The Various Forms of Cow Manure Waste as Adsorbents of Heavy Metals

In recent years, the application of cow manure waste as an adsorbent of heavy metals in water and soil has increased. The analysis of the most effective adsorbents from cow manure as materials that can reduce heavy metals, while being low-cost and easy to produce, is important in the agricultural field. This study investigated adsorbents from cow manure, such as compost, biochar and humic acid, and analyzed the capability of the adsorption mechanisms of Cr, Pb and Cd. The experiments were performed as a function of pH, adsorbent dose, initial metal ion concentration, and contact time. To investigate the mechanism of the adsorption process, the Langmuir and Freundlich models were used. The results showed that the optimum conditions of Cr, Cd and Pb ions were achieved by compost, biochar and humic acid with 83–99% removal. An adsorption isotherm model for compost, biochar and humic acid generally followed the Langmuir and Freundlich models. This study ranks the different forms of cow manure waste in the following order based on their ease of production, high adsorption capacity, and low cost: biochar > compost > humic acid.


Introduction
Livestock is essential for rural livelihoods and the economic sector in developing countries [1]. More than 40-70% of beef, milk, lamb, pork and poultry are produced in developing countries, such as Indonesia. In 2019, Indonesia's agriculture sector had 115 big and small active farms, and the number of livestock, primarily beef cattle, was approximately 183,152 head [2,3]. This enormous number of beef cattle produces a large amount of waste on a daily basis. Manure can thus constitute a valuable supply of nutrients for crops, but it can contaminate water sources when animals are present in high quantities [1]. A large amount of research has been undertaken to investigate how to decrease the negative impact of manure on the environment, with studies looking at the mechanism of adsorption, specific metal ion binding, cation exchange, precipitation, and complexation [4][5][6][7][8]. There are many forms of manure that can have a positive impact on the environment, such as biochar, compost and humic acid [9][10][11][12][13]. Biochar has great potential for increasing heavy metal immobilization in contaminated agricultural regions [14].Compost has many benefits for soil as a result of its thermophilic aerobic microorganism activity, which converts organic material into a hygienic and biodegradable product [15]. Additionally, the decomposition of organic matter, such as humic acid, is an important study area in the adsorption process, increasing mineral nutrient levels and enhancing soil characteristics, such as chemical and microorganism aspects [16][17][18]. Handling animal manure is the most effective approach to mitigate the effects of global warming on agricultural land, as well as a potential solution to agricultural and plantation management. This material has the potential to improve soil conditions and agricultural yields, especially in less fertile soils. The decomposition

Field of Study
Lembang District, West Bandung Regency, West Java, Indonesia, was chosen as the cow manure collection area ( Figure 1). This study applied a survey, a comparison and descriptive approaches to find suitable locations. The locations of animal husbandry facilities were mapped using field surveys. The results of the field survey are mapped on the base map with a 1:50,000 scale Topographical Map of Indonesia (RBI) and the 1:50,000 scale administrative map of the Lembang District. The coordinates of each location were captured using a GPS (Garmin 585, Taipei, Taiwan), and mapped using ArcGIS desktop 10.2. Appl. Sci. 2022, 12, 5763 2 of 13 ing on agricultural land, as well as a potential solution to agricultural and plantation management. This material has the potential to improve soil conditions and agricultural yields, especially in less fertile soils. The decomposition of animal waste can keep water and nutrients in the soil. This study aims to provide important information regarding the effectiveness of cow manure as a cheap and easy to produce adsorbent that can effectively decrease the heavy metals in the environment. This is in line with [19], which focused on the study of available and low-cost adsorbents that could remove heavy metal ions in the environment. The purpose of this research was to examine the capability of different types of adsorbents to reduce heavy metal content. This study provides fundamental information on the different types of cow manure adsorbents, such as compost, biochar and humic acid, and their impact on pollutant reduction.

Field of Study
Lembang District, West Bandung Regency, West Java, Indonesia, was chosen as the cow manure collection area ( Figure 1). This study applied a survey, a comparison and descriptive approaches to find suitable locations. The locations of animal husbandry facilities were mapped using field surveys. The results of the field survey are mapped on the base map with a 1:50,000 scale Topographical Map of Indonesia (RBI) and the 1:50,000 scale administrative map of the Lembang District. The coordinates of each location were captured using a GPS (Garmin 585, Taipei, Taiwan), and mapped using ArcGIS desktop 10.2.

Sample Collection
A sample was obtained at each point ( Figure 1). For laboratory analysis, 2 kg of samples were collected and stored in either a plastic container or a polythene bag, all of which were labeled appropriately. The cow manure was dried in a 60 • C oven, crushed until it passed through a 0.4 mm filter, and finally sealed in plastic containers that were labeled with a code. The samples were ready then for testing.

Production of Compost
The production of compost from cow manure was carried out naturally through incubating cow manure for 2 weeks. The purpose of this was to reduce the water content and to decompose the cow manure material.

Production of Humic Acid
The extraction of humic acids (HAs) from a dry sample of cow manure is the first step in humic acid synthesis. NaOH extraction with a 10:1 comparison was used to remove HA from this substance. For the analysis of HA, the same approach was used in this study as in [20]. The purification of HA from HA extract was conducted in the same way as in [21]. Equation (1) in [22] contains the formula for estimating the Humification Rate (HR) by calculating the HA (% content of humic acid) and FA (% content of fulvic acid) and comparing it to the percentage of organic C content.

Biochar Production from Cow Manure
The first step of producing biochar from the cow manure was to weigh the air-dried biomass samples. They were then pyrolyzed at 400 • C for 90 min in an electrical muffle furnace. Proximate analysis techniques, such as volatile matter, ash content and fixed carbon, as well as the chemical characteristics of biochar, can be seen in the procedures described in [23]. The size of the biochar was 0.4 mm.

Heavy Metal Adsorption by Compost, Biochar and HA
The adsorption of heavy metals (Cr, Pb and Cd) from compost, biochar and HA was analyzed by adjusting the following variables: pH (1-9), contact time (20, 40, 60, 80 and 100 min), weight of adsorbent, by increasing the ratio of the adsorbent (0.5 g, 1.0 g, 1.5 g, 2.0 g and 2.5 g) in the same volume of metal solution, and metal concentrations (Pb: 6, 8, 10, 12 and 14 mg/L; Cd: 1, 3, 5, 7 and 9 mg/L; and Cr: 1, 3, 5, 7 and 9 mg/L). For each treatment, 0.5 g of each adsorbent was dissolved in 50 mL heavy metal solution (Pb 10 mg/L, Cd 5 mg/L and Cr 15 mg/L), shook for 120 min, left overnight and filtered. The heavy metals were analyzed using AAS.

Percent Removal
The percentage removal of heavy metal from the solution was calculated using the equation below [24]: where C 0 and C i are the initial and equilibrium concentration (mg/L) of metal ions in solution, respectively.

Isotherm of Adsorption
The adsorption mechanism plays a critical role in decreasing pollution levels in the environment. To determine the equilibrium adsorptive behavior, we used the Langmuir and Freundlich equations (Equations (3) and (4) below) [24] to measure the adsorption isotherm.

Langmuir Equation :
C e q e Freundlich Equation : ln q e = ln K + 1 n ln C e (4) where q e is the equilibrium amount adsorbed (mg/g) and C e is the equilibrium concentration of metal ions in solution (mg/L). In the Langmuir equation, (mg/g) is the measure of the adsorption capacity under experimental conditions, and b is a constant related to adsorption energy. Freundlich treatment yielded the n parameters, which represent the bond energies between the metal ion and the adsorbent, and K, which represents bond strength.

Morphological Surface of Compost, Biochar and Humic Acid
The analysis of morphological surfaces using SEM-EDX showed that the size and form of the constituent particles of adsorbents varied ( Figure 2). Compost, biochar and humic acids from cow manure exhibited a morphologically irregular form. Most forms in the compost gave a more platy shape than biochar and humic acid. This variation is caused by the difference in processing level in the formation of compost, biochar and humic acids.

isotherm.
Langmuir Equation: = .θ + θ Freundlich Equation: ln q = ln K + ln C where qe is the equilibrium amount adsorbed (mg/g) and Ce is the equilibrium conc tion of metal ions in solution (mg/L). In the Langmuir equation, (mg/g) is the meas the adsorption capacity under experimental conditions, and b is a constant related sorption energy. Freundlich treatment yielded the n parameters, which represen bond energies between the metal ion and the adsorbent, and K, which represents strength.

Morphological Surface of Compost, Biochar and Humic Acid
The analysis of morphological surfaces using SEM-EDX showed that the siz form of the constituent particles of adsorbents varied ( Figure 2). Compost, biocha humic acids from cow manure exhibited a morphologically irregular form. Most for the compost gave a more platy shape than biochar and humic acid. This variat caused by the difference in processing level in the formation of compost, biochar an mic acids. In general, the decomposition process can make a solid material's surface mo rous [25]. In this case, the adsorbents of cow manure had a C:N ratio of 27 after two w of incubation (Section 3.2), which means that the material needed more time to ful  In general, the decomposition process can make a solid material's surface more porous [25]. In this case, the adsorbents of cow manure had a C:N ratio of 27 after two weeks of incubation (Section 3.2), which means that the material needed more time to fully decompose. Based on biochar morphology, the SEM-EDX analysis showed that the biochar had more pores than compost and humic acid, which was caused by an increase in the growth of pores in the biochar samples with the increase in temperature. The increase in temperature in process production could allow a significant improvement in biochar pore characteristics [26]. The morphological form of humic acid was irregular, but mostly of granular and crumb type. This is in line with the results of Rupiasih and Vidyasagar [27], where the humic acid from cow manure, vermicompost, sludge, and sediment showed granular structure (crystal-like forms aggregate in various shapes). Table 1 shows that the major components adsorbed from the three adsorbent materials were C and O. This is in line with the work of Masruhin et al. [28], where the presence of C and O in all materials indicated that there are constituents of polyelectrolyte macromolecules that have functional groups, such as carboxyl (-COOH).

Heavy Metal Adsorption of Cr, Pb and Cd
The chemical analysis results gave the compost material a pH of 6.73, water content of 10.79%, total N of 1.72%, organic C of 46%, K of 0.45%, P 2 O 5 of 0.04%, a C:N ratio of 27, 0.06 mg/L of Cr, 0.10 mg/L of Pb, and 0.003 mg/L of Cd. The results of the proximate analysis of compost showed volatile matter of 82.23% and ash content of 82.34%. Humic and fulvic acid contents derived from compost were 1.84% and 6.63%, respectively. Biochar had a pH of 9, water content of 3.29%, Cr levels of 0.11 mg/L, a Pb level of 0.13 mg/L, and Cd levels of 0.008 mg/L. The nitrogen level was 2.07%, with 0.09% P and Organic C of 30.12%. The ash content was 54.93%, and the volatile matter was 49.98%, according to the proximate analysis. Humic acids had 14.38% water content, 4.04% nitrogen content and 55.06% organic C content. The Humification Rate (HR) from compost was 18.41%. The humification process can be understood as a process of synthesis or resynthesis of organic compounds, which is influenced by various factors, such as the climate, quantity and quality of plant material, and soil management [29]. The HR value shows the ratio of the organic acid content in the material. For the HR analysis, a higher HR value indicates that the formation of humic and fulvic acid content in the soil is greater than the input of fresh organic matter in the soil [22]. The general features of compost, biochar and humic acids at various pH levels in relation to heavy metal adsorption are shown above (Figure 3).
Humic acid, compost and biochar exhibited various adsorption behaviors at each acidity level. Figure 3 shows that humic acid a lower adsorption capacity than both compost and biochar. This was related to humic acid's adsorption mechanism, which was affected by pH. This can also be shown in [30], which suggested that the acidity of the solution altered the mechanism of metal adsorption. As a result of the extraction with NaOH or KOH, the negative charge of humic acids might be saturated by Na or K ions. The acids achieved optimum solubility and stability in suspension as the pH increased [31]. This mechanism is consistent with humic acid's behavior, which was dissolved in base circumstances and separated in acidic conditions [31]. Figure 3 shows that the biochar had a relatively constant adsorption capacity compared with other adsorbents. The morphological characteristics of the adsorbent material ( Figure 2) could also affect the adsorption capacity of heavy metals, where materials with more pores had a greater capability of adsorbing the heavy metals than others. This is in line with [32], which showed that carbonization and calcination produced a more porous adsorbent material. This condition was different from that of humic acid, which produced the same path. In general, the mechanism of the adsorption process was influenced by the surface charge of the adsorbent, the degree of ionization and the speciation of the adsorbate [24]. In general, the behavior of heavy metal Pb in relation to adsorbent humic acid produced a consistent path. The rate of heavy metal adsorption increased when the pH of the solution rose, with the optimum adsorption at a pH of 8. This is in line with [30], which showed that the adsorption behavior of lignin at various pH levels provides a consistent pattern of Pb adsorption.
Appl. Sci. 2022, 12, 5763 6 of 13 mechanism is consistent with humic acid's behavior, which was dissolved in base circumstances and separated in acidic conditions [31]. Figure 3 shows that the biochar had a relatively constant adsorption capacity compared with other adsorbents. The morphological characteristics of the adsorbent material ( Figure 2) could also affect the adsorption capacity of heavy metals, where materials with more pores had a greater capability of adsorbing the heavy metals than others. This is in line with [32], which showed that carbonization and calcination produced a more porous adsorbent material. This condition was different from that of humic acid, which produced the same path. In general, the mechanism of the adsorption process was influenced by the surface charge of the adsorbent, the degree of ionization and the speciation of the adsorbate [24]. In general, the behavior of heavy metal Pb in relation to adsorbent humic acid produced a consistent path. The rate of heavy metal adsorption increased when the pH of the solution rose, with the optimum adsorption at a pH of 8. This is in line with [30], which showed that the adsorption behavior of lignin at various pH levels provides a consistent pattern of Pb adsorption.  The adsorption of all heavy metals rose with an increase of the adsorbent material, as shown in Figure 4. The adsorption of Cr in solution gave a constant path with higher adsorption for three type of adsorbents than other heavy metals, and biochar was gave the higher adsorption that compost and humic acid. According to [33], biochar has a negative charge on the surface that can bind the heavy metal, which increases the adsorption process. The increase in the adsorbate in solution to be adsorbed by materials was responsible for the rise in adsorption metals [34].  The adsorption of all heavy metals rose with an increase of the adsorbent material, as shown in Figure 4. The adsorption of Cr in solution gave a constant path with higher adsorption for three type of adsorbents than other heavy metals, and biochar was gave the higher adsorption that compost and humic acid. According to [33], biochar has a negative charge on the surface that can bind the heavy metal, which increases the adsorption process. The increase in the adsorbate in solution to be adsorbed by materials was responsible for the rise in adsorption metals [34].  The adsorption of heavy metals at varied contact times during each 120 min treatment is depicted in Figure 5 (biochar, composts and humic acids).  The adsorption of heavy metals at varied contact times during each 120 min treatment is depicted in Figure 5 (biochar, composts and humic acids). Figure 5 shows that the metal content in the solution decreased as the contact time increased. This is because the active surface of the adsorbent decreased as the contact time rose, which could decrease the adsorption rate and eventually achieve the equilibrium phase. All adsorbents followed the same path in the adsorption of Cr, Cd and Pb. The adsorption process of biochar and compost was fast in the first 20 min, but gradually slowed down from 40 to 100 min ( Figure 5). The condition caused by the adsorption analytes reached the equilibrium phase between 60 and 100 min of the contact time. This is in contrast with the humic acid adsorption process, which was 1:100 (adsorbent to ml of heavy metals) in the comparison adsorbent, and showed a slowly increasing rate adsorption at different contact times. The contact time of adsorption Pb by lignin got maximum adsorption at half and hour with an adsorption efficiency of 88.77% at pH 5 [30]. On the other hand, in some studies, Pb took approximately 2 h to reach saturation until equilibrium was attained [24]. In terms of material adsorption capacity, the mechanism of increasing the adsorption showed that the analytes were still adsorbed. In general, the adsorption process was influenced by contact time, because the longer contact time can make a higher of adsorption of heavy metals in solution until saturation was reached. This effect was comparable for all of the heavy metals (Cr, Pb and Cd) and was linked to the materials' maximal adsorption, which decreased during the desorption process [30]. The qualities of the adsorbent were one of the most significant factors in the adsorption process. The adsorbent purity, adsorbent surface area (pore volume) and temperature were variables to consider [35].  The adsorption of heavy metals at varied contact times during each 120 min treatment is depicted in Figure 5 (biochar, composts and humic acids).    The adsorption of heavy metals at various metal concentrations is depicted in Figure 6 (biochar, compost and humic acids).
Appl. Sci. 2022, 12, 5763 8 of 13 Figure 5 shows that the metal content in the solution decreased as the contact time increased. This is because the active surface of the adsorbent decreased as the contact time rose, which could decrease the adsorption rate and eventually achieve the equilibrium phase. All adsorbents followed the same path in the adsorption of Cr, Cd and Pb. The adsorption process of biochar and compost was fast in the first 20 min, but gradually slowed down from 40 to 100 min ( Figure 5). The condition caused by the adsorption analytes reached the equilibrium phase between 60 and 100 min of the contact time. This is in contrast with the humic acid adsorption process, which was 1:100 (adsorbent to ml of heavy metals) in the comparison adsorbent, and showed a slowly increasing rate adsorption at different contact times. The contact time of adsorption Pb by lignin got maximum adsorption at half and hour with an adsorption efficiency of 88.77% at pH 5 [30]. On the other hand, in some studies, Pb took approximately 2 h to reach saturation until equilibrium was attained [24]. In terms of material adsorption capacity, the mechanism of increasing the adsorption showed that the analytes were still adsorbed. In general, the adsorption process was influenced by contact time, because the longer contact time can make a higher of adsorption of heavy metals in solution until saturation was reached. This effect was comparable for all of the heavy metals (Cr, Pb and Cd) and was linked to the materials' maximal adsorption, which decreased during the desorption process [30]. The qualities of the adsorbent were one of the most significant factors in the adsorption process. The adsorbent purity, adsorbent surface area (pore volume) and temperature were variables to consider [35].
The adsorption of heavy metals at various metal concentrations is depicted in Figure  6 (biochar, compost and humic acids). The equilibrium phase was the ideal period for maximum adsorption capacity. This condition occurred when the analyte was adsorbed by the whole surface of the adsorbent, and the adsorption remained constant, since the active side of the adsorbent was covered  The equilibrium phase was the ideal period for maximum adsorption capacity. This condition occurred when the analyte was adsorbed by the whole surface of the adsorbent, and the adsorption remained constant, since the active side of the adsorbent was covered with analyte. The analysis of the percent removal based on the mass of the adsorbent showed that the adsorption process of Cr by biochar produced 95.43 to 99.34% removal, by compost ranged from 82.05 to 83.43%, and by humic acid was 90% on average. This condition was similar to the percent removal of Pb by biochar, where the average variable was 99%, compost ranged from 94.75 to 97.32% and humic acid ranged from 90.94 to 92.46%. For heavy metal Cd removal by biochar, the average variable was 99%, compost was between 95.03 and 97.41% and humic acid was 90%. The percent removal for all the treatments can be seen in Figure 7. showed that the adsorption process of Cr by biochar produced 95.43 to 99.34% removal, by compost ranged from 82.05 to 83.43%, and by humic acid was 90% on average. This condition was similar to the percent removal of Pb by biochar, where the average variable was 99%, compost ranged from 94.75 to 97.32% and humic acid ranged from 90.94 to 92.46%. For heavy metal Cd removal by biochar, the average variable was 99%, compost was between 95.03 and 97.41% and humic acid was 90%. The percent removal for all the treatments can be seen in Figure 7. The percent removal of three types of adsorbents from cow manure followed a varied path, as shown in Figure 7, although in general, the percent removal of heavy metals rose with the addition of adsorbent. The percent removal was a significant indicator in determining an adsorbent's success in decreasing pollutants, as it refers to a material's maximal ability to reduce the existing pollutant content. Heavy metals were among the chemicals most commonly found in industrial waste that are harmful to humans. Heavy metals were found in the soil naturally and could not be degraded; they can stay in the soil and water for long periods of time, and rise over time [36]. Heavy metals enter animals and humans through intravascular or extravascular systems (systemic circulation) and disperse throughout the body. Minerals and heavy metals that are not essential for the body's physiological functioning accumulate in the body as toxins that are hazardous to health [37].
The results of the isotherm model study are shown in Tables 2 and 3. The three varieties of cow manure used for the adsorption of heavy metals Cr, Cd and Pb showed R 2 values ranging from 0.60 to 0.96, according to the Langmuir model study (Table 2).  The percent removal of three types of adsorbents from cow manure followed a varied path, as shown in Figure 7, although in general, the percent removal of heavy metals rose with the addition of adsorbent. The percent removal was a significant indicator in determining an adsorbent's success in decreasing pollutants, as it refers to a material's maximal ability to reduce the existing pollutant content. Heavy metals were among the chemicals most commonly found in industrial waste that are harmful to humans. Heavy metals were found in the soil naturally and could not be degraded; they can stay in the soil and water for long periods of time, and rise over time [36]. Heavy metals enter animals and humans through intravascular or extravascular systems (systemic circulation) and disperse throughout the body. Minerals and heavy metals that are not essential for the body's physiological functioning accumulate in the body as toxins that are hazardous to health [37].
The results of the isotherm model study are shown in Tables 2 and 3. The three varieties of cow manure used for the adsorption of heavy metals Cr, Cd and Pb showed R 2 values ranging from 0.60 to 0.96, according to the Langmuir model study (Table 2). The relationship pattern for each material can be described using the calculation from Tables 2 and 3  More than one surface was assumed for systems of adsorption that fit the Freundlich model. This refers to the several surface layers (multilayers) and the fact that the sides were heterogeneous for mechanisms that suited the Freundlich model. When the adsorption process followed the Langmuir isotherm, there was a different behavior in binding energy on each side. Adsorption processes that fit the Langmuir model suggest one-layer adsorption (monolayer). In general, the mobility of heavy metals varied depending on the metal type and the adsorption mechanism that occurred [38]. The adsorption process was influenced by the organic chemical functional groups and surface structures of materials that were altered during the raw material process. In terms of metal adsorption, raw material following the sequence Cu > Zn > Cd > Ni had the potential to be a good material for reducing metals in the environment [39]. Table 4 shows the analysis factors that produced adsorbent from cow manure with different types of forms.  Figure 7. *** Fee of field worker in Indonesia (two working days).
Despite the fact that all types of cow manure have the potential to adsorb heavy metals in solution, Table 4 reveals that the analysis cost from three types of adsorbents was varied. When the process of converting cow manure into other forms that could be used as adsorbents was viewed from an economic perspective (processing time, cost of analysis and adsorbent efficiency) it can be seen that biochar performed the best in terms of time. On the other hand, compost had a longer processing time than biochar and humic acid, but had the lowest production costs. This condition is different for humic acid, which requires 2-3 days to produce but in a small quantity and at a high cost, as a result of its production process being more complex than the other adsorbents. This is in line with [28], in which it was reported that the purification of humic substances might take two days to a week based on time and processing. The cost analysis of producing the three types of adsorbents should be considered in their utilization as adsorbent agents of heavy metals. On the other hand, the range of percent removal for Cd, Cr and Pb as effective adsorbent parameters in the adsorption process showed that biochar was the best material with average of percent removal higher than others (Figure 7).

Conclusions
Compost, biochar and humic acid derived from cow manure are potential adsorbents of the heavy metals Cd, Cr and Pb. The optimum condition was achieved for the time incubation, pH level, weight of adsorbent, and heavy metal solution (Cr, Cd and Pb). Compost, biochar and humic acid followed the adsorption isotherms of Langmuir and Freundlich models. This indicated that the adsorption rates was specific for each heavy metals. In general, the biochar > compost > humic acid adsorption pathway has a high potential for heavy metal removal. Additionally, from an economic aspect, biochar is more useful as an efficient and effective form of cow manure to adsorb of heavy metals. This material has many more pores than the other materials studied (compost and humic acid), and its morphology can increase the adsorption process of heavy metals in solution. The morphology of biochar is different to that of compost and humic acid derived from cow manure, which contain smaller pores, and thus achieve lower rates of adsorption of heavy metals.