Phosphorus (P) is a crucial element for producing crops and is widely used in both recycled manure and inorganic fertiliser. Its cycle has a high impact on the total environment, interfacing the hydrosphere and the pedosphere, and being heavily dependent on the biosphere and anthroposphere. The grey P adsorbents are based on waste materials from the steel industry, which ensure a high rate of P removal but do not allow for its direct reuse as fertiliser. Green P adsorbents are vegetable wastes; they are abundant, locally available, low-cost, and eco-sustainable, but the challenge is certainly their transport. A limitation to the reuse and recycling of agricultural by-products is seeking reusability at all costs, without evaluating the technical and economic feasibility; extra interventions are frequently proposed (i.e., applying high temperatures or adding expensive synthetic molecules to modify the pH). In general, the most promising feasibility is given by its direct use as a soil conditioner or by composting it as a by-product, as the only pre-treatment.
Technologies developed to remove and recover P from P-rich waste streams, such as municipal wastewaters (5–25 mg total P L−1) , are basically of two types: physical–chemical, such as membrane filtration, precipitation, adsorption, ion exchange, or crystallisation, and biological processes (Table 1). These technologies target different P sources, using different engineering approaches that differ significantly in the P recycling rate, pollutant removal potential, product quality, environmental impact, and cost .
|Technologies||Function||Pros||Cons||Constriction||Operative Costs a EUR per 106 L of Treated Water||Cost of 1 kg of P
|Membrane filtration||Semi-permeable selective separation wall||Low energy cost, low capital investment, high productivity||Membrane fouling||Membrane cleaning||42–744||-|
|Ion exchange||Functionalised polymeric
|Suitable for all ions, high productivity||Economic viability||Pre-treatment||42–330||-|
|Precipitation||Salt added||Removal of suspended and dissolved solids||Sodium carbonate management or H2S emissions||Plant maintenance||32–330||1.59|
|Crystallisation||Ca and/or Mg added||Produce granular hydroxyapatite or struvite||-||-||148–305||0.64|
|Coagulation/flocculation||Adding polymers or metal ions||-||-||-||32–330||-|
|Thermochemical treatment of sewage sludge||Mixes the ash with sodium-based salts||Produce P-enrich ash||-||Heavy metal-rich ash||28–180 *||-|
|Biological treatment||Selected bacteria||Low cost, high productivity||Additional treatment before P recovery||High concentrations of organic substrate||32–330||-|
|Adsorption||Surface phenomenon of molecular interaction||Low cost, high productivity||Reduced ability to remove organic P||Surface area and selectivity of adsorbent; contact time||42–130||-|
Based on the “grey” removal of P using adsorbents derived from by-products or the waste of the industries of steel, aluminium, or other material, or by their modifications, were carried out because of the availability of these materials and their chemical affinity with phosphate ions. Additionally, if often very efficient in P removal, P recovery by the adsorbent is carried out with strong acids or hydroxides. Therefore, many of these adsorbents, also meeting the criteria of a “circular economy” because the adsorbent is fully recovered and reusable several times, do not fit exactly with the concept of a “green treatment” . On the other hand, relatively few works have described typical adsorption processes and the ability of adsorbents derived from agricultural waste to recover important anions such as P, arsenic (As), and chromium (Cr) (VI) anions. The “green removal” agricultural waste products are proposed as bio-based solutions to recover and reuse directly on farmland soils. Indeed, the feasibility of using the recovered materials in agriculture has not received much attention, however, due to their low cost, adsorbents from recovered agricultural materials deserve further study and still need major research. The great advantage in the removal of the P anions consists of the possibility, in the eventuality of a strong bind that does not allow for the recovery of P and reuse of the adsorbent, to use the product obtained as a fertiliser or a substrate. This opportunity, evidently, is not conceivable in the case of Cr or As. Table 2 lists some agricultural waste materials on which experiments have been conducted to evaluate their Cr(VI) and As(V) removal capabilities. As (V) and Cr (VI) are only considered because, in these oxidation states, they behave as anions, CrO42− and AsO43−, as well as P (PO43−), and thus, these three elements behave similarly in adsorption processes. The adsorption mechanisms of other PTEs (Pb, Fe, Zn, …) were not considered because their behaviours are essentially those of cations.
Biosorbents tested to recover Cr(VI) work under extremely acidic pH conditions (pH 2). Therefore, the development of this solution is a challenge from an economic and chemical point of view because maintaining a pH 2 in an actual plant is far from the concept of a “low-cost and green solution”. The same cannot be said for the biosorbents that have been tested to remove arsenate, which work in a pH range between 4 and 9, ensuring the promising performance of the biosorbent.
|Adsorbent||Modification||Pollutant Removed||Adsorption Capacity
|Ficus auriculata leaves||Unmodified||Cr (VI)||94.3||2.0||30|||
|Milled olive stones||Unmodified||Cr (VI)||2.3||2.0||-|||
|Olive stone||-||Cr (VI)||53.3||2.0||30|||
|Date pit||-||Cr (VI)||82.6||2.0||30|||
|Cellulose derived by rice husk||Treated with alkaline humic acid||Cr (VI)||19.3||5.0||25|||
|Exhausted coffee waste||Unmodified||Cr (VI)||686||3.0||25|||
|Raw rice straw||Unmodified||Cr (VI)||8.0||2.0||30|||
|Date palm trunk||Graft with diethylenetriamine and triethylamine||Cr (VI)||129.8||3.5|||
|Sludge Biomass||Immobilised with calcium alginate||Cr (VI)||116.1||5.0||25|||
|Sugarcane bagasse pith||Immobilised with Na-alginate||Cr (VI)||52.8||2.0||25|||
|Black wattle tannin||Immobilised with nanocellulose||Cr (VI)||104.6||2.0||25|||
|Cactus mucilage||Unmodified||As (V)||2.8||5.0–9.0||-|||
|Powder of stem of Acacia nilotica||Unmodified||As (V)||50.8||4.0–7.0||-|||
|Sorghum biomass||Unmodified||As (V)||2.8||5.0|||
|Opuntia ficus indica fruit powder||Unmodified||As (V)||85–92||6.0–7.0||-|||
|Mucilage cactus||Unmodified||As (V)||98||30|||
|Cactus mucilage (non-gelling extract)||Mixed with sodium alginate and CaCl2||As (V)||97.1|||
|Cactus mucilage (gelling extract)||Mixed with sodium alginate and CaCl2||As (V)||101.6||[]|
To take into account the environmental sustainability of the adsorbent and its usefulness and easiness for reintroducing P into the environment, over recent years, researchers have proposed some adsorbents from waste materials of the agricultural sector with good properties that would enable sustainable P recovery, both environmentally and economically. These waste materials or by-products of agricultural processing, with or without further modification, are considered environmentally friendly, low-cost, and highly selective with high adsorption capacities . The agricultural by-products that can be used to adsorb P, and then used as fertiliser or substrate are various: apple and black currant pulp, tea scraps, banana pith, sugar cane pith, coffee pulp , orange peel, potato peel, tangerine peel, onion peel, palm peel, hazelnut peel , exhausted coffee, corn cobs, rice hulls, corn straw and sawdust, rice straw and husk, sugarcane bagasse , almond shells, palm shell charcoal, hazelnut shells, peanut shells, eggshell or apricot kernels, and sunflower seed shells . Numerous attempts have been made to develop new anion exchangers by grafting positively charged amino groups onto the polymer chains of agricultural residues, such as sugar cane bagasse , corn bracts , raw walnut wooden shells and raw almond wooden shells , and wheat straw . These studies have shown that the absorption capacities of the charged materials were significantly increased compared to raw materials. The reuse of agricultural waste in the form of classic fertilisers (pellets, for example) is not yet sustainable, either economically or agronomically. The main problem is the elemental composition: all the elements of plant nutrition should be present and in balanced relative quantities .