Temperature can modify solubility, surface tension, and sorption thermodynamics and hence the interaction. High temperatures, during ageing may favour the presence of oxygen-rich functional groups C–O, C–OH, and CO ^[1][71] and the interaction with hydrophilic PHs as well as their desorption ^[2][72].

Among all the environmental factors regulating the interaction, pH represents is largely dominant, since it affects MP surface charges and drug speciation as well ^[3][73]. A significant group of drugs, such as antibiotics, are polar and ionizable, and thus speciation is driven by pH. Drugs can be cationic, anionic, and zwitterionic species and will be sorbed depending on the surface charge ^[4][74]. They are 85–95% ionizable and weak acids or bases ^[5][75]. The electrostatic interaction is the main sorption mechanism of PHs by MPs. This is a physical mechanism driven by the pH of the aquatic system, normally 5–9, when the surface charge of the MPs varies among positive, negative, or neutral, and the polymers tend to aggregate ^[3][73]. However, at the common pH value of estuaries and coastal water, 8.1, there would be a tendency for many PHs to desorb from the MPs due to the repulsion ^[6][76].

The point of zero charges, pH PZC, of MPs is below 7, and hence, at the common pH of water systems, the surface is negatively charged. The pH dependence of the surface charge was shown by the point of zero charges, PZC, experiments: When the pH of the system was above the PZC of PE, PP, and PS, the polymers carried negative charges ^[7][8][49,77]. The effect of solution pH on the sorption of tetracycline by PE, PP, and PS was studied by Xu et al. ^[7][49]. Tetracycline is present in cations at low pHs, anions at high pHs, and zwitterions at pH 5–7, with the highest electrostatic attractions for the zwitterionic species and the maximum sorption at a pH level of 6. When the pH increased, sorption decreased as tetracycline and polymers became negatively charged and repulsed. In the same way, Guo et al. ^[9][50] showed that the sorption of sulfamethoxazole increased with pH from 3 to 5; at pH 5–7, sulfamethoxazole is neutral and sorbed by hydrophobic interactions with the non-polar polymer surface. At pH > 7, sulfamethoxazole is anionic and is repulsed by the increased electronegativity of the polymer. This is explained by the electrostatic and hydrophobic attractions since the antibiotics became positively charged in an acidic pH, whereas the electrostatic interaction became less significant at an alkaline pH as the antibiotics became negatively charged. Zwitterionic or neutral drugs interact with non-polar plastic surfaces through hydrophobic and van der Waals forces at a neutral pH. At a high pH, as in marine environments, drugs are present as anions, which enhances their electrostatic repulsion with polymer surfaces. Thus, the pH of the aqueous system affects zwitterionic strength and electrostatic repulsion. Other similar results are shown in the literature. Guo et al. ^[10][54] studied the sorption of Tylenol on PS, PVC, PE, and PP. They found that the sorption capacity in the case of PS and PVC decreased gradually with the increment of pH from 3.0 to 7.0, while PE and PP showed a minimal difference. This is likely due to the electrostatic attraction. In another study ^[11][78], the pH decreased to less than 3.0, and protonation of PE and PS surfaces and enhanced adsorption of PFOS, perfluoro octane sulfonic acid, was observed. Thus, the pH is critical in influencing the sorption process between MPs and PHs depending on their surface charges ^[12][79]. Elizalde-Velázquez et al. ^[13][36] studied the sorption of three non-steroidal, anti-inflammatory drugs, ibuprofen, naproxen, and diclofenac, on PS, ultra-high-molecular weight PE, average molecular weight, medium density PE, and PP. The sorption process was pH-dependent, probably due to the molecular equilibrium effects that affect not only MPs’ surface charge but the compound’s speciation as well. Only under acidic conditions were the pharmaceuticals highly sorbed onto the MPs, a process ruled by hydrophobic interactions. Thus, solution pH affects overall chemical reactivity, biochemical and physicochemical properties, equilibrium condition, and toxicity ^[14][80]. McDougall et al. ^[15][81] studied the desorption of three groups of PHs, cationic, anionic, and neutral, from PE-simulating gastric and intestinal fluids. The pH of the solution was the most important factor driving PH desorption, influencing speciation of the active compounds and MP surface charge. The desorption of the cationic compounds in gastric fluids was more evident and up to 50% due to the reduced surface charge of the polymers under low pH, revealing that PHs sorbed to MPs can be bioavailable ^[15][81].

The role of ionic strength on PH sorption depends on the type of adsorbent, adsorbate, electrolyte, and solution chemistry ^[16][17]. Sorption decreases with ionic strength, as was the case of ciprofloxacin ^[16][17], sulfamethoxazole ^[4][74] and sulfamethazine. Polymers are normally negatively charged and hence hydrophilic and influenced by the levels of cations, such as Na⁺ and Ca²⁺, in an aqueous medium that can bind electrostatically, disturbing the charge equilibrium of surfaces. Salts also increase the viscosity and density of water, hindering the mass transfer of drugs with ions being sorbed more easily ^[10][54]. Wan et al. ^[17][82] observed that MgCl₂ promoted the aggregation of PS and hence decreased tetracycline sorption via electrical double-layer compression or elimination and modulating repulsive forces and reducing accessibility to PHs ^[16][17].

An opposite effect of ionic strength can be also observed: the so-called salting-out and ionic complexation effect ^[18][83]. Lu et al. ^[6][76] observed a threefold increased sorption of 17β-estradiol and 17α-ethynylestradiol when the salinity of seawater was doubled and explained using solubility decreases of hydrophobic PHs with an increase of salts and the enhanced hydrophobic interactions of drugs with MPs. The presence of ions also promotes cationic bridging, especially with multivalent ions. On the other hand, polyvalent ions may favour the formation of ternary complexes through the bonding of the drugs with specific PH functional groups of the adsorbent, which enhances the sorption properties ^[19][84]. In the case of non-steroidal, anti-inflammatory drugs, NSAIDs, Elizalde Velasquez ^[13][36] reported higher sorption of ibuprofen, diclofenac, and naproxen on PP, PS, and PE in freshwater than that in seawater due to the high salinity content of seawater. The increase in salinity affects the aggregation state of the polymer. In parallel, it also increases the hydrophobic interaction between the drug and the polymer via the salting effect, i.e., decreasing the concentration of PHs in the solution ^[20][55].

The effects of dissolved organic matter, DOM, the fraction of organic matter in a solution that passes through a 0.45 μm filter, on the binding of drugs are poorly studied ^[13][36]. DOM can induce contrasting effects on PH sorption by MPs in the function of their properties enhancing or reducing sorption ^[7][49]. Few studies show increased sorption. Zhang et al. ^[21][60] studied the sorption of oxytetracycline on beached polystyrene foams in the presence of humic acid, HA, and fulvic acid, FA, as the two representative dissolved organic materials and showed increasing sorption with DOM. They found that fulvic acid promoted significant oxytetracycline sorption to the beached foams due to the complexation role of humic acid, acting as a bridge with both the beached foam surfaces and oxytetracycline molecules. Lu et al. ^[6][76] recently observed increased sorption of hormonal steroid compounds by MPs due to DOM–MPs complexation. Chen et al. ^[22][85] showed the formation of a copolymer between the carbonyl group of DOM and the aromatic moieties of the polymer through π–π interaction, which enhanced the electrostatic sorption of the positively charged oxytetracycline to the copolymer. Migration and stability of MPs might be considerably aided by DOM ^[23][86] due to the formation of a coating, eco-corona layer on the MPs, which inhibits the aggregation of plastic particles via electrostatic repulsion and steric forces ^[23][86]. However, mobility and stability can be reduced with the presence of DOM, as they can create a coating layer on the plastic surface. There are studies by Xu et al. ^[7][49] reporting decreased sorption of tetracycline on PS, PP, and PE when DOM increased due to complexation with the hydrophobic or hydrophilic DOM parts, which changes the partitioning between the polymer surface and water ^[24][87]. DOM and drugs may compete for the limited sorption sites on MPs ^[25][88] with the desorption of PHs from MPs ^[26][89] or can block the pores on the MPs’ surface with their large molecules, hindering the access of drugs ^[27][90]. Another interactive role is played by the concomitant presence of polyvalent metals: Through PHs, stable ternary complexes can form in the water metal cations and DOM ^[28][91].

Biofouling is the process of microbe colonization of MPs through extracellular polymeric substances, EPS. This induces changes in morphology and physicochemical properties ^[29][92]. There is still very limited knowledge on the effect of biofouling on MPs–MPs interatom. PHs must pass from the water into the biofilm and then to the polymer material ^[30][93]. PHs interact via hydrophobic partitioning into the biopolymer or through binding onto the sorption sites of the heterogenic EPS ^[31][94]. EPS is rich in ionizable functional groups, such as carboxyl, phosphoryl, amino, and hydroxyl, that increase the sorption of metal ions via MPs ^[32][95], and in this way, metals have a synergistic effect on the sorption by forming a PHs–metal–EPS complex through an ionic bridging effect ^[33][96].

MPs in water are exposed to physical and chemical weathering, such as photooxidation, temperature variations, friction, and saltwater corrosion. They crack into smaller particles with reduced hydrophobicity ^[3][73]. Ageing occurs through ultraviolet-induced photodegradation, physical impacts, and biodegradation ^[34][97]. Many kinetic models and batch sorption data reveal how modified MPs have a higher sorption capacity than pristine polymers ^[3][73]. Weathering processes increase surface area, accessible binding sites, and hence PHs sorption ^[34][97]. In this regard, however, the data are not always in agreement, Table 12. Huang et al. ^[34][97] studied the role of MP ageing, PS, on the interaction with sulfamethoxazole, SMX, and the β-blocker propranolol (PRP) and using red tilapia as the model fish. The authors reported a 0.27- and 0.16-fold increase in the specific surface area and average pore volume, respectively, and the formation of more carbonyl on the aged PS. Ageing increased PRP accumulation by 82.3% in the brain, whereas it decreased the SMX level by 46.1% in the gills. Even the response of the model was different: In the case of PS–PRP, the stress was alleviated by the ageing with reduced neurotoxicity and lipid peroxidation damages, whereas in the case of SMX-aged PS, co-exposure resulted in higher inhibition of cytochrome P450 enzyme activities. Thus, the interactive effect of MPs and the drug varies with the intrinsic features of the drug, exposure strategy, selected endpoints, biological models, and environmental conditions. One key issue, as stated by Huang et al. ^[34][97], is that there is no significant literature on the interactive effects of aged MPs and drugs on aquatic organisms where the experiments are carried out with commercially pristine MPs, that is, with homogeneous size distribution and regular shape. If ageing in laboratory settings has shown evident increases of SSA and the oxygen load and consequently more intensive drug sorption on polymers, it is very likely to hypothesize side chain effects of the toxicological interaction between MPs and drugs.

The effects of the biotic and abiotic degradation events depend on the chemical nature of the plastic, the loads, the hydrophilicity of the surfaces, and the environmental conditions ^[39][102].

In water, the excess of hydrogen atoms favours the formation of a phenolic hydroxyl group on aged MP surfaces ^[40][103]. Through chemical transformation, the richness of oxygen-containing functional groups increases, carboxyl, hydroxyl, ketone, and ester, and this confers polymers hydrophilic properties ^[41][104], making them capable of more adsorb the hydrophilic molecules of PHs ^[42][29]. If the weathering process increases, the negative charges of the polymer surfaces and the sorption of cationic PHs via electrostatic interaction increases ^[3][73]. The increased hydrophilicity was highlighted by Liu et al. ^[43][58] who showed functional group alteration on PS and PVC via UV irradiation and ciprofloxacin sorption increase. Zhang et al. ^[21][60] also showed a twofold increase in the adsorption capacity of beached PS foams for oxytetracycline, OTC, concerning virgin PS foams, and the adsorption included electrostatic interaction, multivalent cationic bridging mechanisms, and H-bonding interaction. In the same way, ageing can also sustain other mechanisms, such as π–π, electrostatic interaction, hydrogen bonding, ion exchange, and complexation ^[21][60]. Other authors, reported decreased desorption of atorvastatin and amlodipine from weathered PS due to the increased electrostatic interaction ^[12][79].

Ageing also modifies polymer crystallinity since the weathering process enhances the percentage of the crystallization region and reduces the amorphous plastic portion, but the effects on PHs sorption are poorly known ^[44][105].