Similarly, SO2 or H2S gas modified agricultural wastes bio-chars produced -C=S and -S-S groups, which formed strong complexes with Cd2+ and Hg2+ . Depending on experimental conditions, methods to introduce S-containing functional groups can either increase or decrease specific surface area and pore volume of carbonaceous adsorbents ; however, any reduction in sorption capacity associated with the loss of surface area and porosity is generally ofset by the increased S-containing functional groups. Sulfur-functional groups also interact with other functional group types in synergist adsorption mechanisms. When -COO-, −NH and -C-S were co-introduced to the surface of sugarcane bagasse during modification procedures, these groups interacted to retain metals through ion exchange and complexation reactions . The interactions among these functional groups greatly improved the overall sorption capacity for many environmental pollutants.While O-, N- and S-containing functional groups are the predominant reactive groups controlling adsorption performance, other functional groups may also enhance the adsorption performance of agricultural waste products . Most of the -C=C groups exist in benzene rings and react with pollutants through π-π interactions. Aniline addition modification enhanced the -C=C content of corn stalk and strengthened π-π interactions betweenadsorbent and adsorbates,plant pot with drainage thereby increasing the adsorption capacity for acid red and acid orange dyes .
Fe-containing functional groups are typically introduced onto agricultural waste materials to prepare magnetic adsorbents, especially in the preparation of bio-char. Doping with magnetic materials creates an adsorbent that can be easily separated from solution to remove bio-char-bound pollutants. These Fe oxides were highly efficient for the removal of As3+ and As5+ through specific sorption reactions . Additionally, Fe3O4 reacts with H2O creating a polar structure that can retain pollutants by hydrogen bonding. P-containing functional groups are generated on adsorbents by H3PO4 modification and are capable of retaining pollutants primarily by ion exchange mechanisms. Tang et al. demonstrated the presence of a large number of -P-O functional groups on corn stalks after H3PO4 treatment. The -P-O group was effective in retention of methylene blue through ion exchange. Additionally, the -P-O group efficiently retained U6+ from solution via ion exchange . Adsorption of uranium changed the morphology and the structural characteristics by decreasing the distributed interstices, voids and crystal structure in the surface of the modified rice stem through interaction with -P-O.Adsorption mechanisms are primarily afected by specific surface area, pore volume/connectivity and the amount/type of surface functional groups. Physical adsorption takes place by weak Van der Waals’ attraction forces and H-bonding, whereas chemisorption occurs through stronger chemical bonding between adsorbates and adsorbents. Chemisorption generally plays a more prominent role in pollutant removal from aqueous solution than physisorption for modified agricultural adsorbents . Notably, although no functional groups or chemical bonds take part in the fixation of pollutants during the precipitation process, chemical reaction occurs during the formation of the solid products casused by the adsorbents.
The retention of pollutants through precipitation is still a chemisorption process. Many O-, N- and S-containing functional groups are suitable for the removal of cationic pollutants, whereas O- and N-containing groups are more suitable for adsorption of anionic pollutants. Fe- and S-containing functional groups often coexist with modified adsorbents surface and were used to binding divalent cationic heavy metals via complexation . Moreover, chemical precipitation usually occurred as well for the pollutants retention due to the formation of stable crystal structures. Complexation and ion exchange are the main adsorption mechanisms for O-containing groups , whereas N- and S-containing functional groups interact with pollutants primarily through complexation . Studies involving modifications to increase O- and N-functionality of agricultural wastes are much more common than those addressing enhancement of S-functionality, due to the prominence of O- and N-functional groups and their higher adsorption affinity compared to S-functional groups, especially for anionic pollutants. The main mechanisms for pollutant interactions with modified agricultural wastes are portrayed in Fig. 3. Complexation and ion exchange are often the most important adsorption mechanisms for interactions between functional groups and various pollutants. These chelate-forming or electron-donating functional groups readily react with pollutants to form complexes. Electrostatic attraction is a prominent adsorption process, especially under acidic or alkaline conditions where dissociation/ionization of functional groups initiates interaction with oppositely charged pollutant compounds. For carbonization-treated agricultural wastes, π-π interactions with organic pollutants and alkaline precipitation mechanisms are improved due to the increased C=C content and high alkalinity after modification . Moreover, carbonization generally increases the specific surface area of the modified materials, which favors physical adsorption , H-bonding and van der Waals’ attraction.
Given the multi-functionality of most modified adsorbents, the overall adsorption process will involve a combination of several mechanisms.Cost is an important consideration when selecting an agricultural waste and modification treatment. Currently, few studies have assessed cost estimates of modified adsorbent, thereby limiting an evaluation of material and processing costs. A few studies demonstrated that modified agricultural wastes showed a good price advantage compared with commercial adsorbents. For example, the cost of citric acid modified sugarcane bagasse was 4.76 $ kg−1 , much lower than commercial activated carbon . Further, Kannan and Sundaram determined that the price of carbonised adsorbents prepared from rice husk or straw was ~5 times cheaper than commercial activated carbon. In addition, some modified adsorbent materials exhibited excellent pollutant removal efficiency over at least three reaction/ recovery cycles , allowing for reuse potential that further reduces their lifetime costs. Moreover, many feedstocks for bio-adsorbents originate from agricultural wastes that would otherwise be discarded. Hence, the reuse of these solid waste materials is beneficial for utilization of waste materials as well as reducing waste disposal costs, which is often a more substantial cost than the modification costs. Other studies demonstrated that bio-char preparation could generate energy , thereby of setting production costs partly . Based on the available studies, almost all cost-benefit analyses indicated that agricultural waste modification provides a low-cost option for preparation of bio-adsorbents for environmental remediation. Most studies of bio-adsorbent preparation were limited to laboratory evaluations of synthesis costs, which do not account for several associated costs. Te costs of raw materials, transportation, modification chemicals and energy are the main factors determining the final preparation cost. Many of these costs are site and time specific making it difficult to produce a universal cost estimate . For example, Salam et al. calculated that the total cost of HCl-modified peanut husk was L.E. 5000 t −1 , a factor of two lower than commercial activated carbon at that time. Additionally, bioadsorbents will have variable efficiencies for retention of a given pollutant, wherein a higher cost,pot with drainage holes but higher efficiency product will actually have a lower application cost per unit of pollutant removal. More research is required to assess the costs of modified agricultural waste materials, not only for their production costs, but also for the cost per unit of pollutant removal and including a complete life-cycle analysis that accounts for externalities, such as greenhouse gas impacts.The main purpose of current modification procedures is to alter the physical and chemical characteristics on the surface of agricultural wastes to enhance pollutant adsorption. Future studies will undoubtedly devise new techniques to improve the efficiency and selectivity of the modified adsorbents for targeted applications. In addition, some adsorbent modification processes generate a series of potentially toxic by-products that must be addressed to minimize negative environmental impacts. Thus, optimization studies are required to generate low-cost adsorbents that are tunable for efficient and selective retention of targeted pollutants using green-chemistry principals. This will require systematic studies followed by physicochemical characterization of the modified materials and finally testing of their efficacy for retention of various pollutants under a wide range of environmental water conditions.
Rigorous physicochemical characterization is a critical intermediate step in this overall process. First, characterization provides the details for how various modification steps affect the physicochemical properties of the modified products. Second, understanding the physicochemical properties of the modified products is critical for mechanistically understanding the materials ability to retain various pollutants. Once sufficient characterization data has been systematically compiled, it will serve as a toolbox for fine tuning modification techniques to optimize materials for efficient and selective retention of specific pollutants. Although the modification methods discussed above are effective to improve the adsorption performance of agricultural wastes, there are some drawbacks that need attention. High alkali concentration may cause an excess elimination of covering materials from the cellulose surface and delignify the fibre extremely due to the hydrolysis, which can negatively afect the strength of the fibre. Concentrated acid oxidation has strong corrosiveness and was shown to decrease the surface area of oxidized adsorbent, which may reduce the porosity and efficacy of the adsorbent material for retention of some pollutants. Esterification and etherification creat high cost of the organic solvents used, the catalyst would also need to be recovered after modification. Carbonization not only consumes a lot of electricity, but also reduces the content functional groups of adsorbent at a high temperature. Magnetization can reduce surface area due to the block of magnetive particles in the pore of adsorbent. Surfactant and grafting have strict requirements for preparing environment and are prone to produce formation of undesirable block co-polymers. In all, the effectiveness of the treatment depends not only on the adsorption environment, but also on various preparation conditions. The conditions for the production of low-cost adsorbents after surface modification for higher uptake of pollutants need to be optimized further. Therefore, we still need to refine these modification methods to reduce their shortcomings and make them exhibit greater potential in preparing agricultural waste based adsorbents. Although modified agricultural wastes have been proven effective in retention of various pollutants in water, remediation actions that leave the pollutant-adsorbent compounds in the water environment have not been fully investigated. Over time, pollutants retained by bioadsorbents and accumulated in aquatic sediments may be released back to the water column upon changes in pH, temperature, ionic strength, redox conditions and bio-adsorbent degradation. Thus, bio-adsorbents that can be effectively extracted from the water environment, such as magnetic compounds, will provide a more permanent solution for pollutant retention and extraction from environmental waters. While modified agricultural wastes show great potential for the removal of various pollutants under laboratory conditions, far fewer studies have demonstrated their efficacy in real-world environmental waters, such as industrial/municipal waste waters. The composition of waste waters is much more complicated than that of synthetic waste waters used in many laboratory studies, which undoubtedly will affect the adsorption performance of the bio-adsorbents. Testing common bio-adsorbents across a wide range of environmental waters will provide fundamental knowledge to optimize applications for real-world use. Finally, most previous studies focused on the single adsorbent-adsorbate system, while there is a paucity of studies evaluating simultaneous use of multiple bio-adsorbents for retention/extraction of multiple pollution types. The coexistence of different pollutants is a common scenario in environmental waters and interactions among pollutants often affect the adsorption performance of various adsorbents. Moreover, there is a distinct paucity of research on the removal of rare and emerging pollutants , which are of considerable concern for human and aquatic ecosystem health. Finally, greater effort is required to rigorously investigate adsorption/ retention mechanisms. In addition to traditional sorption isotherm and kinetic studies, the emergence of advanced analytical methods, such as spectroscopic, microscopic, isotopic and molecular techniques, offer great potential for advancing our understanding of adsorption mechanisms. We are currently at the cross-roads of advancing from a more trial-and-error approach to developing a cook-book approach for designing modification techniques to achieve targeted materials for selective removal of pollutants.Children and other community members living within agricultural communities have an elevated risk of exposure to pesticides during pesticide spray seasons . This has been shown even among people who do not work in agriculture but reside near crops . These studies have described greater urinary metabolites of pesticides and lower acetylcholinesterase activity during the spray season compared to pre-season levels. Organophosphates and carbamates are commonly used insecticides in agriculture which are designed to inhibit the activity of AChE, leading to cholinergic overstimulation of the nervous system . Alterations of the cholinergic system can induce physiological alterations in the cardiovascular system . The relationship between pesticides and blood pressure is unclear, with a limited number of studies reporting positive associations between blood pressure and pesticide exposure constructs based either on selfreports or biomarkers , while some evidence of negative associations has also been described .