# The Various Forms of Cow Manure Waste as Adsorbents of Heavy Metals
## 3 Results and Discussion
### Heavy Metal Adsorption of Cr, Pb and Cd
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]. Figure 3: The adsorption of heavy metals Cr (**a**), Cd (**b**) and Pb (**c**) at various pH levels. The adsorption of heavy metals at varied contact times during each 120 min treatment is depicted in Figure 5 (biochar, composs 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]. Figure 4: The adsorption of heavy metals Cr (**a**), Cd (**b**) and Pb (**c**) at various weights of adsorbent. The adsorption of heavy metals at various metal concentrations is depicted in Figure 6 (biochar, compost and humic acids). Figure 5: The adsorption of heavy metals Cr (a), Cd (b) and Pb (c) at various contact times. Figure 6: The adsorption of heavy metals Cr (a), Cd (b) and Pb (c) at various concentrations of metals. 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 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%.