Recovery of nutrients from sewage using zeolite-chitosan-biochar adsorbent: Current practices and perspectives
Graphical abstract
NZs: Natural zeolites
Introduction
About 70 % of phosphorus (P) and 75 % of nitrogen (N) constitute the overall sewage composition [1]. These components would impose hazardous effects if it is not further treated. The adverse effect of N ionic compound can potentially cause blue-baby syndrome or methemoglobinemia in infants and the accumulation of N and P in sewage can lead to eutrophication [2]. The removed nutrients are either lost through biological uptake and/or released to the atmosphere by converting the ammonium ion (NH4+) to nitrogen gas [3,4]. Due to the accumulation of unexploited nutrients, the focus has been a shift to nutrient recovery involving the N and P as a solution to high carbon footprint produced during the manufacturing process of fertilizer as about 500 million tons of ammonia (NH3) produced annually and every 1 ton of NH3 produced, will generate 1.87 tons of CO2. Moreover, phosphate rock cannot be replenished and will be used up in 30–300 years [[5], [6], [7], [8]].
The study approach to recover N and P has been done by physical, chemical, and biological methods. Nutrients can be recovered by chemical precipitation and it requires a tight pH range for optimum efficiency due to the potential volatilization of free ammonia at pH > 9.8 [9]. Physical processes for nitrogen recovery include membrane process [10], filtration [11], adsorption [12] and stripping (vacuum stripping) [13] or thermal stripping [14]. The biological process was also done by bio-electrochemical systems (BES) but the limitation lies in the membrane fouling and substrate variation [15,16]. As compared to other processes, the adsorption process has the easiest controllability, simple operation, and design but requires a downstream process such as desorption [[17], [18], [19]].
Adsorption using NZs, chitosan, and biochar has been used in nutrients removal due to its simple process including for sewage treatment [[20], [21], [22], [23]]. NZs are the naturally occurring minerals that are abundant and low-cost resources [24,25]. NZs are crystalline hydrated aluminosilicates with a framework structure containing pores occupied by water, alkali, and alkaline earth cations such as K+, Na+, Ca2+, and Mg2+ that contributes to the high cation exchangeability [[26], [27], [28]]. The ranges of cationic exchange capacity (CEC) ranges from 0.64 to 2.29 meq/g [25]. Pore sizes of NZs vary between 0.3 and 1 nm resulting in porosities between 0.1 and 0.35 cm3/g respectively [29]. Owing to this microporous structure, the surface area tends to be very high ranging from 300 to 700 m2/g and attribute to the excellent AC of NZs [29,30].
Chitosan, a natural and environmentally friendly biopolymer has been used as a biosorbent [31,32]. Low-cost chitosan can be produced from the deacetylation of chitin, extracted from shells of crustaceans, skin fungal cell walls, insects, organisms from industries, fungal chitosan, and wine yeast [[31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]]. Among the carbon materials, activated carbon, carbon nanotubes, fullerenes, and graphene showed high AC but they require high production cost compare to the production of biochar [[43], [44], [45]]. The reported feedstocks for biochar production are from various fruit peels, plant parts, seeds, shells, crop residue, wood chip, algae, manure, sewage sludge, and municipal waste. The capability of biochar from those feedstocks to treat ammonia nitrogen and other contaminants are well documented [22,[46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]].
The ability of NZs, chitosan, and biochar has been proven to treat various contaminants but has few drawbacks when being utilized as adsorbents. The low affinity of NZs towards negatively charged nutrients did not demonstrate appreciable AC [58]. Chitosan is a cationic biosorbent that shows a high affinity towards anionic nutrients such as nitrate (NO3−), nitrite (NO2−), and phosphate (PO43-) but, it has low AC towards positively charged nutrients such as NH4+ [31,59,60]. Furthermore, the AC of chitosan is highly dependent on the pH [41,61]. Studies indicated that carbon char recorded low AC because it has a non-polar surface. During the carbonization process, the transformation of macropores (> 50 nm) to micropores (< 2 nm) has attributed to the non-polarity of carbon char that is determined by the concentration of surface oxygen complexes [62]. During the transformation, the formation of oxygen complexes mostly occurs minimally at the outer surface or edge of the basal plane compared to the surface plane of the graphitic structure that has been attributed to low polarity [62]. The non or low polar carbon char can adsorp non-polar nutrients, but inadequate for the adsorption of polar nutrients such as ammonia nitrogen (N-NH3) and (NO2−) [63,64]. In order to increase the surface oxygen that promotes the polarity of carbon char, surface modification such as physical and chemical treatment has to be carried out [65]. Moreover, the application of these single adsorbents suffers from enormous transfer resistance due to large diffusion lengths during the adsorption process [66]. Hence, the application of organic materials such as chitosan with inorganic NZ, as well as biochar, may increase the AC by compositing these three adsorbents [58,66,67].
There are several reviews focused on the modification of NZs, chitosan, and biochar into hybrid adsorbents. The reviews mainly focused on the hybrid by combining two single adsorbents and focus on the synthesized routes as well as performance in the adsorption process. It is shown that, through the hybridization of adsorbents, there has been an improvement in the AC and affinity towards new contaminants. Some studies revealed that hybrids have higher AC in contrast with single adsorbent [18,68,69]. However, the application of hybrids in nutrient recovery by leaching (desorption process) has not been reviewed. Hence this review will emphasize the modifications of NZs, chitosan biochar into hybrid adsorbents, and the application of various hybrids in nutrient recovery.
Section snippets
Zeolite: Properties, modification, and adsorption process
Generally, zeolite can be divided into two which are NZs and commercial zeolites. Both zeolites possess similar composition but different crystalline structures [70]. The crystalline structure of NZs composed of eminently fine crystals or particles of clay minerals that exhibits chemical properties of colloids [[70], [71], [72]]. Pristine NZs has been applied industrially in water remediation [28], cement and concrete production [73], agriculture [74], and energy application [75]. NZs is
Chitosan: Properties, modification, and adsorption process
Chitosan, a derivative of chitin has better properties compared to chitin. Chitosan is produced through partial deacetylation of chitin which is done by deacetylase hydrolyses most of the N-acetamido group in the chain via reaction with NaOH. The parameters in the process such as the concentration of NaOH, temperature, and oxidation period will determine the properties of chitosan [31,37]. Chitosan is chemically defined as a copolymer consisting of two residues;
Biochar: Properties, modification, and adsorption process
Biochar is a carbonaceous material produced during the thermochemical decomposition of organic biomass feedstock in the presence of little or no oxygen [48,49,127,128]. Biochar can be produced by pyrolysis, hydrothermal carbonization, gasification, torrefaction, and microwave heating varying in temperature and duration [[129], [130], [131], [132], [133]]. Feedstock pretreatments and biochar pretreatments can greatly alter the physicochemical properties of pristine biochar [48,[134], [135], [136]
Hybrid adsorbents: Properties, modification, and adsorption process
Modification of NZs, chitosan, and carbon char into hybrid adsorbents will combine the physical and chemical properties of both organic and inorganic material. This is targeted to increase the density of sorption sites, flexibility upon pH changes, selectivity towards both cationic and anionic nutrients [126,127]. The properties, pretreatments of adsorbents, modification parameters, and performance in nutrients removal are summarized in Table 4. The hybrid adsorbents are the combinations of
Nutrients recovery process
About 90 % of conventional sewage treatment constitutes of P and N load. The proportion of nutrients are 11 % in the sewage sludge through primary settlement, 28 % is present in the biomass which is eliminated in the excess sludge and another 50 % can be removed with adsorption and chemical precipitation [17]. Furthermore, the wastewater from the dewatering of sludge contains about 30 % of nitrogen loads and there is a high opportunity for N recovery [3]. Nutrients-rich water can be utilized as
Future works
Further future works such as the hybridization of NZs, chitosan, and biochar can be synthesized. Its physicochemical properties, nutrient removal, and recovery need to be investigated. The Life Cycle Assessment (LCA) of recycling the nutrients to be used as fertilizers should be considered before the application [189]. Lastly, the hazards that will be imposed due to the utilization of leached nutrients from hybrid adsorbents as fertilizers should be outlined before real application.
Conclusion
Hybridization of double adsorbents that are made up of NZ, chitosan and biochar is done by initial protonation of chitosan by acid. However, there is no hybridization of triple adsorbents being reported. The addition of crosslinker and metal, types and acid concentrations, ratio, and temperature used for protonation of chitosan, as well as pyrolysis temperature, will affect the physicochemical of hybrid adsorbents. Hybrid adsorbents are bonded by the electrostatic force of positively charged
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The authors would like to acknowledge Universiti Malaysia Sabah for funding this work under the grant SDK 0044-2018 and SPB0001-2020.
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