsorption as A Function of pH
The pH is determinant for the adsorption of heavy metals for biochar, since it shows that the adsorption occurs through electrostatic attraction. In Figure 3 is represented the adsorption as a function of pH for the three biochars. The Pb 2+ adsorption increases with the increase of pH from 3 to 5. However, when pH increase from 7 to 9 the Pb 2+ adsorption decrease. The Cd 2+ removal percentage is the same for the pH range evaluated. The optimal pH for the removal of Pb 2+ and Cd 2+ is performed the best at pH 5. The adsorption decrease below pH 5 because an excess of hydrogen ions is generated, producing a competition with the positively charged metal ions towards the same places on the adsorbent surface [37]. Removal also decreases when pH is higher than 7, the pH increase promotes the formation of anionic hydroxide complexes that decrease the concentrations of free Pb 2+ ions [62]. The occurrence of the hydrolysis reactions is represented by Equations (10)-(12): [Pb 2+ + OH − → Pb(OH) + , -pk a = 6.48] (10) [Pb(OH) + + OH − → Pb(OH) 2 , -pk a = 11.16] (11) [Pb(OH) 2 + OH − → Pb(OH) 3 − , -pk a = 14.16] Kumar et al. [61] and Coelho et al. [63] determine pH 5 is the best for lead and cadmium removal using activated carbon obtained from cashew nuts in India and Brazil, respectively. The behavior of the biochar evaluated in this study represents an excellent possibility for drinking water treatment with typical pH values between 6.5 and 7.5.

Effect of Adsorbent Dose
The BCM using 14 g L −1 allowed 86% of Pb 2+ removal, but BCH and BZM with 12 g L −1 removes a maximum of 74% and 85%, respectively (Figure 4). There are no differences between the BCM doses used for Cd 2+ removal. The BCH and BZM using 14g L −1 obtained over 90% of Cd 2+ removal. The percentage of removal increases with mass biochar increase. The greater the biochar mass the more available spaces for adsorption. However, when the equilibrium is reached, no matter how much biochar is used, the removal percentage does not increase. The aggregation or partial agglomeration of the adsorbent particles in higher concentration promotes this behavior [6,64,65]. N'goran et al. [37] reported the optimum Pb 2+ removal using activated carbon from cashews and shea nuts using 12 g L −1 . Coelho et al. [63] reported experimental essays using 12 g L −1 for activated carbon prepared from Brazilian cashew shell.

Adsorption Kinetics
The equilibrium sorption for Pb 2+ and Cd 2+ on the biochar was reached within 45 min at which was obtained the highest removal value ( Figure 5). BCM developed the higher adsorption rate 0.0029 mg −1 g min −1 which developed the highest Pb 2+ removal. The removal percentage of biochar: 95.96% for BCM, the 87.37% for BCH and 67.77% for BZM (Table 3). The adsorption capacity depends on the specific surface because the BCM (1224 m 2 g −1 ) is higher than BZM and BCH. The BCM has a great surface to retain Pb 2+ and Cd 2+ from water. The kinetic parameters of biochar evaluated in this study are summary in Table 4. So, the adsorption mechanism of Pb 2+ and Cd 2+ removal on biochar can be determined [65]. Taking into account the R 2 value near to 1 the experimental sorption data are best fitted to the pseudo-second order kinetic model. So, Pb 2+ and Cd 2+ removal on biochar is performed by chemisorption. Furthermore, Pb 2+ and Cd 2+ removal by biochar is governed by physisorption due to the electrostatic attraction previously discussed as an effect of pH [66].
Chemical sorption reactions occur through chemical bonds at specific functional groups which are irreversible. Previous studies attributed the Pb 2+ and Cd 2+ removal is performed by ion exchange reactions. The three biochar of this study contain some exchangeable metals on the surface such as: Na + , K + , Ca 2+ and Mg 2+ that allow the exchange. The adsorption of lead and cadmium by ion exchange reaction with those exchangeable ions can be described by Equation (13).
Furthermore, some complexation reactions occurred between Pb 2+ and Cd 2+ and the functional groups existing on the surfaces of coals. Biochar contains some organic groups containing oxygen that promote the Pb 2+ and Cd 2+ sorption on the biochar [67]. B-COOH and B-OH (B: biochar) represents the functional surface groups of BCM, BCH and BZM. The Pb 2+ and Cd 2+ adsorption is expected to occur by complexation reactions described in Equations (14) and (15) [29].
Furthermore, it has been found in previous reports that some precipitation reactions may occur due to the components of biochar [28,68]. However, in this study we do not have evidence of this fact. The low Pb 2+ and Cd 2+ concentrations used for the study make it improbable to detect them by SEM-EDX characterization.

Adsorption Isotherms
The Pb 2+ and Cd 2+ removal by biochar increase with the increase of initial metal concentration. Taking into account the R 2 , the experimental sorption data of both Pb 2+ and Cd 2+ on biochar are best fitted to Freundlich isotherm. The highest Pb 2+ and Cd 2+ sorption on biochar is obtained by BCM (Table 5). Accordingly, to Freundlich isotherm model the adsorption is performed by physical mechanisms (Figures 6 and 7). Physical adsorption denotes the existence of an energetically heterogeneous surface. The occurrence of Van der Waals forces and the metal adhesion to the porosity is determinant for adsorption. The Langmuir isotherm establishes chemisorption as the basis mechanism. Chemisorption takes place in a homogeneous layer, indicating finite sites on the adsorbent's surface specific for the adsorbate [13,28]. As it was reported before, Pb 2+ and Cd 2+ adsorption on the biochar occurred by specific ion exchange and complexation reactions represented by Equations (13)- (15).   The results of this study suggest the simultaneous occurrence of physical and chemical adsorption for the Pb 2+ and Cd 2+ removal. Physisorption, is the mechanism that governed the Pb 2+ and Cd 2+ caption on biochar.

Conclusions
The yield of BCH ≈ 78.64% > CZM ≈ 61.23% > BCM ≈ 60.16%. The moisture content, the effective particle size, ash content and volatile material of biochar is in accordance with the standards for granularly activated carbon. BCM has the higher Pb 2+ and Cd 2+ adsorption capacity. The physicochemical characteristics of BCM are responsible for the high adsorption behavior. BCM has the highest specific surface area of biochar: BCM ≈ 1224 m 2 g −1 > BZM ≈ 778 m 2 g −1 > BCH ≈ 652 m 2 g −1 . The Pb 2+ adsorption capacities of biochar were: BCM ≈ 2.528 mg g −1 > BCH ≈ 0.532 mg g −1 > BZM ≈ 0.453 mg g −1 . The Cd 2+ adsorption capacities of biochar were: BCM ≈ 0.314 mg g −1 > BZM ≈ 0.155 mg g −1 > BCH ≈ 0.049 mg g −1 . The experimental data were best fitted to the Freundlich isotherm model and the pseudo-second order kinetic model. The adsorption is denoted by physical mechanisms: Van der Waals forces and the biochar porosity. The adsorption on biochar is also promoted by means of chemical reactions, complexation and ion exchange.
The biochar evaluated has great potential to be used in the treatment of water, allowing Pb 2+ and Cd 2+ removal from drinking water. The low cost and availability of the raw material makes it an interesting proposal for water treatment. Furthermore, adsorption is a low-cost and easy-to-implement technology in treatment systems for drinking water. Although the energy requirement to obtain the biochar is important, the social benefit is also very relevant for developing countries, because with this application we are able to avoid serious diseases in children and adults, which is of evident public health interest. Furthermore, on the other hand, a sustainable use of waste is achieved, that otherwise would have to be managed, incurring significant costs.